A synchronous phase modifier is a synchronous motor which is not connected to any load.  The motor can be made to behave like an inductor or a capacitor by decreasing or increasing the excitation. 

Synchronous Phase modifiers are used to regulate the voltage in transmission lines.

Series Capacitors Shunt Capacitors
Used to reduce the line reactance Has no impact on the line reactance
Voltage rise only occurs across terminals.  Can be used to raise the line voltage
No effect on system stability Improves system stability
Does not affect the power factor Used to improve power factor

The ratio of capacitive reactance to the inductive reactance in a transmission line is called the percentage compensation of the Transmission Line.  The inductive reactance is mainly caused by the nature of the load.  The capacitive reactance is caused by the line capacitances and by series capacitors, if any.

Submarine Power Cables are used to carry power underwater.  They are used to connect small islands with the mainland.  Sometimes, the route through the water is shorter than through land.  Submarine Cables are used for both AC as well as DC transmission. 

AC is used up to 80 km while DC is used for long distance transmission.  Submarine cables are mostly gas filled cables.  The cable laying is done by special ships.  XLPE cables fpr submarine transmission are also available.

AC cables can be laid as three separate cables or as a single 3 phase cable.

Offshore wind farms are connected to the mainland by submarine cables.   

Damage to these cables by shipping is a matter of concern.  The routes which contain submarine cables are specially marked in nautical maps.

Gas filled cables are used in many applications.  In these cables, the insulation is filled with gas, usually nitrogen.  The inert gas fills up the voids which may form in the insulation.  There are low pressure and high pressure variants in this type of cable. 

The gas is filled in the insulation and is enclosed by means of a sheath.  Since Nitrogen is easily available, any losses can be easily replaced during installation. 

However, the gas has a low dielectric strength.  It cannot be used at very high voltages.  Gas filled cables can be used up to 275 kV.

Oil Filled cables have almost replaced Gas filled cables in recent times.  However, there are situations where oil filled cables cannot be used such as undersea applications and in hilly terrain.

High Voltage Dielectrics are used for insulation of cables at high voltages.  These dielectrics are subjected to very high stresses and temperatures.  They also have very demanding reliability requirements. 

Some of the common properties of High Voltage Dielectrics are

  1. Dielectrics should be flexible as this reduces the bending radius.  A lower bending radius enables cables to take turns in small spaces. 
  2. The Permittivity should be low so that the charging current is as low as possible
  3. The power factor of the dielectric should be low so that the heat generated is minimum.
  4. The Impulse strength of the insulation should be high.

Impregnated Cables are cables which have insulation which is impregnated with an insulating material.  For instance Paper insulation can be impregnated with oil or synthetic fluid with good dielectric properties.  The impregnated insulation is enclosed in a lead sheath to prevent the ingress of water. 

Some  cables have insulation which requires the insulating compound to be maintained at a particular pressure.  This may require the installation of pumps at periodic locations. 

Oil impregnated cables have special channels for the flow of oil.  The oil which is pressurized fills up any void which may develop in the insulation. 

Modern Impregnated Cables do not require pumps or any pressurizing devices. 

Oil impregnated cables can have their own cooling systems if used at voltages above 525 kV

Dust in electrical Equipment can affect the operation of the equipment.  Dust can result in tracking which causes current to flow over the insulation eventually leading to failure.  Dust on the surfaces of conductive parts such as breaker poles or relay contacts can affect the conductivity and cause heating and voltage drops

Dust when combined with oil or water in the atmosphere can form acids which can corrode and weaken the insulation. 

A layer of dust on the surface of an equipment can affect the heat transfer and prevent adequate cooling. 

Hence, equipment which are to be installed in dust prone environments should be adequately dust proof.  Panels should have filters which prevent the entry of dust. 

Tracking is a phenomenon in Electrical Insulation.  Tracking refers to the flow of current over the surface of the insulation.  Tracking causes heating which results in damage to the insulation.  As current gets a path to the ground or another energized conductor, tracking can lead to a flash over. 

Tracking is caused by many factors.  One principal reason is dust. Dust forms a layer over the insulation, this when combined with moisture in the atmosphere provides a conductive layer for the current to flow.  Other factors which can cause tracking are humidity which can cause condensation, temperature, air pressure, etc. 

Tracking can be prevented by ensuring proper creepage distance.  Equipment should be kept at the right temperature and humidity. 

