Trip Circuit Supervision in Circuit Breakers

Trip circuit supervision in Circuit breakers is an vital part of any protection scheme. If the trip relay fails to operate, it may result in upstream tripping or even in damage to equipment. Trip circuit supervision makes sure that the tripping coil of a circuit breaker is always in the healthy condition. 

The Trip circuit supervision is particularly important in breakers which have only one trip coil.   The Trip circuit supervision relay continually measures the resistance of the trip coil of circuit breakers. It also measures the control voltage of the trip coil and gives and alarm when the control voltage falls to low levels. 

The Trip circuit supervision relay injects a constant current through the trip coil of the breaker and measures the voltage drop across the coil. Thus, the relay is able to measure the resistance of the coil. 

The Trip circuit supervision relays can also monitor more than one breaker coil. 

If the Trip circuit supervision Relay detects a fault, it activates the breaker failure logic which can activate a backup breaker if installed or cause the tripping of upstream breakers.

Circuit Breaker Operating Time Measurement

The operating time of a circuit breaker is crucial in any protection scheme. A circuit breaker that takes too long to open will seriously compromise protection causing damage to equipment and people.

Hence, circuit breakers should be periodically tested to see whether they operate at the correct operating time specified by the manufacturer.

Being mechanical devices, circuit breaker are made of numerous springs, washers, and linkages. These components can get jammed, the lubricating oil in the linkages can evaporate or lose its properties. The lubricating oil can mix with dust and form a viscous deposit. This can cause partial seizure and is particularly true for circuit breakers which are kept in the closed or open position for very long periods of time.   

The Circuit Breaker operating time can be measured by special testing equipment known as the Time interval meters. These instruments measure the time between the signal to open and the actual interruption of the current.   The opening time of the contacts belonging to each individual phase is measured.

Making and Breaking Capacity of a circuit breaker

         Making capacity of a circuit breaker is the maximum current which the breaker can conduct at the instant of closing. The making capacity is considered to the peak value of the first cycle when there is an imaginary short circuit between the phases.

         When there is a short circuit in the line and the breaker is closed, the peak value of the first cycle is the most severe from an electrodynamic perspective. This value is in kA. The making capacity is expressed as a peak value as the dc offset during fault conditions is taken into account.

         Breaking capacity of the circuit breaker refers to the maximum current in rms value the circuit breaker can interrupt. This is also in the order of kA. 

         The making capacity of the circuit breaker is usually greater than the breaking capacity of a circuit breaker as breaking an electric circuit is difficult due to arcing which occurs and which has to be quenched.

Measuring Contact Resistance in Circuit Breaker Contacts

The contacts in the circuit breaker need to checked periodically to ensure that the breaker is healthy and functinal. Poorly maintained or damaged contacts can cause arcing, single phasing, and even fire.

The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact measurement check.

The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.

The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 300 A through the contacts and measuring the voltage drop across it. This test is done with a special contact resistance measuring instrument.

Then, using Ohm's law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records.

Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in a damaged conditions.

Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.

Stuck breaker protection

Stuck breaker protection is a situation in which a circuit breaker fails to operate even after receiving a tripping signal from a relay or a switch. Stuck breaker can undermine the protection scheme and can cause damage to machinery and is a danger to personnel. 

Common reasons for a circuit breaker not opening are a disconnection in the trip circuit or a mechanical problem with the circuit breaker. In these conditions, there needs to be a backup protection device which can interrupt the fault and isolate the system. In some cases, the entire section of the bus to which the breaker is connected is de-energized to interrupt power. 

A simple Stuck breaker protection schemes functions by sensing the position of the circuit breaker through the limit switches in the circuit breaker. The protection system waits for the open status from the circuit breaker after the open signal has been given. If the signal is not received within a preset time, the scheme assumes that the breaker is stuck and initiates backup measures.

However, this system has its limitations. The system cannot detect a situation where the current continues to flow despite the breaker having tripped. This can occur due to situations where the arc has not been quenched (failure of the arc extinction system) and the current flows even though the contacts have mechanically separated. 

To ensure proper feedback of the interruption of the current, advanced stuck breaker schemes sense the current as well as the position contacts of the circuit. This ensures that an accurate feedback of the breaker status.

Classification of Circuit Breakers

Circuit breakers are used in a wide range of applications. They are used in many environments and can handle currents and voltages of different ranges.

Circuit breakers are classified on the basis of different criteria. Some of the classifications are below

Based on the interrupting medium

Air circuit breaker
Oil circuit breaker
SF6 circuit breaker

Based on type of action

Automatic circuit breakers
Non-automatic circuit breakers

Based on the method of control

Locally controlled circuit breakers
Remotely controlled circuit breakers (remote control can be mechanical, pneumatic or electrical)

Based on the type of mounting

Panel mounted circuit breakers
Remote from panel mounted circuit breakers
Rear of panel mounted circuit breakers

Based on location

Outdoor circuit breakers
Indooor circuit breakers

Based on voltage

Low voltage circuit breakers
Medium voltage circuit breakers
High voltage circuit breakers

Air Circuit Breakers

Air Circuit Breakers are Circuit breakers where air is used as the medium of extinguishing the arc.  The air is usually compressed and kept in a cylinder.  When the breaker operates and the contact separation occurs, the arc is driven into special arc chutes by means of compressed air which is blown through specially designed nozzles. 

Air Circuit Breakers are mostly used in the LV range.  They can interrupt currents of several thousand amperes. 

Air Circuit breakers are provided with an inherent current sensing mechanism like the thermo-magnetic release.

Arc Chute used in Air Circuit Breakers

The Arc chute is a component which is used to weaken and quench the arc.  The Arc Chute contains a number of splitters which split the arc into a number of sections.  This increases the length of the arc and quenches it.  The arc which is formed during the separation of the fixed and the moving contact is driven into the chute by means of the pressurized air.  The arc chute is usually made of composite refractory materials. 

Air Circuit Breakers are available in both Three pole and Four pole versions.

Air Circuit breakers are used widely in the industry for the protection of facilities and transmission lines.  They are also used for protection of electric equipment such as transformers, motors, etc.  Air circuit breakers are also used in mines and on board ships. 

Vacuum Circuit Breakers

Vacuum Circuit breakers are a very popular type of circuit breaker used in the industry. Vacuum Circuit Breakers or VCBs as they are popularly known use vacuum as the quenching medium.

Vacuum Circuit breakers work by opening the contacts in an evacuated chamber. The vacuum in the chamber ensures that there are very few ionisable molecules which can sustain the arc. The arc is, thus, unable to sustain itself and is extinguished.

Vacuum circuit breakers have been used up to 36 kV and can interrupt up to 4000A. When the arc is initiated during contact separation, metallic vapour is produced from the contacts. If the contacts are of soft metal, very little vapour is produced. While this ensure quick arc extinction. The current waveform can get chopped and lead to high voltage transients. If too soft metals are used in the contacts, the arc would cause erosion of the material. Hence, the contacts are made of materials which are neither too soft or hard. Common materials used are alloys Copper-bismuth or copper-chrome.

Live Tank circuit breakers and Dead tank circuit breakers

Live Tank circuit breakers are circuit breakers in which the interrupting chamber is at the line potential. The interrupting chamber should therefore be provided with insulated supports. The centre of gravity of these circuit breakers is higher, hence live tank circuit breakers need extra support for seismic capability (ability to withstand earthquakes)

In dead tank circuit breakers, the interrupting chamber is at ground potential. The conductors enter the interrupting chamber through insulated bushings. Maintenance activities are easier to conduct as the interrupting chamber is at ground level. Seismic capability is higher as the interrupting chambers are at ground level. 

Generator Neutral Breaker

The Generator Neutral Breaker is used in systems, which are grounded through low resistances or solidly grounded (without a resistance). In such systems, the fault current in the line due to an earth fault will be high.

