Induction motor rotor - construction

The rotor of the induction motor has a core which is made of electrical steel.

The bars which constitute the squirrel cage are typically made of aluminium or copper. The bars are placed in slots on the rotor core. There is no need for insulation between the bars and the core as the voltage developed in the squirrel cage is very low.induction motor rotor

The rotor bars are skewed in order to prevent magnetic locking. Magnetic locking is also known as cogging.

Magnetic Locking can also be prevented by ensuring that the number of rotor slots is not equal to the number of stator slots.

Motor Bearings – Functions and Types
The bearings can be one of the two types, a plain bearing (sliding contact) or an anti-friction bearing (rolling bearing), depending on the design parameters of the machine element, each of the two types of bearings, plain and anti-friction, is available for design with linear motion, radial loads and axial loads.

Bearings may be classified into three general classes
Guide or flat bearings, which support linear motion in machine tables and slides.

Thrust bearings

Thrust bearings, which support rotational motion in machine elements that have axial loads i.e., the load is applied along the central axis of the rotating shaft

Radial bearings

Radial bearings, which support rotational motion in shafts with radial loads i.e., the load is applied along the radius of the rotating shaft.

Anti Friction or Roller Element bearings

Anti – friction bearings or roller – element bearings, as they are often called, use a rolling element (ball or roller) between the loaded surfaces.
Anti-friction bearings are divided into two categories,
a) ball bearings
b) Roller bearings.

Ball bearings have five general types:
Guide, Radial, Thrust, Self – aligning and Angular contact.

Roller bearings have four general types: Cylindrical, Thrust, Spherical and Taper.

Roller and Ball bearing types
Guide bearing: The ball guide bearing is used for linear motion where very low co-efficient of friction and extreme smoothness in operation are desired.
Radial bearing: The first radial bearing is the single – row, deep – groove ball bearing, most widely used anti – friction bearing. Second radial bearing is the cylindrical roller bearing is capable of carrying larger radial loads at moderate speeds than those carries by radial ball bearings using the same size bearing.
Thrust bearing: First the ball thrust bearing is designed for axial (thrust) loads only – no radial loads. Second spherical roller thrust bearing is capable of very heavy axial loads as well as moderate radial loads.
Angular contact ball bearing: The shoulders in this provides for thrust (in one direction only) that is larger than the single row, deep radial ball bearing can handle.
Taper roller bearing: A pair of taper roller bearing is capable of handling both very large axial and radial loads.

Shell bearings

In case of very large motors, shell bearings are used. Grease is used as the lubricant in case of roller and ball bearings.

For Shell bearings, lube oil is used as the lubricant.

The bearings are usually designed to withstand radial loads. However, in some applications such as in the use of gears and belts, the motor may also be subjected to axial loads. In such roller bearingscases, bearings such as angular ball bearings which can withstand axial loads can be used.

Laminations in Transformer Core and Motor Stator

 Electric machines, especially AC machines such as transformers and alternators are exposed to alternating magnetic fields during operation.
This alternating magnetic field causes the induction of eddy currents in the core of transformers and the stator of motors. The eddy current creates a loss of energy in the form of heat loss and hysteresis loss.

In order to avoid this, the core of transformers and the stator of motors and generators are made of a set of laminated steel sheets. Silicon Steel is used. This steel is cold rolled and has special grain orientation. Each steel sheet is around .3 mm thick.

The sheets are insulated on both sides and laid of top of one another. This arrangement ensures that the eddy current is reduced as it cannot flow over a wide area of cross section. The laminated surfaces need to be very clean. Presence of foreign particles can cause laminar faults which lead to core damage.

Eye Bolts in Motors-An Overview
An Eye bolt is an important component of the motor. It is used in lifting the machine. The eye bolt consists of a loop at one end and a threaded end at the other. The threaded end is screwed into the motor body. eye bolt

When the motor or alternator is to be lifted, a sling is connected to the eye bolt and the machine is lifted using a crane. It is important to note the capacity of the eye bolt. Every Eye bolt has a WLL ( Working Load Limit). If this is exceeded the eye bolt will fail.

This can result in injury to crew members or even death. Check the capacity of the eye bolt before lifting.

The eye bolt is intended only for vertical lifting. Angular lifting (lifting in an angle) will cause the bolt to fail quickly. shouldered eye boltAngular eye bolts will have the safe Working load limit. As the angle of lifting increases, the WLL decreases. Thus, while the WLL will be maximum at vertical, it decreases as the angle increases. The angle of lift should be calculated and the safe Working Load Limit determined.

There are special types of eyebolts which can withstand angular load (up to a certain degree, usually 45 degrees. Check the angle with the manual).

These are known as shouldered eye bolts. Check that the machine has shouldered eye bolts before lifting the machine.

Grease in Electric Motors
Grease is the most widely used lubricant in electric motors. Grease is used in motors with ball and roller bearings.
The function of grease is to minimize friction and wear,to prevent corrosion and to prevent the entry of foreign objects which can contaminate the bearing.

Grease, thus, has a sealing effect. Grease is a semi-solid lubricant. It is composed of a base oil, additives and a thickener. The base oil can be synthetic or natural. Synthetic oils are used in applications with high temperatures and longer regreasing intervals.

The function of the thickener is to prevent the base oil from leaking. Thickeners are usually metallic soaps. Additives include oxidation and corrosion inhibitors, anti-wear agents.

The grease in motors will have to be replaced over time. The regreasing intervals are based on the bearing manufacturers' recommendations.