The aging of electrical equipment is caused principally by the aging of the insulation.  The insulation made of materials such as rubber, paper or polymers deteriorates over a period of time.  The molecules of the material break down.  The material loses its mechanical and electrical properties.  It finally gives in, leading to failure. 

Aging of insulation in Electric machines has a number of reasons.  Some of them are

  1. Partial Discharges
  2. Tracking
  3. Treeing
  4. Electrolysis
  5. Dielectric Heating
  6. Space Charges


Aging of Electrical Insulation can be monitored by periodically testing the insulation such as Insulation Resistance, Polarization Index, Tan Delta test, Surge Test, etc.

V/Hz is the ratio of voltage to Frequency in Electric Machines.  In Electric Machines such as motors, generators and transformers, the drop in frequency results in a decrease in the reactance of the stator winding and the core.

In a motor, when the frequency drops, the stator reactance reduces.  This causes a higher current to flow for the same voltage.  This can result in increased flux and increased torque. 

In the machine core, the reduction in reactance can cause more eddy currents to flow.  This can result in over heating and damage to the laminations in the core of the stator.  This can permanently damage to the machine. 

This is a condition known as overfluxing. 

Overfluxing protection is an important part of any protection scheme. 

V/Hz is a mode of control in AC drives.  For a motor to run at constant torque, its flux should be constant.  This is achieved by the V/Hz control.  Since the flux can change when the frequency drops, the voltage also reduces during reduced frequency.  This ensures that the flux in the stator field of the motor is always constant. 

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 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.

The Locked Rotor Amperage is a specification which specifies the current in the rotor of a motor when it is locked. 

When the motor is a standstill and the power is given to the terminals, the current is maximum in the rotor. As the speed increases, the current drops.  The Locked Rotor Amperage is typically 3 to 8 times of the Running Current Amperes.

The LRA is also dependent on the input voltage during start up.  As such, the Locked rotor Amperage value is used to select the type of starter for the motor.

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 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

Puncture Voltage

The voltage at which the insulator punctures is called the puncture voltage.  When the puncture voltage is exceeded, the insulator is permanently damaged as the current flows through the inside of the insulator.

Creepage distance

The creepage distance refers to the distance over the surface between the conductor and the ground.  The creepage distance in insulators is increased by corrugating the body.  The creepage distance depends on the voltage.  Higher voltages will require higher creepage distances. 

Flashover Distance

The Flashover distance is the minimum distance below which a flash over can occur. 

Flashover Voltage

The voltage above which a Flashover can occur is called the Flashover voltage.

AC Servomotors DC Servomotors
Suitable for Low Power applications Used for High Power Applications
They have low efficiency They have high efficiency
The operation is stable and smooth Noise is produced during operation
It has low Maintenance as there is no commutator Relatively more maintenance is required due to the presence of the commutator.


Creepage refers to the minimum distance between two conductive parts through the surface of the insulation.  Creepage is an important characteristic as reduced creepage will result in tracking along the surface of the insulation. 

Tracking refers to the flow of current along the surface of the insulation.  Tracking causes localised heating and carbonization of the surface.  This, eventually, leads to failure of the insulation. 

The Comparative Tracking Index (CTI).  The comparative tracking index gives the minimum voltage which can cause tracking across the surface of an insulation.  Creepage also depends on contamination of the surface, humidity, corrosive chemicals and the altitude in which the equipment is installed. 


Clearance refers to the shortest distance through air between two conductive parts.   If the Clearance is less than required, it will result in flashover due to dielectric breakdown in air.  Dielectric breakdown is dependent on other factors as well such as humidity, contamination, altitude and temperature.

The creepage is calculated based on the value of the rms voltage while the Clearance is calculated based on the peak voltage.

Altitude is very important in the selection of Electrical Equipment.  Electrical Equipment in General are designed for an altitude up to 1000 m above sea level.  At higher altitudes, the density of air decreases.  This results in a decrease in the dielectric properties of air.  The dielectric property of air depends on the density, temperature as well as pressure. 

At high altitudes, all three parameters decrease.  Thus the insulation provided by air decreases.  The insulation coordination should be reviewed.

Hence, there is a risk of flashovers and arcs.  Circuit breakers and other current interrupting devices should be derated at high altitudes. 

The operating altitude should be kept in mind when designing equipment. 