The current flowing through the equipment due to an earth fault can be limited if a breaker is connected in series with the neutral. This breaker is opened simultaneously with the armature and the field breaker. This will bring the fault current to zero quickly.

Pre Insertion Resistors in Circuit Breakers

Circuit breakers used in switching of long transmission lines have a resistors which is pre-inserted between the contacts before the contacts are closed. This resistor is called the Pre-insertion resistor. The function of this resistor is to limit the initial charging current of the line. The resistance of the line is around 500 ohms.

Once the closing command is given to the breaker, the resistor is first connected across the contacts. This resistance in series limits the line current. A few milliseconds later, the contacts are closed. 

While opening the breaker, the pre-insertion resistor is first disconnected before the contacts are opened by the circuit breaker. Pre-insertion resistors are also used in lines which have transformers to limit the high inrush current.

Primary and Secondary Protection Schemes

Protection Relay systems are classified into two types.

Primary relaying Equipment and
Secondary relaying Equipment

The Primary relay protection equipment is the first line of defence. The secondary relay scheme comes in line when the primary relay system fails to act.

The Primary relay protection scheme can fail due to reasons such as

Failure of DC tripping voltage supply.
Failure of Current or voltage signal to the relays.
Failure of the Circuit Breaker.
Failure of the internal mechanism of the Protection Relays. 

The Secondary relay Protection scheme is intended to operate in the event of a failure of the primary supply. Hence, the secondary relay protection scheme should be totally independent of the primary. The current and voltage signals, the power supply of the relay, the output to the breaker should all be independent of the primary protection scheme. The secondary protection scheme has a time delay greater than that of the primary relay protection scheme. 

Non-directional Earth fault Protection using Residual Current Relay

The Residual Current based earth Fault relay works by measuring the vector sum of the three phase currents. 

Under healthy conditions, the vector sum of the three currents is zero. In the event of an earth fault, however, the fault current flows through the ground and hence, the vector sum of the currents is not equal to zero. This is known as the residual current. This current can be used to operate the earth fault relay.

The connection of the earth fault relay consists of three current transformers connected in parallel to each other. This kind of earth fault protection is also known as unrestricted earth fault protection.

The residual current protection is usually set to operate at around 10% of the nominal current. For fault currents lower than this value, as may be the case in high resistance grounded generators and transformers, the sensitive earth fault relay is used. This is because, the three current transformers used in the residual current protection may not be exactly identical in response, even if they are from the same manufacturer. Thus for very low setting, there is the risk of false operation of the relay due to errors in the current transformers. 

Since the sensitive earth fault relay uses one Core Balance Current Transformer instead of three individual current transformers, it can be set to lower values of earth fault current.

Voltage dependent overcurrent protection

Overcurrent protection is a crucial component of the generator protection scheme. Overcurrent protection is used to protection the generator against overloading. It is also used to isolate the generator in the event of a short circuit fault.

However, there is one issue to be considered when designing a protection for a generator. In the event of a short circuit, the fault current is very high for a few milliseconds after a fault. This heavy current causes the generator voltage to drop. This drop in voltage causes the current to decay. Therefore, a high overcurrent setting may not operate in the event of a short-circuit.

To solve this problem, voltage dependent overcurrent relays bias the overcurrent setting with the measured voltage. That is, at normal voltage, the overcurrent relay operates if the current exceeds the setpoint. However, if there is a voltage drop, the overcurrent setting also progressive decreases according to the biasing. Thus, at lower voltages, the current required to operate the relay is very low.

A variation of the voltage-dependent relay is the voltage controlled. This relay has an undervoltage setting and a overcurrent setting. The overcurrent setting is set at a value less than the rated current of the generator. For the relay to operate, both the undervoltage and the overcurrent need to occur at the same time. This can occur only at the instant of a short circuit.

Sensitive Earth Fault Protection

         The sensitive earth fault relay is a protective device that works by measuring the residual current across the three phases in a system. This is done using a Core Balance Current Transformer (CBCT).  In the ideal condition, the residual current will be zero as all the currents flow through the three wires and their magnetic fields cancel each other out.

         In the event of a fault, the residual current over the three phases will not be equal to zero as the current from the faulted phase flows through the earth.

         The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding. High resistance grounding restricts the earth fault current to less than 10A. High resistance grounding enables electrical systems to continue running when one of the phases is faulted. This prevents interruptions to the power supply. This kind of earthing system provides time to identify and isolate the fault.

         Once an earth fault occurs in the high resistance grounding system, an alarm needs to be generated and the fault needs to be traced. For this a reliable protection which detects earth faults even when the fault current is very low is necessary. Undetected earth faults in this system are dangerous as a second earth fault in another phase may result in a short-circuit. Conventional earth fault relays may not be accurate in detecting an earth fault at such low current values.

         The sensitive earth fault protection, as the name suggests, is a highly sensitive relay. It can sense currents as low as 0.2% of the CT secondary current.

         The sensitive earth fault relay may be configured to either generate an alarm or a trip signal.

Anti-pumping relay

The anti-pumping relay is a device in circuit-breaker whose function is to prevent multiple breaker closures. For instance, if the operator gives the closing command to the breaker by pressing the close button and the breaker closes. However, a fault in the system causes the breaker to trip. Since the close command is still in the pressed condition, there is a chance of the breaker closing again and being tripped by the relay multiple times. This can damage the closing mechanism of the breaker. The anti-pumping relay prevents this by ensuring that the breaker closes only once for one close command from the control panel.

Directional Power Relay

The directional power relay is used to protect a synchronous generator, running in parallel, from motoring. Motoring occurs due to the failure of the prime mover such as a turbine or an engine driving a generator that is connected to the grid. The generator which is running at the synchronous speed will continue to run at the same speed. However, the power required to keep the generator running along with the prime mover will be drawn from the mains. Hence, power flows in the reverse direction i.e. bus to generator. This condition is called reverse power.

Directional power operation may cause damage to the prime mover. Hence, reverse power protection is a vital part of the generator protection scheme.

The reverse power relay operates by measuring the active component of the load current, I x cos φ. When the generator is supplying power, the I x cos φ is positive, in a reverse power situation it turns negative. If the negative value exceeds the set point of the relay, the relay trips the generator breaker after the preset time delay.

The typical setting for reverse power is 4% in case of turbines and 8% in case of diesel engines. The time delay can be set from 2 to 20 seconds.

The Vector Surge Relay

The vector surge relay is used to decouple synchronous generators from the grid utility in case of grid failure.

Synchronous generators are generally operated in parallel with the grid utility. This ensures greater reliability and enables the generator to export power to the grid. In this condition, there is a chance, of a momentary interruption of the grid supply which may result for a few milliseconds. Such temporary interruptions can be caused to mal-operation of the circuit breakers on the grid transformer side.

For a synchronous generator, running in parallel with the grid utility, such a temporary interruption and restoration of the supply can be dangerous. As the restoration of the supply can be asynchronous i.e. the generator and the grid are now not in a synchronised condition. The can lead to the consequences of wrong synchronization such as damage to the generator or the prime mover.

The vector surge relay prevents this condition by decoupling the generator from the grid as soon as the grid supply fails. This is an extremely fast acting relay with an operating time of less than 300ms from relay operation to breaker opening.


The vector surge relay functions by monitoring the rate of change of the rotor displacement angle of the generator. During parallel operation there is an angular difference between the terminal phase voltage (Up) and the internal synchronous voltage of the generator (Ui). This is due to the fact that the generator rotor is magnetically coupled to the generator stator and is forced to rotate at the grid frequency. The angle between the vector of the mains voltage Up and synchronous electro-motive force is known as the rotor displacement angle.

This angle is constantly varying and is dependent on the torque produced by the generator rotor. In the case of the grid failure, there is sudden change in the rotor displacement angle.