Another limitation caused by altitude is in heat dissipation.  As the air grows less dense, the heat carrying capacity of air decreases.  This results in overheating of the equipment.  Thus equipment and components may require larger heat sinks or cooling equipment with increased capacity. 

  Solid State Relays Electromechanical Relays
Noise No Noise Noise due to contact switching
EMI No EMI as no switching is involved EMI due to arcing during switching
Zero Voltage switching Possible Not Possible
Resistance to shock They are resistant to shock and vibration These relays can be affected by shock and vibration.
Temperature sensitivity Sensitive to Temperature less sensitive to temperature
Heat Generation Heat is generated due to higher resistance Low resistance in the contacts. Hence, less heat is produced
Heat dissipation Heat Sink is required Heat sink is not required.
Leakage Current Solid State Relays do not completely turn off.  There is a slight leakage Current.  This can affect downstream circuits. No leakage current.  The relays can be completely switched on or off.
Lifetime very long lifetimes. Relatively short lifetime

Reduced Voltage Starting in Motors refers to the practice of reducing the motor at a reduced voltage.  This is done to limit the heavy inrush current during starting.  The voltage is reduced by connecting resistors or reactors in series. 

Soft Starters can also be used to reduce the voltage during starting. 

The disadvantage of using reduced voltage starting in motors is that as the voltage is low during startup, the torque is also low at starting.  This method of starting may not be suitable for applications such as conveyors, lifts, etc which require the motor to start with the load. 

Types of Reduced Voltage Starters are

  1. Resistance Starters
  2. Reactance Starters
  3. Autotransformer Starters
  4. Soft Starters
  5. Star Delta Starters

Oil Directed Air Forced Cooling is an updated version of Oil Forced Air Forced Cooling (OFAF).  IN ODAF, the oil is directed through special channels in the core and close to the windings.  This makes the cooling more effective.  The hot oil is then passed through the fins which are cooled by fans. 

Oil Directed Air Forced Cooling is used in transformers with large capacities. 

Oil Forced Water Forced Cooling of Transformers is a method of cooling Transformers in which oil is forced through the transformer by means of pumps.  The oil which is thus forced through the transformer collects the heat from the transformer.  This oil is then passed through a cooler (heat exchanger) which has water as the other medium.  The water which collects the heat from the oil is pumped to a cooling tower where the heat is dissipated.

The OFWF method of cooling is similar to OFAF (Oil Forced Air Forced) method of cooling. 

When a temperature is at no load, its temperature is slightly greater than the ambient temperature.  When the transformer is loaded the temperature rises.  The temperature rise rating of a transformer gives the maximum value to which the temperature of the transformer would rise. 

Dry type transformers are usually available in three standard temperature rises, 80C, 115C or 150C.  Liquid filled transformers have ratings of 55C and 65C.  These values are based on a reference value of 40C.

For instance, a transformer with a temperature rise rating of 80C will reach a maximum temperature of 120 C (40+80) during operation. 

The lower the temperature rise rating, the better is the ability of the transformer to withstand momentary overloads.  Thus a transformer with a temperature rise rating of 80C will have a better overloading capacity than a transformer with a temperature rating of 120C.

The temperature rise rating gives and idea of amount of heat produced and the amount of heat removed.  Transformers with lower temperature rise ratings use windings with lower resistivity. 

Transformers with low temperature rise are used in special applications such as in underground installations, air conditioned buildings

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.

  Induction Motors Synchronous Motors
Speed The speed varies below the synchronous speed and can be adjusted The speed is constant and rotates at the synchronous speed
Construction Simple and Rugged Construction A slightly complex construction
Starting It is self starting Special Starting Procedures are required
PF control Cannot be used for PF improvement Can be used for PF improvement
Excitation Does not require external excitation Requires DC excitation for rotor
Efficient Less Efficient More Efficient
Cost Cheaper More expensive

Feature Open Loop Control System Closed Loop Control System
Effect of Output on Input No effect on input The input signal affects the controller output into the system
Stability Very Stable The response changes as the input signal changes.
Response to external disturbances No reaction to disturbances.  The Open Loop control works on fixed output The output of the controller adjusts itself in response to the input signal
Ease of Construction The controller is easy to construct Controller is difficult to construct as it is complex
Cost Cheap Expensive
Bandwidth Small Bandwidth Large Bandwidth
Maintenance Low Maintenance More Maintenance is required. 
Feedback There is no Feedback Feedback is always present. 