This causes a surge in the generator voltage shown in the figure. The relay works by monitoring the time taken between the zero-crossings in the waveform. Under normal operation, the time interval between two consecutive zero-crossings is almost constant. During the grid failure, the vector surge which occurs causes a delay in the zero-crossing. This delay is detected by a highly sensitive timer inside the relay and the relay operates.

The relays are usually set to operate for a change in the rotor displacement angle of 0 to 20 degrees

Opto-coupler Relays

Optocoupler relays are relays in which the changeover of contacts is effected by the switching on or off of a light source which is linked to a SCR or a TRIAC. The SCR or the TRIAC is switched on or off when the LED is switched unlike conventional relays, where it is done electromagnetically.

These relays are faster than electromagnetic relays. More importantly, the provide isolation between the control and the power circuits.

These relays do not have any moving parts which can deteriorate due to arcing or operational wear. However, they are expensive over conventional relays and hence still find limited application.

Another advantage of the Solid state relay is that it can open an AC circuit when the sinewave crosses zero. This ensures that the back-emf is minimum and this ensures that there are no voltage kicks in the opposite direction when the circuit is open due to inductance. This is because the Triac or the SCR used in the solid state relays conduct current till the waveform reaches the zero point even after the optocoupler LED has been switched off. This prevents premature failure of the contacts.

The Advantages of Optocoupler relays include

Smaller Size

Faster Response time
Noiseless operations as there is no mechanical movement of the contacts.
Optocoupler relays can withstand high vibration compared to conventional relays
They do not generate Electromagnetic radiation as there is no coils to be energized.

However, they also have some disadvantages.

They are more expensive
They generate heat and require special heat-sinking fixtures.
They cannot switch on very low currents
When Solid state relays fail, they fail in the "closed position". In this situation, the machine which is connected will continue to be in the operating condition and there will be difficulty in isolating it. Electromagnetic relays usually fail in the "open" position.

Difference between AC and DC relays

AC and DC relays work on the same principle, that of, electromagnetic induction. However, there are some differences in construction. DC relays have something known as the freewheel diode which acts to discharge the emf built in the inductance when the coil is de-energized. (Click here to read about the phenomenon of freewheeling.) AC relays have cores which are laminated to prevent losses due to eddy current heating.

Another more conspicuous difference between a DC relay and an AC relay is presence of the Shading Coil. In AC relays, the alternating current supply changes direction about 100 times a second. At each instance, when the sine wave passes through zero, the current flowing through the coil becomes zero. This results in a loss of magnetism for a few milliseconds. When this happens about 100 times a second, the repeated drop and pickup of the coil produces a noise known as chattering. This also leads to the making and breaking of the relay contacts leading to disturbances in the connected electric circuits.

A shading coil is a coil with high remanence. thus when the magnetism of the coil collapses when the current becomes zero. The shading coil still retains the magnetism. Thus, ensuring that the contacts do not drop off.

Reverse Power Relay - Function and Operation

A reverse power relay is a directional power relay that is used to monitor the power from a generator running in parallel with another generator or the utility. The function of the reverse power relay is to prevent a reverse power condition in which power flows from the bus bar into the generator. This condition can occur when there is a failure in the prime mover such as an engine or a turbine which drives the generator.

Causes of Reverse Power

The failure can be caused to a starvation of fuel in the prime mover, a problem with the speed controller or an other breakdown. When the prime mover of a generator running in a synchronized condition fails. There is a condition known as motoring, where the generator draws power from the bus bar, runs as a motor and drives the prime mover. This happens as in a synchronized condition all the generators will have the same frequency. Any drop in frequency in one generator will cause the other power sources to pump power into the generator. The flow of power in the reverse direction is known as the reverse power relay.

Another cause of reverse power can occur during synchronization. If the frequency of the machine to be synchronized is slightly lesser than the bus bar frequency and the breaker is closed, power will flow from the bus bar to the machine. Hence, during synchronization(forward), frequency of the incoming machine is kept slight higher than that of the bus bar i.e. the synchroscope is made to rotate in the "Too fast" direction. This ensures that the machine takes on load as soon as the breaker is closed.

Setting the Reverse Power Relay

The reverse power relay is usually set to 20% to 50% of the motoring power required by prime mover. By motoring power we mean the power required by the generator to drive the prime mover at the rated rpm. This is usually obtained from the manufacturer of the prime mover (turbine or engine).

Grading of Overcurrent Relays

Grading of over-current is the adjustment of the settings of the over-current relays to ensure discrimination and selectivity. Consider a radial feeder with multiple feeders in series. An over-current protection relay is installed at every breaker location. When a fault occurs at any given point, only the relay located closest to the fault should operate.  This is known as grading.

There are different types of grading. They are
1)   Current Grading
2)   Time Grading
3)   Time-Current Grading

Current Grading

Current Grading refers to the discrimination achieved by reducing the current setting as we move towards the power source. This ensures that the relay closest to the fault trips first. The downside of this arrangement is that the fault current does not always vary with the location. Hence, it is not possible to accurately discriminate between the relays.

 Time Grading

Time Grading refers to the discrimination achieved by varying the time delay for the different relays. In this method, the relay farthest from the source has the shortest time delay and the time delay increases as we move towards the source. That is, the source breaker will have the highest time delay. This will work in systems where the fault current is uniform across the system. However, this type of grading will not be sufficient in systems where the fault current varies with the location of the fault.

Time-Current Grading System

The Time current grading system is the most widely used method of Grading. This method uses a combination of Time and Current grading to achieve discrimination. In this method, the time setting varies with the fault current. A severe fault will have a shorter time delay  while the delay will be more for a mild fault.

Selection of Current Setting

The current setting is determined by first calculating the current during a fault. This is done by a procedure called the fault level calculation. The current during a fault will depend on the number of power of upstream power sources. Thus the fault current at minimum generation and the fault current at maximum generation should be calculated. A three phase fault during maximum generation will cause maximum fault current while a fault between two phases during minimum generation will result in minimum fault current.

Each section of the distribution should serve as a backup for the immediate section downstream. The setting such that the relay operates for a fault at the adjoining section during minimum generation. The current setting is lowest at the feeder farther from the source and increases towards the source.

Voltage Supervision Relays

The Voltage Supervision Relay is an integral part of any protection system. The voltage supervision relay protections systems from undervoltage and overvoltage. Overvoltage in a system can result in serious damage to insulation and equipment while undervoltage can cause motors to draw more current and reduce the speed of the motors, disturbing the process.

Besides protecting against overvoltage, the voltage supervision relay can also be used to detect earth faults as the phase to earth voltage is distorted when there is an earth fault in one of the phases. Voltage supervision relays can generate alarms when the voltage is low or high in only one phase. This is also known as phase asymmetry.

In motor circuits, the voltage supervision relay protects against single phasing. Single phasing can cause serious damage to motors.

A simple auxiliary relay can also be used to generate alarm for undervoltage. When the voltage drops, the relay can drop off thus generating an alarm or a shutdown.

Rotor Earth Fault Relay

The Rotor Protection relay is used in synchronous motors and generators to identify the presence of an earth fault in the rotor winding. While the winding in the rotor is insulated from the ground during normal operation, the Rotor is subjected to stresses due to vibration, heat, etc. These stresses can cause the winding to give way in a particular place and the winding can get earthed.

While a single earthing in the winding is not immediately damaging. It sets the stage for damage if a second failure should occur. The second earthing can cause a short-circuit through the rotor causing extensive damage to the rotor and the winding.

The currents produced during a rotor earth fault can cause excessive vibration and disturb the magnetic balance inside the alternator. These forces can cause the rotor shaft to become eccentric and in extreme cases cause bearing failure.