The requirements of an effective control system are


The control system's response should be swift.  This is necessary to ensure that the process variables do not stray away from the set point which can affect the process. 


The number of oscillations should be as low as possible.  Oscillations can cause disturbances in the system.  The damping of the system should be optimum.


Accuracy refers to the ability of the system to detect even the smallest deviation and initiate a corrective response.   The sensors and the error sensing algorithms should have a high resolution.


Noise refers to undesirable interferences in the signal of the transducer.  There are many reasons for noise.  Electromagnetic Interference (EMI) is one of the reasons.  A good Control system should be able to filter the noise and process only the principal signal. 


Any control system should be stable about the set point.  It should maintain the process variable at the set point.  It should not react to changes in the surroundings, temperature or any other external parameter.  It should react only to the principal input signal. 


Bandwidth is dependent on the frequency of operation.  The Bandwidth should be as large as possible for a good frequency response of the system.

Open Loop and Closed Loop Control Systems

Open Loop Control

Open Loop Control is a type of control in which the output is not given as feedback to the controller.  The Input is totally independent of the output of the controller.   Thus the control regulates the process without taking into account the state of the output.  Examples of Open Loop controls are Traffic signals at intersections, Bread toaster, Volume control in a Television set, etc

Closed Loop Control

Closed Loop control is one in which the output is given as feedback to the input. Thus the input is increased or decreased depending on the output.  This ensures that a set point is maintained.  Examples of Closed Loop Controls are Automatic temperature control in an air conditioner, level controller in boiler, a voltage stabilizer, etc. 

Manual and Automatic Systems

Manual Control System

A Manual Control System is one in which the process is manually controlled to achieve the set point.  An example would be a tank with a pump.  If a person switches on the pump every day to fill the tank to a particular level and stops the pump once the level in the tank has been reached, it is called manual control

Automatic Control System

In Automatic control, the process is automatically controlled.  If the pump has a timer which switches on the pump for 15 minutes every day to reach a certain level in the tank, the control system can be said to be automatic. 

Both open loop and closed loop control systems can be manual or automatic.

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.

Heat Sequencers are switching devices used in Electric Furnaces.  Sequencers work by sequentially switching the heating elements on or off.  electric furnace sequencer

If all the heating elements in a furnace were to be switched on at the same time, they would trip the circuit breaker. 

The sequencer avoids this by switching the elements in sequence.  The sequencer consists of a thermostat.  The thermostat senses the furnace Temperature and activates the elements. 

The thermostat sends a current to a small element in the sequencer.  This elements heats up a bimetallic strip which closes and supplies power to the heater element.  When the supply to the heater coil is cut off, the bimetallic strip cools and open the circuit.  The heating element is thus switched off.

The sequencer is thus able to switch on and off the heater by means of bimetallic strips. 

Microwave ovens have become and indispensable part of our kitchen.  These useful appliances are able to heat and cook food quickly and efficiently.  They are compact and clean. 

Microwaves work by heating the food with microwaves.  Microwaves are waves which are situated between the infrared and radar waves in the electromagnetic spectrum.  microwave

The microwaves are generated by a device called a magnetron.  The magnetron generates the microwaves at a frequency of 2.45 GHz and a wavelength of 12.2 cms.  The waves generated by the magnetron are focused at the food by means of a waveguide. 

The magnetron is powered by a high voltage transformer.  A microprocessor controls the entire process while a fan cools the components.

The Microwave oven works on the principle of dielectric heating.  A dielectric is a material which can be polarized.  All food materials contain water.  The water molecule has charge.  When it is placed in an alternating electric field, the molecule moves back and forth as the direction of the electric field changes. 

This produces heat which cooks the food. 

The food is placed in a cooking chamber,  The chamber is enclosed in a metallic cage which forms a faraday cage.  This is to ensure that the radiation does not come out. 

Microwave cooking is efficient as the food alone gets heated.  The heating is uniform, unlike conventional cooking where the heat travels from the outer surface of the food to the inner parts.  This eliminate localized overcooking. 

Microwave cooks at a lower temperature.  This prevents the formation of tars and char in the food. 

Incoloy is an special alloy.  It is used in high temperature applications and is known for its corrosion resistance.  Electric Heaters are sheathed with Incoloy as it offers corrosion protection at high temperatures.  The Incoloy sheath encloses the heating element such as nichrome.

Incoloy is composed of Nickel, Iron and Chromium.  There are small additions of Molybdenum and copper as well. 

Incoloy has excellent corrosion resistance properties.  It resists stress corrosion as well as localised pitting.  It is resistant to both reducing and oxidising acids. 

Such chemical resistance is required for electric heaters as the medium they will be heating can be corrosive.  Incoloy protects the heating elements from damage from corrosive media.

Electric Ovens are used in industries to dry products.  They are an important part of many industrial processes.   industrial oven

Ovens are used for curing, for heat treatment, for drying and for many other industrial processes.

There are different types of ovens used in the industry such as Cabinet Ovens, Conveyor Ovens and Truck-In Ovens.

Ovens are categorized into Gravity Convection and Forced Air Circulation Models.  In Gravity Convection Ovens, air circulates within the ovens by means of convection.  In Forced Air circulation models, Air is drawn from the outside and circulated inside the furnace. 

Furnaces are available in a wide range of temperature ranges.  Furnaces are generally heated by an electric heating element.  The elements are made of materials such as nichrome.  Temperature Controllers regulate the temperature in the oven.

Rotating Diodes are diodes fitted in the rotor of the synchronous machine between the excitor and the main field winding.  The function of the Rotating Diodes is to rectify the AC output of the excitor into DC which rotating diodecan be used to magnetise the main field windings and poles.

These diodes are connected in the shape of a normal three phase rectifier with six diodes (sometimes twelve diodes are used).  The Diodes are called Rotating diodes as they are mounted on the rotor which is rotating.  The diodes do not rotate by themselves. 

The DC output fromrotating diode block the rectifier assembly is usually collected from two rings in the assembly.  The Diodes are protected by a Varistor against voltage spikes during sudden load fluctuations.

The synchronous machine is used widely in industry both as a generator for power generation as well as a motor for driving industrial equipment such as conveyors, pumps, crushers etc.

The synchronous machine has the following parts


The Stator of the Synchronous Machine consists of the Core.  The core is made of electrical steel.  It is made in the form of insulated laminations which are stacked together.  This prevents the flow of eddy currents in the core.  The core has slots.  The windings of the core are placed in the slots. The windings are made of copper.  The winding are preformed and placed in the slots.


The rotor of the Synchronous Generator consists of a number of poles.  The number of poles depends on the speed and frequency of the machine.  The relation between the number of poles and speed or the frequency is N=120 x f /p ( where N is the speed in rpm, f is the frequency and p is the number of poles).

The rotor of the synchronous machine can be either salient or non-salient in construction.  In salient pole rotors, the poles are protruding from the rotor while in non-salient pole construction, the rotor windings are placed in slots machined in the rotor (See article on Salient and Non-salient Rotor Construction).Alternator Block Diagram


The excitor can be imagined to be a small generator placed in the rotor.  The excitor provides the excitation power for the the excitation.  The excitor consists of a field winding in its stator.  The armature winding of the excitor is placed in the the rotor of the machine. 

Rotating Rectifiers

The output of the excitor is a 3 phase AC supply.  This supply is rectified by means of the rotating rectifiers which are fitted on the rotor shaft.  This rectifier assembly converts the AC power into DC.  The rectified DC is then supplied to the main rotor windings. 


When the machine is first started, residual magnetism in the excitor field winding induces a voltage in the excitor stator.  This 3 phase supply is rectified and fed to the field windings of the main rotor poles.  These poles get magnetized and induce the output voltage in the stator.

The control of the machine output voltage is done by a device called the AVR ( Automatic Voltage Regulator).  The output voltage of the machine is connected through a potential transformer to the AVR.  The AVR then regulates the excitation input based on the feedback received.  This a closed loop control.

Electromechanical Relays are the oldest type of relays.  Electromechanical, have been gradually been replaced by Static and then by Numerical Relays. 

However, Electromechanical Relays are still being used.