Hence, it is necessary that any earthing in the rotor is detected at the earliest.

In slip ring rotors, carbon deposits on the slip rings may compromise the insulation resistance of the rotor. Hence, the slip rings need to be inspected for any deposits.

The Rotor Earth Fault Protection Device consists of a current injection device which applies an AC voltage to the rotor winding by means of a slip ring fitted on the rotor. The current is applied to the rotor through a coupling capacitor. In the normal condition, the system is floating and the current flowing through the device is zero as the resistance is high.

When a fault occurs, the current increases causing the relay to operate. The relay can be configured for alarm or trip depending on the criticality.

Any Protection Relay should fulfill the following functional Characteristics.

Selectivity and


Reliability means that the relay will act when it is required to act. This is ensured by making sure


Sensitivity refers to the characteristic of the relay to act when the actual fault conditions occur. Sensitivity is usually represented in terms of the minimum volt-amperes required for the relay operation.


The relay should act according to the present time delay. It should neither operate too fast or too slow. If it is too slow it can cause damage to the equipment, if it operates too fast it may unnecessarily trip equipment for transient faults.


Selectivity refers to the ability of the relay to discriminate between faults. This is critical as only the smallest possible section of the power system should be taken out of line in the event of a fault. The relay should be able to discriminate between a transient fault and a through fault. It should be able to differentiate between downstream faults and immediate faults.
that all the components of the protection from the voltage and current signals to the dc power supply for the trip circuit to the internal components of the relay are checked for for functionality and integrity. The failure of any one of these components can result in the failure of the relay to act affecting the reliability.

Classification of Relays

Relays can be classified on the basis of their function into five broad categories. They are Protective, Regulating, reclosing synchronism Check and Synchronizing, monitoring and Auxilliary.

Protection Relays
Protection relays are used in generators, transformers, feeders, transmission lines, etc. The primary function of these relays is to continually monitor a specific parameter such as current, voltage or power and to generate alarm/isolate the system or device in the situation of deviation from set limits for the parameter or a fault. For instance, an overcurrent relay may be programmed to operate when the current in a feeder exceeds a certain predetermined limit. These relays generally obtain their feedback from current or voltage transformers.

Regulating Relays
These relays are used to regulate a specific parameter such as the output voltage of a transformer. These relays operate a control equipment such as the tap changer of a transformer. These relays are not designed to respond to fault conditions.

Reclosing Relays,
These relays are used to put the system into operation. These relays are used to synchronize lines and feeders. These relays usually are used in connecting different components of an electrical distribution system such as generators, feeders, transformers, etc. They also come into play when restoring the system after a fault.

Monitoring relays
These relays are used to monitor conditions in a system such as the direction of power flow and generate alarms when there are deviations. Examples include the low forward power relay which generates an alarm when the power in a direction falls below the minimum set points. They are also used to monitor the continuity of systems such as pilot wires.

Auxilliary Relays
These relays are used generally for contact multiplication. The single contact available in a relay is used to trip a number of breakers. Besides, these relays are also to isolate the relay from other equipments such as breakers.

The Universal Torque Equation for Protection Relays

The Universal Torque Equation is a equation which governs the application of all types of relays. The equation has variables and constants which can be ignored for specific functions.

This equation can be used to describe the operation of any Electrical Relay by changing the signs of some of the terms or ignoring them entirely.

For example, to describe the overcurrent relay, K2 and K3 can be considered zero while K will be negative as it is used to describe the restraining torque.

The Equation will then become


In the case of a directional power relay, K1 and K2 can be considered to be zero while K can be considered to be negative.

Capacitor Protection Relays 

Capacitors Protection Relays are dedicated relays which are designed to provide a range of protection functions for capacitors.

Capacitors are widely used in power systems for VAr regulation and PF control.  They are also used for filtering harmonics.

Capacitor banks need to be protected against overload by harmonic currents. They also need to be protected against system overvoltages.

Capacitor Protection Relays consist of a number of different protection elements such as overcurrent, overvoltage, differential protection, etc.   They also have protective interlocks such as preventing an energised capacitor from being connected to the network.

Types of Relays

Electric Protection Relays are vital components in the protection scheme. Protection Relays protect an equipment such as a Transformer or a Generator from internal and external faults such as overvoltage, overcurrent, earth fault, etc.

Relays have been in existence since the early years of Electrical Engineering. Relays can be configured for instantaneous operation or delayed operation. 
There are three broad categories of relays based on the principle of functioning. 

They are Electro-Mechanical Relays, Numerical Relays and Solid State Relays

Electro-mechanical Relays

Electro-mechanical Relays are the oldest type of relays. These relays are simple in construction. They are easy to adjust. These Relays consist of a disc, usually made of aluminium, which rotates when there is a fault. The rotation occurs due to the presence of eddy currents caused by current and voltage coils. 
When a fault occurs, the rotating disc rotates and closes the alarm contacts which generate the alarm. If the fault is severe or persistent, it closes the tripping contacts which issue the tripping command to the circuit breaker. 
The disadvantages of these relays is that the values tend to drift over time, due to the effects of heat, vibration and aging on the relay components. These relays are gradually being replaced by numerical and solid State Relays

Numerical Relays

These relays are electronic relays. They do not have moving parts. In the Numerical Relay, the analog values are converted into numbers. The alarm and the trip values are also fed into the relay and stored as digital values.   Numerical relays are also called digital relays. The microprocessor monitors the field values and generates the alarm or the trip command. Numerical Relays are programmable. The behaviour and the characteristics or these relays can be programmed. Numerical Relays are also multifunctional which means that the same relay can be used for overvoltage as well as overcurrent protection. 
Modern Numerical Relays can also communicate with protocols such as Ethernet, RS 485, etc. They can store historical data of trends and events. This feature will be helpful in analyzing a fault condition or a blackout. Timestamping also enables sequential registering of events.

Static Relays

Static Relays are analog relays. In static relays, the voltage or the current from the field is converted into rectified voltages and currents. These values are then processed by means of op-amps, transistors, etc and the output signal is generated. 

Common Terms in Relay Protection

Common Terms in Relay Protection are

Pick Up Value

Pick up Value of relay refers to the value above which the relay will generate the alarm or the trip.

Operating Time

The Operating Time is the time which is allowed to elapse after the pick up value has been exceeded before the output is given. 

Reset Value

Once the relay operates, it has to be reset. The relay resets after the measured value falls beyond a certain value. This value is below the pick up value in case of functions such as overvoltage or overcurrent. In case of functions such as undervoltage or underfrequency, this value is above the pick up value.

Reset Time delay

Reset Time delay is the time taken by the relay to reset after the reset value has been reached.

Reach of the relay

This is a term used in distance protection. The distance Relay operates when there is fault in a cable.   Reach of the relay refers to the distance till which the relay can sense the fault. 

VA Burden in Protection Relaying

Protection Relays serve to protect equipment and circuits from abnormalities such as overcurrent, overvoltage, overloading and under reactance. The input to the protection Relays are the current and the voltage of the system. Using these two basic parameters, the relays are able to calculate a host of values such as kW, kVAr, pf, etc.
Thus, every protection relays needs an input from the Potential Transformer or the current transformer or both. When the relay is connected to the circuit of an instrument transformer, it becomes a load. The Potential or Current Transformer acts as the source.  
When designing the Protection Scheme, we must ensure that the Potential Transformer or the Current Transformer does not get overloaded. This is done by adding the VA burden of each protection relay in the system. Every Relay will have the VA burden mentioned in the manual. The total VA burden imposed by all the relays should be calculated. The VA capacity of the instrument transformer should be greater than this. 
While designing a system, the instrument transformer should have an excess capacity of 10% of the present load. This is an allowance for future relays which may be added to the system. 
If the transformer is overloaded, the voltage and current signals will not be accurate. 
The wires used in the protection system should be of sufficient thickness so as not to added unnecessary burden on the transformers.

Coreless DC motors

Coreless dc motors are small motors which do not have a core.  The armature winding is self-supporting wound in the form of a basket.

These motors can have extremely high acceleration due to reduced weight of the rotor.  The rotor armatures also have low inductance.  This helps in extending the life of the brushes and the commutator.

These rotors have low inertia and low noise and vibration and thus can be easily controlled.

The downside to these motors is that the rotor gets heated quickly as it does not have the core which can serve as a heat sink.  These motors can be used only for low power application such as the motor in magnetic tape devices, optical rotary encoders, etc..

Motor Selection Guide - Online is a website that helps engineers quickly select a motor based on their requirement.  You just have to only key in the required speed and torque and specify the type of motor you are looking for, induction, dc, synchronous, etc.

The site launched in 2008, now claims to have a database of over 10000 configurations.  The site is brand independent and includes specifications from all leading brand. 

You can also test the characteristics of motors which fit your search online.

Flame Proof Motors

Flame Proof motors are used in hazardous locations usually in chemical, petroleum and gas industries, where the environment may contain gases or inflammable vapours. Electric equipment are notorious for being the source of sparks during switching. This could be catastrophic in areas where inflammable gases are present.

Hazardous areas classified into zones depending on the possibility of the presence of gases and combustible dust-air mixtures.

The following are the classification for hazardous areas

Zone 0, An Explosive atmosphere is continuously

Zone 1, An Explosive atmosphere is present for 1000 hours in a year.

Zone 2, An Explosive atmosphere is present for 10 hours in a year.

Flameproof motors are specially designed so that any explosion which occurs within the motor is contained within the motor frame itself. This prevents any secondary explosion in the surrounding area. The flame-proof motor is specially enclosed within a protective shield which covers the entire motor including the bearings.

Flame proof motors are heavier that motors of similar rating due to the increased weight of the reinforced design of the end shields so as to withstand the impact of an explosion.

Flame proof motors have special arrangements for cooling. Since conventional air circulation is not possible with these motors, special arrangements such as using the internal air to carry heat to the surface of the motor where the heat is transferred to the external air by means of special tubes have been devised.

Another method of cooling is the use of a coolant such as water which circulate in the stator and rotor of the motor. This water is brought to the near the inside of surface of the motor where it is cooled by means of the atmospheric air.

The cooling system ensures the isolation of the internal cooling medium from the external atmospheric air.

Totally Enclosed Fan Cooled (TEFC) Motors

The Totally Enclosed Fan Cooled Motor is the most widely used type of motor.  It consists of frame which totally encloses the motor.  Thus there is no airflow between the inside of the motor and the outside.  However, the frame is not airtight.

The term "Totally Enclosed" means that the motor is dust proof.  It is however not submersible.

Cooling occurs by means of a fan which blows air over the frame.  The air which passes over the frame removes the heat created by the motor.  The enclosure has fins to maximize the surface area to enhance cooling.

Open Drip Proof (ODP) Motors

Open Drip Proof Motors(ODP)  are motors which are used in applications where there may be dripping water.  These motors are covered by a metal enclosure which ensures that any water which drips will not flow into the motor.

Drops of water which fall at an angle of 15% will be caught by this enclosure.  Open drip proof motors usually have a fan for cooling the motor.  These motors are relatively inexpensive and used widely in the industry.

Open Drip Proof motors are used in clean, dry environments.  Sometimes, the air from the blower is made to pass through a filter before entering the motor

Can Single Phase motors be reversed by changing the polarity?

Single phase motors have two windings, the main winding and the auxilliary winding. The auxiliary winding is used to start the motor and may be disconnected once the motor picks up sufficient speed.

Reversing a single phase motors cannot be done by reversing the polarity of the supply to the entire motor. To reverse the single phase motor, the polarity of the supply to only one of the windings needs to be changed.

This can be done by reconfiguring special links which may be provided in the terminal box of the motor.

Reluctance Motor-An Overview

A reluctance motor works on the principle of reduced reluctance.  Reluctance motors have high power density.  However, their downsides are low efficiency, low torque and low pullout torque.  However, they are ideal for small power applications such as hard disk drive motor, Analog electric meters, etc.

The reluctance motor works on the principle that a piece of iron placed in a magnetic field will rearrange itself such that the reluctance of the magnetic path is minimal.

The Reluctance motor consists of a wound stator.  The rotor is made of laminated material in which poles are cut so that a salient pole rotor is produced.  The number of rotor poles is made less than the number of stator poles.

When a three phase supply is connected to the stator, a rotating magnetic field is set up. The rotor tries to align itself in a minimum reluctance path with reference to the magnetic field of the stator.  As the stator magnetic field keeps rotating, the rotor moves along with it.

Reluctance Motors are classified into

Switched Reluctance Motors and

Synchronous Reluctance Motors

Fractional Horse Power motors

Fractional HP Motors are motors whose power rating is less than one horse power i.e. 746 watts.  Fractional Motors range from an output of 1/20th horsepower to 1 horse power.  Motors less than 1/20th horse power are called sub-fractional horsepower motors. 

Fractional motors find wide application in automobiles for rolling up windows, windshield wipers, etc.  Induction motors, synchronous motors and dc motors can be used as Fractional HP motors. 

Fractional HP Motors also find wide application in household appliances.   Fractional Horse Power motors used in household application such as exhaust fans, blowers etc are usually single phase.  They are generally of the split phase or the capacitor run type. 

Extremely low speeds can be obtained using Fractional HP motors by means of suitable drives.

Stepper motors and servo motors are also types of Fractional HP motors.  Fractional HP motors are also available as geared motors.

Frameless Motors

Frameless motors as the name suggests, are motors which do not have a frame.  The stator and the rotor are delivered separately and are assembled to the load.  Thus, the motor does not have bearings.  The rotor is directly coupled to the load while the stator is coupled to the frame of the machine.  These motors are custom designed to meet the speed requirement.

Thus this arrangement eliminates the need for the coupling.  This increases the stiffness of the power train of the machine.  It also eliminates torsional application.  The size of the motor is also considerably reduced to around one seventh. This reduced weight for a given power output reduces inertial in the machine and enables quick movements and direction reversals. 

A hand tool with Frameless Motor
Frameless motors are widely used in Hand tools, Medical devices, in satellite technology and in semiconductor equipment.

Vibration motor used in cellphones

Vibration motors are motors which deliberately generate vibration.  These motors are used in mobile phones to create vibration alerting the user to a call or a message.

They are also used widely in the industries such as in construction industry to vibrate the concrete so that air pockets are not formed which the concrete is solidifying.  They are also used in mixers to prevent material from being left behind after the mixing process is over.

Vibration motors are also used in flour mills, pharmaceutical, food industries, etc to facilitate the smooth flow of materials in conveyors, hoppers and other mixing equipment. 

These motors are constructed just like normal motors.  However, they have a mechanically unbalanced weights attached to the output shaft.  These unbalanced weights create vibration.  The frequency and the magnitude of the vibration can be changed by modifying the shape and weight of the counterweights.

Split Phase Motors

A single phase induction motor is not self starting.  This is because the ac supply creates a pulsating magnetic field in the stator core and not a rotating magnetic field which is required for an induction motor to be self starting.

Many methods are used to start a single phase motor.  One such method is the split phase method.  The split phase motor has a main winding in the stator.  In addition to this, it has a starting winding wound in the stator.  The starting winding is connected parallel to the main winding.  When the supply to the windings.  The starting winding is displaced 90 degrees from the main winding.

This is because the starting winding has a higher resistance and occupies less space in the stator.  It is wound over a few slots in the stator and is usually placed above the main winding in the slots. The main winding has many turns and thus has a higher reactance.  Thus it lags behind the starting winding. 