The Advantages of Electromechanical relays are-

  1. They are quick acting and can be reset fast.
  2. They are simple in construction.
  3. They are reliable. 
  4. The values can be easily set.  No special programming device is required.
  5. People can be trained on these relays easily

The disadvantages of Electro-mechanical relays are

  1. The VA burden of these relays is higher than static and numerical relays.  Hence, the Potential and the Current Transformers should have a higher capacity.
  2. These relays require to be calibrated periodically and tested.
  3. These relays suffer from the effects of age.  As time passes, the springs and the linkages inside the relay grow weak.  This causes the setting values to drift.  This can result in maloperation and false trips.
  4. These relays do not have the directional feature. 
  5. The speed of operation is limited by the mechanical inertia of the moving components. 
  6. Multifunctioning is not possible.  One relay can perform only one function.

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. 

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. 

Materials are classified based on their response to a magnetic field as Ferromagnetic, Paramagnetic and Diamagnetic materials.

Ferromagnetic Materials

Ferromagnetic materials are materials which have a large susceptibility to magnetic fields.  These materials can get magnetised when placed in an external magnetic field.  They experience strong attraction to magnetic fields.  This behaviour comes from the large magnetic domains in these materials.  In the demagnetized condition, these domains are aligned in different directions.  However, when they are placed in a magnetic field, these domains get aligned in the same direction.  Thus, the material as a whole is magnetized. 

Examples of Ferromagnetic materials are iron, cobalt and nickel. 

Paramagnetic Materials

Paramagnetic materials experience a weak attraction to magnetic fields.  This magnetic field comes from the magnetic moments of unpaired electrons in the materials.  There are no domains in paramagnetic materials.  Examples of paramagnetic materials are magnesium, lithium and tantalum.

Diamagnetic materials

These materials have negative susceptibility.  These materials are slightly repelled by magnetic field.  There are no unpaired electrons in these materials.  All the electrons have paired and hence the magnetic moment of these electrons cancel each other.  Copper, Gold and Silver are examples of diamagnetic materials. 

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

Synchronous Motors are used in the following situations

  1. Applications which require constant Speed
  2. 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

Solo Mode

Synchronous Generators are operated in a standalone pattern where the machine is simply connected to a load.  It is said to be operating in Solo Mode.  In Solo mode, the active and reactive loads are determined by the distribution. 

Island Parallel Mode

When the generator is running in parallel with more than one generator in a synchronized condition, it said to be operating in island parallel mode.  It is called "island mode" as the machine is not connected to the grid utility.  In Island Parallel mode, the total plant load is determined by the distribution.  However, the individual machine loads can be adjusted.

Grid Parallel Mode

When the generator is running in parallel to the utility, it is called Grid Parallel Mode.  Once synchronized with the grid, the plant operator does not have any control over the frequency and the voltage.  However, the active and reactive power on the machines can be controlled from the plant. 

The three different modes have different mechanisms for control of the active and the reactive power. 

Hunting refers to the periodic variation in speed, voltage or power factor of the machine.  Let us say a synchronous generator is rated for 1500 rpm.  When hunting occurs, the speed will vary below and above the set speed, 1500 rpm.  For instance, it may vary from 1300 to 1800.

In voltage hunting, the voltage varies periodically. 

Hunting may be severe or mild. 

Hunting causes a variation in output.  It also results in instability across the network.  The hunting in one machine can cause hunting in another machine. 

In Synchronous machines, there can be active power hunting as well as reactive power hunting.

Active power hunting occurs when the active power (kW) varies. 

  1. The common causes of active power hunting are
  2. Speed Control Issues with the Prime mover
  3. Issues with fuel input to the prime mover
  4. External Power Fluctuation in the Grid

Reactive Power Hunting

Reactive Power Hunting occurs when there is a periodic variation in the reactive power (kVAr) and the Power Factor

The common causes of Reactive Power Hunting are

  1. Loose connection in the excitation circuit of the alternator
  2. Faulty AVR (Automatic Voltage Regulator)
  3. External reactive power fluctuation in the grid

Active Power Hunting can be rectified by adjusting the speed controls and the fuel system of the prime mover.

Reactive Power Hunting is directly related to the generator.  Check the excitation circuit inside the generator.  Check the AVR wiring and change the settings if required.

No load Hunting and On-Load Hunting

No Load hunting refers to the variation of the speed and the voltage when the machine is running on no-load.  That is, when the generator circuit breaker is open.

On Load hunting is when the hunting occurs when the circuit breaker is closed and the machine is on load.

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.

A Synchronous Machine designed to supply reactive power is known as the synchronous condenser.  The synchronous condenser or a synchronous capacitor as it is sometimes called is a motor which runs in an overexcited condition.  The synchronous condenser is not connected to any mechanical load. 