Split phase motors are used where low to moderate starting torque is sufficient.  They are designed up to 1/3 hp. 

The starting winding is only used when the motor is starting.  The winding is connected through a centrifugal switch.  When sufficient rpm is reached, the centrifugal switch operates and isolates the winding. 

Types of single phase motors.

Resistance Split Phase motors
It has Low Starting current.  The starting torque is moderate.  Applications include Fans, grinders, centrifugal pumps, washing machines

Capacitor Start Motors
The starting torque is high.  Hence, it is used for applications with heavy connected loads such as pumps, conveyors, compressors, etc.  It is the most widely used of all single phase motors.  It is used for motor sizes up to 6 kW.  The capacitor is in line during starting  The capacitor is disconnected by a centrifugal switch as the motor approaches the rated speed.

Permanent Capacitor Motor
The starting torque is high.  The power factor is good due to the presence of the capacitor at all times.  The efficiency is high and the torque produced is smoother.  Ceiling Fans use this type of motor

Two Value Capacitor Motors
This motor is also a capacitor based motor.  However, here there are two capacitors instead of one.  One capacitor is in line during start of the motor while the other capacitor comes in line when the motor is running.

Shaded pole motor
The Shaded pole motor works on the principle of a shading ring which is fitted to the stator poles. This ring varies the reactance and creates two fields.  This motor is simple in construction.  It is used for fractional kW applications such as in hair dryers, table fans, etc.  It has low starting torque and lower power factor.

Resistance split phase motor

The resistance split phase motor has two windings in parallel.  One of the winding is the main winding.  The other is the auxiliary winding.  There is a centrifugal switch in series with the auxiliary winding such that the winding is disconnected at around 75% of the synchronous speed.

The auxiliary winding is made of thin wire of a few turns such as that it has low reactance and high resistance. This creates a magnetic field with a displacement of 30 degrees.  This produces a torque which is moderate and can be used in motors up to 250 watts.

Permanent Split Phase motor

A permanent Split phase motor consists of a main winding and an auxiliary winding.  The auxiliary winding has a capacitor in series.  The auxiliary is always in circuit.  It produces a flux which is displaced from the main flux by 90 degrees.

The permanent split phase motor suffers from torque pulsations at main speed.  The permanent split phase motor is used up to 1/4 horse power.  The starting torque is relatively lower.  Hence, the permanent split phase motor is not used for applications which require high torque.

Universal Motor 

Universal Motors are motors which run on both AC as well as DC power.  These motors find wide application in household electric appliances such as mixers, blenders.

The small size, high power and speed of universal motors make them ideal in these applications.  The Universal Motor is the motor of choice for washing machines as they can be used to agitate the drum by easily reversing direction.

A universal motor is a dc series motor which usually has some modifications which enable it to run efficiently on AC as well.  Running the motor on AC leads to eddy currents being developed in the core of the motor.  Hence, the core and poles of universal motors are made of laminated sheets.  The windings of the motors are made thicker to reduce the reactance. 

Like all DC machines, universal motors suffer from the effects of commutation such as sparking.  Hence, universal motors are used mainly below 1000W.  The high speed of the universal motor enables it to deliver a higher power even while having a small size.  Universal motors are usually kept coupled to the loads such as gears or blowers as they tend to run at very high speeds on no load conditions.

Speed Control is usually done by electronic means.

NEMA Classification of motors

The National Electrical Manufacturers Association (NEMA)  has classified motors design in the following manner based on starting current, rotor torque, breakdown torque and slip.

They are

NEMA design A

maximum slip of 5 %
medium to high starting current
normal locked rotor torque
normal breakdown torque
suited for a broad variety of applications
Typical applications : fans and pumps

NEMA design B

maximum 5% slip
low starting current
high locked rotor torque
normal breakdown torque
Typical Applications
common in HVAC application with fans, blowers and pumps

NEMA design C

maximum 5% slip
low starting current
high locked rotor torque
normal breakdown torque

Typical application : High inertia starts - as positive displacement pumps

NEMA design D

maximum 5-13% slip
low starting current
very high locked rotor torque
Typical application : cranes, hoists etc.

Vertically Mounted Motors
Vertically Mounted Motors are used for applications such as pumps and blowers. These motors are similar to their horizontal counterparts, except that they have few differences in the design. The most obvious example is in their ability to withstand axial loads.vertically mounted motors

Horizontally mounted motors do not have to withstand axial loads. Hence, roller bearings or ball bearings can be used. In vertically mounted motors, angular ball bearings are usually used to help the motor withstand axial thrust.

In case of higher thrust loads, special thrust bearings will be used.

Double Squirrel Cage Motors

One of the main drawbacks of the squirrel cage induction motor is that it has very low starting torque. Hence, for applications requiring high starting torques, the slip ring motor is the only choice.

However, by providing a double squirrel cage rotor to the induction motor, the starting torque of the motor can be increased.

The double squirrel cage induction motor consists of a rotor which has two cages. The outer cage consists of rotor bars with high reactance and low resistance. The inner cage, on the other hand, consists of rotor bars with low reactance and high resistance.

When the motor is started, the slip is high. As a result, the frequency of the currents induced in the rotor is high. This causes the rotor currents to flow in the outer cage. Thus the torque of the motor increases due to the high initial resistance in the outer cage.

When the motor reaches the rated speed, the slip between the stator and the rotor decreases and the frequency of the rotor currents drop. This causes the reactance of the inner cage to drop. Thus the current flows through the inner cage.

During normal operation, current flows through both the inner and the outer cages as the reactance is very low.

Thus, the double cage induction motor provides excellent speed torque characteristics during start-up.

The double cage induction motor is used in applications such as mixers and crushers in the industry where high initial torque is necessary. It is cheaper than the slip ring motor and does not require the complicated and costly starter circuits like the slip ring motor.

Applications of Squirrel Cage Motors

Squirrel Cage induction motors find wide applications in industries and in homes. Their rugged constructions and low maintenance makes them the motor of choice for many requirements.


Lathes and turning equipment
Industrial Drives
Fans and Blowers
However, the downside of squirrel cage motors is that they draw a very heavy current when switched on due to the absence of back-emf. Hence, they require special starters.

They also have low starting torque. Hence, they are seldom used in applications such as lifts and cranes.

Another disadvantage of the squirrel cage motor is the poor speed control. However, with the advent of Variable Frequency drives, this disadvantage has been overcome.

Types of DC Motors

DC Motors can be categorized into four types depending on the connection of the field and the armature windings:

DC Shunt Motors
In these motors, the field and the winding are connected in parallel. They are used in applications where there is minimal change in speed as the motor is loaded. The provide medium torque while starting.

DC Series Motors:
These motors are used for applications requiring high starting torque. Here, the field and the winding are connected in series. These motors can be used in applications requiring high starting torque such as in traction related applications. The load on these motors must never be reduced to zero as this may result in excessive speed.

Permanent Magnet Motors:
These are used in applications which require greater reliability. Here, the field is made up of permanent magnets. The efficiency of the motor is higher. Speed can be controlled by varying the voltage of the armature.

Compound wound motors:
These motors combine the features of shunt and series motors. They have on field winding connected in series to the armature and another field winding connected in parallel. They provide a heavy starting torque. This kind of motor can be used for loads which are not sensitive to speed variations.

DC Compound Motors

A compound motor is a combination of shunt and series motor i.e., a series field winding, wound with heavy copper conductor on top of the shunt field winding. The series field winding is connected in series with the armature. So that its mmf will be proportional to the armature current and in the same direction as the shunt field mmf

Typical compound motors designed for industrial application obtain approximately 50% of their mmf from the series field wen operating at rated load.

There are two types of compound motors connection,

If the connection to the series and shunt winding is in such a way that their respective mmfs are additive is called cumulative compound motor.