The synchronous Condenser differs from other synchronous machine by its large synchronous reactance and large field windings.  The field current fed into the machine will be high as the the machine usually runs overexcited.

Synchronous capacitors support the network voltage and additional short circuit power capacity. 

Synchronous capacitors enable very fine reactive power control.  Ordinary capacitors can only provide stepped control.  Synchronous condensers also do not generate switching transients or harmonics. 

The Synchronous Condenser is usually constructed with two or four poles.  Synchronous Condensers can be cooled with air, water or with hydrogen. 

An induction Generator is an induction machine which is run above its synchronous speed.  In this condition, the induction generator draws reactive power from the lines and supplies active power. 

The induction generator can be made self-excited by connecting a capacitor bank in parallel to the load.  The capacitor supplies the required reactive power while the load draws the active power.

This is known as the standalone induction generator. 

The capacitor bank provides the reactive power requirements of both the load and the induction generator. 

Standalone induction generators are used in remote locations and connected to small water turbines or wind mills.

The voltage drop in a line can be compensated by a capacitor booster.  A transformer is connected in series to a line.  The secondary of the Transformer is connected to a capacitor. 

The booster is used to boost drops caused by low power factor loads. Voltage drops due to high power factor loads cannot be improved with this method.  This cannot also be used to improve voltage drops due to loads with leading power factor as it may cause the voltage to drop further.

Furnace Transformers are Transformers which are specially designed for powering Arc Furnaces.  These Transformers are designed to withstand high currents and severe voltage fluctuations.  They are also designed to withstand higher than normal temperatures.Furnace Transformer

The windings are mechanically strengthened to withstand the huge forces generated due to the flow of high currents.  Reactors are often used to smoothen the fluctuations.  Operation of the Furnace breaker which can trip frequently generates surges and operational over voltages.

Furnace Transformers are designed to provide On load or no-load tap Changer.  They also have a built in reactor for long arc Stability.  The bushings can be air cooled or water cooled.

Transformers for DC Furnaces have an attached rectifier.  DC Furnace Transformers are also designed to withstand the harmonics generated by the rectifier operation. 

Furnace Transformers are generally used in the steel industry for smelting iron and refining steel. 

Cable deterioration due to oil is a area which has gained notice only in recent years.  Many people think of oil as a harmless and inert substance.  However, the opposite is true.  Oil attacks the plasticizers in the polymer which forms the insulation.  Plasticizers are substances which give polymers the firm shape.  When plasticizers are damaged, the cable insulation softens.  This exposes the conductors and results in a short circuit or a ground fault.

Oil is a substance which is widely present in the industrial environment.  Even if the cables do not come in direct contact with the oil, oil droplets which drift in the air can settle on the surfaces of the cable insulation and can initiate the degradation process.

Hence, it is essential to ensure that the cables used in industrial environments are resistant to oil. 

Cables are classified on the basis of their resistance to oil as UL62, UL Res 1, Ul Res 2, UL AWM 21098


Flexible Industrial Cables are cables which have been specially designed to have very small bending radius.  Flexible cables are used in installations where space is a constraint.  They are used in small control panels and connection boxes where there is little space.flexible cables

The conductor is made of finely stranded bare copper.  The insulation is made of a special PVC compound. 

Flexible Cables are also resistant to oil, acids and water.  Flexible cables are available for power, instrumentation and data applications. 

Flexible cables are used applications such as CNC machines, grinding machines,  conveyors,  assembly lines and control panels

The insulation in flexible cables is usually enclosed in a jacket for extra protection. 

The flexible cables are shielded for RFI and EMI protection. 

Submersible Wires are used in underwater applications.  Typical examples are submersible pumps.  Other applications include mining, dewatering and naval pumps. submersible wire

Submersible wires are made of copper like ordinary wires.  However, they have special PVC insulation which is impervious to water, oil and acids.  The insulation is provided with a separate jacket for added protection. 

Stripping refers to the process or removing insulation from the conductors.  Stripping should be done without damaging the conductors. 

The insulation is removed by a hand held stripper or by a stripping machine.  The details of the length of wire, the size of the conductor are fed into the machine.   The length of the wire to be stripped is also fed into the conductor.  Special sensors monitor the length of the stripped conductor and whether the conductor is damaged.  wire stripper

Stripping machines can perform full strips, Half strip, Inner processing of conductors.