If the series field is reversed with respect to the shunt field, its mmf will subtract from the shunt field mmf, causing the net flux to decrease with increasing load, resulting in excessive speed, which is differential compound motor.

Stabilized - shunt motor
Compound motors, whose series field are designed to provide just enough mmf to nullify the equivalent demagnetizing mmf of armature reaction and provide a very slight speed droop, are called stabilized – shunt motors. The series field winding of such machines generally have one – half to one and half turns / pole and depending on the application, provide approximately 3 to 10 percent of the total field mmf at rated load. The speed of stabilized – shunt motors is fairly constant, with only a slight droop in speed with increasing load. Stabilized – shunt motors are used in applications that require a fairly constant speed and a moderate starting torque.
Reversing the direction of rotation of compound or stabilized – shunt motors is accomplished by reversing the armature branch or reversing both the series field and the shunt field.

AC Servomotors and DC servomotors-A comparison

AC Servomotors DC Servomotors
Suitable for Low Power applications
They have low efficiency
The operation is stable and smooth
It has low Maintenance as there is no commutator

Used for High Power Applications
They have high efficiency
Noise is produced during operation
Relatively more maintenance is required due to the presence of the commutator.

Asynchronous motors

Asynchronous motors are motors which run below the synchronous speed.  The term "asynchronous motors" is usually used to refer to induction motors.

Motors which run at super synchronous speed are also asynchronous motors.  The doubly fed induction motor is an example of a super synchronous motor.

By "asynchronous" we mean that the speed of the rotor is not equal to the synchronous speed of the rotating magnetic field of the stator.  The difference between the speed of the rotor and the speed of the stator is called slip.  In asynchronous motors, the slip is not equal to zero.  In synchronous motors, the slip is zero.

Synchronous motors are motors that run a a constant speed, known as the synchronous speed.  The speed of the synchronous motor is constant regardless of the load.

Applications of Synchronous Motors

Synchronous Motors are used in the following situations

Applications which require constant Speed
Applications which require load as well as power factor improvement

Synchronous Motors are used in applications which require constant speed.  Examples can be escalators which need to rotate at a constant speed as people get in and get out.  In industries, synchronous motors are used in systems which have to operate in synchronism such as in bottling plants, in robotics and in conveyors.

Fractional Synchronous motors are used for small applications such as in microwaves, electric clocks and in data storage devices.  Fractional Synchronous motors do not have wound excitation system.  The rotor contains permanent magnets which run in synchronism with the stator.
Synchronous Motors for Power Factor Correction.

Synchronous Motor operated in an overexcited condition can be used for power factor correction.  The overexcited synchronous motor draws active power from the grid and supplies reactive power.  This helps improve the power factor of the system.  A motor run in this manner is called a Synchronous Condenser

Speed Torque Curve of an induction motor

The speed torque curve of an induction motor is a plot of speed on the x-axis and torque on the y-axis. When the motor is started, the initial torque is about 250% of the rated torque. This is the torque required by the motor to overcome the inertial of the standstill. As the motor picks up speed the torque drops to the pull-up torque. If the pull-up torque of a motor is less than the torque requirement of the load coupled to it, the motor will stall and over heat.

The breakdown torque is the maximum torque which can be developed by the rotor before it overheats. The breakdown torque needs to be high for loads with high inertia and which are susceptible to overloads such as conveyor belts.

The full load torque is the torque produced by a motor operating at the rated speed and load. Exceeding the full load torque causes reduction in the life of the motor.

When the motor is run on no load, the rotor speed reaches the synchronous speed. The slip becomes zero and the motor runs at zero torque

Crawling in Induction Motors

The supply given to an induction motor may have harmonics present in it. These harmonics will have their own torques in addition to the synchronous torque. Let us consider a supply with odd harmonics. The 3rd harmonic will be absent in 3 phase systems. Hence, we only have to consider the 5th and 7th harmonics. The other higher order harmonics can be neglected.

The torque produced by the 5th harmonic rotates in the opposite direction. Thus, the forward torque is given by the sum of torques produced by the primary frequency and the 7th harmonic.

The rotating field of the 5th will rotate at one fifth of the synchronous frequency (Ns/5). However, the torque produced by the 5th harmonic rotates in the reverse direction. Similarly, the 7th harmonic will rotate at one seventh of the synchronous frequency. The torque produced by the 7th harmonic is maximum at 1/7th of the supply frequency.

When some poorly designed motors are started with load, the motors may not reach the nominal speed. The motors will get stuck at 1/7th of the nominal speed.

This phenomenon is known as crawling. Crawling can be overcome by properly selecting the number of rotor bars in the rotor of the induction motor

Methods of Starting Synchronous Motors

Synchronous motors used widely in the industry. Synchronous motors provide constant speed. The synchronous motor consists of a wound rotor and a stator. The stator winding is energized from the power supply. This sets up the rotating magnetic field. The rotor gets magnetized when the field winding is energized. During operation, the rotor is in synchronism with the rotating magnetic field of the stator. Hence, the name, synchronous machine.

The synchronous machine, however, is not self-starting. The synchronous machine has to be rotated to near the synchronous speed of the stator before it can "catch" the stator field and begin rotating on its own.

There are many different methods employed for Starting Synchronous Motors.

Pony Motor

The pony motor is an induction which drives the rotor of the synchronous motor. Once the speed reaches the synchronous speed, the field winding is switched on. The pony motor is then decoupled and the synchronous motor runs on its own.

Damper windings

Damper windings or amortisseur windings are special windings which are fixed on the salient pole of the rotor of the synchronous motor. These windings work in a similar manner to the squirrel cage winding in induction motor. Thus the synchronous motor starts as an induction motor. The rotor runs at a speed slightly lower than the synchronous speed. When the speed comes close to the synchronous speed, the field winding is switched on and the rotor gets locked to the stator magnetic field and the machine runs as a synchronous motor.

Starting using Variable Frequency

Synchronous motors which are electronically controlled can be started by supplying a reduced frequency to the stator winding. This generates a slowly rotating magnetic field in the stator. The rotor of the synchronous machine is able to follow this magnetic field. Once the rotor starts to rotate, the frequency is gradually raised to the power frequency. The synchronous motor can now run at the normal frequency.

Constant Torque Operation in Motors

Constant Torque Operation refers to the operation of the motors at a fixed torque value.  The torque supplied by a motor is dependent on the load.  However, a motor which has a constant flux is considered to be running at constant torque.

For a motor to run at constant torque it has to be driven by an AC drive.  AC drives are able to vary the frequency and the voltage such that a constant V/Hz value is obtained.  The V/Hz is the ratio of voltage to the frequency in an electric machine.

When the frequency of the motor is adjusted, the stator reactance changes.  This reduces or increases the stator current.  To correct this, the voltage has to be adjusted.

The conveyor is an example of an application which requires a constant load.

Derating in Motors

Derating refers to the operation of equipment at reduced capacity or speed.  Derating in motors can be caused due to the following reasons.


When frequency increases, the speed increases and the torque decreases.  If the frequency increases by 5%, the speed increases by 5% while the torque decreases by 10%.


Voltage has a direct relation with torque.  When the voltage falls the torque also reduces.  Equipment has suddenly rotate faster or move faster due to voltage fluctuations.


As the altitude increases, the density of air decreases.  This reduces the ability of air to transfer heat and cool the motor.  Thus if the motor is to be operated above 1000 metres above sea level, it has to be derated.

Ambient Temperature

The ambient temperature is also a factor in derating.  If the ambient temperature is high, the insulation may reach its maximum temperature limit quickly.  Hence, the motor may have to be derated.

Braking methods in Induction motors

Braking in induction motors refers to quickly bringing the speed of the motor to zero. Braking can be categorized into two broad categories viz. mechanical braking and electrical braking.

Mechanical braking involves stopping the shaft by means of a braking shoe. When the braking is to be done, the supply to the motor is cut off and the brake is applied to bring the motor to a halt.

Mechanical braking used in cranes and hoists. It is also used in elevators when the elevator has to stop at a specific floor of the building.

Electrical braking involves stopping the motor using electrical means. Most electrical braking systems have a mechanical brake to hold the shaft in position once the machine has been stopped.

There are two main types of Electrical braking.

Dynamic braking
Regenerative braking


Plugging involves reversing the supply in two of the phases. For instance, R and Y can be interchanged. This leads to a torque being developed in the opposite direction to the rotation of the motor. This causes the motor to stop at once. Once the motor stops, the reverse supply is cut off (to prevent the motor from running in the opposite direction). The rotor is secured by a mechanical brake.

Dynamic Braking can be classified into DC injection braking, AC dynamic Braking and Capacitor Braking.

AC dynamic Braking

In AC dynamic braking, the supply to one of the phases is cut off. Thus the motor runs as a single phase motor. This induces negative phase sequence components in the supply and the motor stops. Another method is to give the remove one phase and give the same phase to two terminals. For instance, two terminals will have 'Y' phase and one will have 'B' phase.

DC injection braking

In DC injection braking, a separate rectifier circuit produces a dc supply. When the brake is to be applied, the ac supply to the stator is disconnected and a dc supply is given to two of the phases. The dc voltage in the stator sets up its own magnetic field. The conductors of the rotor which is rotating will cut the magnetic field. As the conductors are short circuited, a high current is produced. This causes a braking torque to be produced in the rotor. The current produced in the rotor is dissipated as heat. This system can be used only when the rotor can withstand the heat which will be produced when the brake is applied.

Capacitive Braking

Here the AC supply to the stator terminals is cut off and the terminals are connected to a three phase capacitor bank. The capacitors will excite the induction generator. This sets up a magnetic field which will cut the rotor bars. The rotor energy is thus converted into heat and the motor is stopped.

Regenerative Braking

In Regenerative braking, the supply frequency to the stator is reduced. This is possible with VFDs where the frequency can be varied. When the supply frequency is reduced, the synchronous speed of the motor is reduced. When the synchronous speed falls below the rotor speed, the induction motor works as an induction generator and power is supplied back to the terminals. The energy in the rotor is thus recovered. Due to the loss of energy, the rotor slows down and stops.

Efficiency of the Electric Motor 

The efficiency of the Electric Motor is about 92 %.

The efficiency is lower for smaller size motors. The efficiency for smaller motors can drop to around 80%

The losses in the motor are the iron losses due to the magnetic field, the copper losses due to the current, the mechanical losses (friction and windage losses) and the stray losses such as the harmonic losses.

Why should the dc series motor never be run without any load ?

The dc series motor develops a very heavy torque during start-up. The motor relies on the connected load to restrain the speed.

Hence, if the dc series motor is run without any load. it may result in overspeeding which can cause serious damage to the motor .

What happens when an induction motor is run above the rated speed ?

When the induction motor is run above the synchronous speed which is the speed of the rotating magnetic field, it works as an induction generator. That is, it generates active power kW while it still consumes reactive power kVAr in order to establish the magnetic field.  This usually happens when another prime mover such as a wind turbine is coupled to the motor shaft.

The slip (Ns-Nr) then becomes negative.

Another scenario where the motor can overspeed is when the frequency of the input power is itself increased by means of a Variable frequency drive. Then the motor is said to be running like an induction motor but at a higher speed.

The torque characteristics may vary with the varied speed. The rotor, gears and the coupling may experience increased centrifugal force which can cause damage. Hence, the overspeed limits need to be ascertained from the manufacturer.

Thermal Protection in Motors

Thermal Protection is an important protection in motors. Motors can get heated due to overloading, high ambient temperature, variations in power quality, etc. Thermal overload can result in stator overheating, faulty operation and in some extreme cases even fire. Hence, all motors need to be fitted with protection against thermal overload.

Thermal overload protections can be classified into three types viz. Bimetallic, Magnetic and temperature sensing protection.

Bimetallic Protection
In bimetallic protections, a strip of two metals which are attached to one another is used. The motor current is made to pass through the strips. As current passes through the strips, the strips heat up and expand. Since, the strip is made up of two different metals and these metals have different rates of expansion, the strip bends in one direction. When the temperature of the strip reaches a particular value, it activates a mechanism which trips the motor.  This kind of protection is widely used and is simple in construction.  However, this method is not suitable in applications which require frequent starting and stopping of the motor.

The bimetallic protection gets reset faster than the motor cooling temperature and it may thus permit the motor to be started again when the motor has not sufficiently cooled from the thermal overload.

Magnetic Protection
This consists of a magnetic element whose field strength is a function of the motor current. When the motor current exceeds a preset value, the electromagnet inside the relay operates and trips the motor. The downside of this kind of protection is that it does not take into account ambient operating conditions such as temperature and ventilation which play an important role in the temperature rise of motors.

Temperature based thermal overload protection
This is method of protection that is relatively new. This method involves actually measuring the temperature of the motor and those of the winding hotspots using a temperature sensor such as the RTD (Resistance Temperature Detector). This method uses direct temperature sensing and is the most reliable and accurate, though, it is more expensive.

Stalling in Induction Motors, its effects and prevention

Stalling of the motor refers to a condition, where the motor is unable to rotate. This condition can be caused either due to any obstruction in the load or due to any problem with the motor such as bearing seizure, etc.

This condition is also known as locked rotor.

When the speed of the rotor decreases to a very low value or stops completely due to stalling, the slip of the induction motor increases. This causes higher voltage and consequently higher current to be induced in the rotor windings.

The stator currents also increase. The equivalent of the motor stalled condition is that of a transformer whose secondary is short circuited.

The high current drawn will cause damage to the windings and cause the rotor to heat up.

Stall protection devices work by monitoring the motor current and the speed. If the motor draws higher current at a preset low speed, the relay is activated.

Negative Phase sequence in Induction motors

     Negative phase sequence in induction motors is caused due to unbalanced voltages in the supply voltage applied on the stator terminals or unbalanced windings.
     Negative phase sequence components create a rotating magnetic field in the stator which moves in the opposite direction. This causes a decrease in the torque developed by the motor. The motor will thus have to draw a higher current for the same mechanical load.

     The rotating magnetic field which rotates in the opposite direction induces voltages in the rotor. These voltages have a frequency that is double the system frequency. Since the frequency of this rotor voltage is higher, it flows on the surface of the rotor due to the skin effect and causes surface heating which can lead to motor damage.

     Negative phase sequence relays can identify negative phase sequence condition and trip the machine. Negative phase sequence relays work by using a special filter which filters out the positive sequence and the zero sequence components. The filtered negative phase sequence voltage alone is measured. When the measured negative phase sequence voltage exceeds the set value, the relay trips the motor.

Shaft Currents in Motors and Generators

The magnetic field in a motor or a generator is ideally conducted along paths that are symmetrical. However, sometimes the magnetic fields within an electric machine such as a motor or a generator is asymmetrical. This asymmetry can be caused usually by variation in the quality of the steel used in the motor construction or in some cases to shaft eccentricity.

These asymmetrical magnetic fields which are varying over time can induce currents in the shaft of the motor or generator. These currents which are induced in the shaft tend to flow from one end of the shaft to the other through the bearings and then through the earth.

When these currents flow through the bearings, the tend to cause arcing and consequent pitting in the bearings. This can lead to failure of the bearings.

Shaft currents can be prevented by insulating one of the bearings. A Teflon layer is usually placed between the shell of the bearing and the bearing housing. This ensures that the shaft voltage induced does not have a return path. This prevents shaft currents from flowing.