Particularly difficult are special conductors such as ribbon cables and coaxial cables. 

Crimping refers to the fitment of the lugs on to the wire or the conductor.  Crimping is a very important part of wiring.  Poorly crimped conductors can cause loose contact, arcing and malfunctioning in the circuit.Crimping machines  Crimping is done by deforming the lugs to suit the circumference of the wire.  crimping tools

Crimping can be done manually using special hand-tools.  There are also dedicated special crimping machines for larger cables and conductors. 

There are also bench mounted Crimping devices.  These devices are pneumatically operated. 

Pressure Sensors are calibrated using a pressure calibrator.  A pressure calibrator is a pneumatic devices which develops a pre-set pressure.  Pressure Calibrators are usually manually operated. 

The Pressure Calibrator is connected to the pressure switch or transducer to be calibrated.  The reference pressure is applied through the calibrator.  The output of the transducer is checked for the appropriate signal. 

For instance, a 1.10 bar pressure with an output of 4.20 mA should develop 12mA when a 5 bar pressure is applied. 

If the sensor output is not proper.  The sensor can be adjusted using the zero and span adjustments.

Five Point Calibration

The Five point Calibration is a method usually adopted in pressure sensor calibration.  The pressure sensor is tested at 0%, 25%, 50%, 75% and 100% of the range.  Thus, a pressure sensor with a range of 0 to 12 bar will be tested at 0 bar, 3 bar, 6 bar, 9 bar and 12 bar.

Enamelled Wire is a copper wire which has been coated with a layer of enamel.  The enamel serves as the insulation.  Enamelled copper wire is used to wind insulation. 

There are many types of enamels used to coat the winding.  Polyvinyl Acetal, Polyesterimide, polyester are some polymers which are used as the enamel coating in the wires.  The thickness of the coating depends on the voltage rating.  The enamelled copper is subjected to a Breakdown voltage test during manufacturing.

Enamelled Copper is also known as magnet wire.  Enamelled copper wires have high space saving factor.

Enamelled Copper wires should have good flexibility and conductivity.

The advantages of enamelled copper wires are the small size which enable easy winding.  They also have high conductivity and resistance to corrosion. 

Aging in Transformers refers to the the reduction in the usable life over a period of time.

The main components of the Transformer are the windings, the core and the insulation.  Of these, the windings and the core are made of metal.  The windings are made of copper while the core is made of silicon steel.   The aging in the metallic parts of the transformer is considered to be negligible.   They are not subjected to the aging process. 

The aging of the transformer is thus considered to be related to the aging of the insulation.  The insulation of most transformers is usually made of cellulose paper. 

The factors which affect the aging of the insulation are


Oxygen in the air causes degradation. 


Moisture or water content in transformer oil comes from two sources.  The moisture in the atmosphere can be removed by the silica gel in the breather.  Another source is the cellulose itself.  As the cellulose breaks down due to aging, water is released into the oil.  This water can be removed by periodically filtering the transformer oil.


Acids are formed, due to the degradation of the oil.  The acids formed in the oil affect the insulation and reduce its life.


Heat causes the degradation of the oil.  High temperatures also cause the insulation to break down.  Hence, it is necessary to ensure that the transformer is operating within the temperature limits.  The cooling system of the transformer should be properly functioning

Generator Step-up Transformer units are used to increase the voltage of a generator and connect the supply to a bus bar.  For instance, if the generator has a voltage of 3300V and the busbar a voltage of 6600V, the Step Up Transformer will have a ratio of 3300/6600.

Generator Transformers are also used to limit the fault level of the generator in case of a fault.

Transformers used for these applications are called Generator Step-up Units.  These Transformers are designed to operate at near full load. 

Every time a fault occurs in the grid, these transformers are subjected to stress.

As these transformers are connected to the grid, they are subjected to constant voltage fluctuations.     Voltage fluctuations cause stress to the transformer windings.  These windings have to be specially designed to withstand these stresses.  Localized heating is another issue which is a result of over excitation by the generator during voltage fluctuations.  Localized heating can damage the metallic accessories of the transformer. 

Hence, the magnetic circuit is specially constructed to have very low leakage flux. 

Generator Step Up Transformers are available in both single and three phase units.  They can also be designed to withstand wide temperature variations.