Deep Bar Rotors in Induction Motors

Deep Bar Rotors are used in induction motors to increase the torque during starting. Deep bars indicate that the bars which comprise the cage in the rotor are deeper than those in normal rotors.

When an induction motor is started, the slip between the rotor and the stator is high. Thus the frequency of the rotor current is high.

This high frequency results in high reactance in the lower layers of the deep bar. Hence, most of the current flows in the surface of the rotor bars. This results in high current density and increased resistance. This resistance produces high torque during starting.

When the motor reaches its rated speed, the slip frequency drops and the reactance reduces. The current now, flows uniformly across the entire cross section of the rotor bar. The resistance in the rotor drops and the motor runs normally.

Natural Ester Oils for Transformer Cooling

Natural Ester Oils or Esters are used in cooling Transformers. These ester based oils are created by a reaction between a tri-alcohol and fatty acids. Natural Oils have higher flash and fire point. They are chemically stable. They are also biodegradable.

 Another important advantage is the water solubility. Natural ester oils have a water solubility which is 20 to 30 times that of mineral oils. Thus water is drawn from the insulation and absorbed in the oil. This increases the life of the insulation.    They are thus considered an effective alternative to mineral oils which are environmentally risky and prone to fire accidents.

The downside is that Natural Ester Oils are more expensive. They also have high viscosity which results in slower flow through the transformer and results in reduced heat transfer. Natural Ester Oils are also prone to oxidation which reduces the lifespan. Natural ester oils are used in temperate climates. At cold climates, the flow rate is reduced. They are used in distribution and Traction Transformers.

Synthetic Ester based oils are artificial oils made by select ingredients. These oils can be tailored to meet the exact requirements of the application. Synthetic ester oils can be made to resist oxidation. They can be used in cold as well as temperate climates. 

Silicone Coolant in Transformers

Transformers generate tremendous amounts of heat. This heat can be cooled by air in case of small transformers. In large transformers, more substantial cooling media are needed. Mineral oil is widely as a cooling medium. Another medium of cooling transformers is Silicone Transformer Oil.

Silicone Transformer Oil has high flash point. Hence, it can be placed closer to buildings as the risk of fire is reduced. Silicone fluid is also self extinguishing. The heat and smoke produced in the event of a fire is minimal. It is not hazardous and environmentally friendly. It can be recycled. Its base is polydimethylsiloxane is a chemically inert material. 

It does not form sludge and down not break down over its lifecycle. The Transformer has a longer life with reduced maintenance. 
The downside is that Silicon Transformer Oils is not biodegradable. 
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Oil Leakages in Transformers

Oil Leakages are a frequent issue in Transformers which if not properly addressed can result in serious accidents such as fire or explosions. Oil can also cause accidents due to slipping.  

Besides, Transformer oil is an environmental pollutant. Spillage of Transformer oil can result in severe penalty for environmental violations.  

Oil Leakages can be detected visually most of the times. However, some minor leakages are not always detectable by the naked eye. Special techniques such as the application of special fluids which indicate oil leakage by changing colour.

Once an oil leakage is detected, it needs to be arrested and the point of leakage should be plugged. Welding the leakage would require switching off the transformer and draining the oil. 

Transformer oil leaks can be rectified by the application of special polymeric compounds and putties which cure and seal off the leakage.  

Leakages can be prevented by ensuring that the elastomeric components of the transformer such as the gaskets, O rings, etc are replaced at the scheduled replacement intervals. The Transformer should be properly painted and any damage to the painting should be properly rectified.  

In addition to oil leakages, in transformers filled with Nitrogen the leakage above the oil surface can result in the leakage of nitrogen. The leakage of the inert gas can be confirmed by applying a soapy solution and observing the bubbles. 

Regeneration of transformer oil

Transformer oil regeneration refers to the treatment of old transformer oil. Oil Gets contaminated due to the entry of moisture and the formation of sludge. The contaminated oil in the transformer Regenerated. Regeneration involved degasification, dehydration and filtration. The old transformer oil is regenerated by passing it through columns containing fuller's earth. Fuller's earth is a type of clay which removes the impurities in oil when it is passed through it. 

The impurities are thus removed without the use of any chemical. The oil which is purified can be reused in the tank. After a certain number of times of cleaning, the Fuller's Earth can be reactivated.  The oil is drawn from the transformer, purified and sent back to the tank. Thus the transformer need not be taken offline. 

Significance of Acidity in Transformer Oil

Acidity in Transformer oil is measured by the TAN (the Total acidity number). Acids cause the breakdown of oil which results in sludge formation. 
Thus, measuring the acidity of Transformer is an important parameter. The acidity is expressed in Total Acidity Number (TAN) which is the milligrams of Potassium Hydroxide (KOH) required to neutralize the acid present in one gram of transformer oil.
Oils which have a high value of TAN will have to be treated.
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Sludge in Transformer

Sludge in Transformer is formed as the oil breaks down. Sludge attacks the cellulose of the transformer windings causing it to deteriorate. It reduces the oil circulation inside the transformer. 
Sludge also forms a layer on the transformer winding and reduces the heat transfer. These lead to higher transformer winding temperatures.
Sludge thus has to be removed. The transformer oil can be replaced or filtered.
See Also: Transformer oil Deterioration

Nitrogen in Transformers

Oil filled Transformers usually have a conservator which maintains the level of oil. The conservator has a layer of air on top of the oil. This air communicates with the atmosphere through a breather containing a desiccant,usually, silica gel.  

In some transformers, the tank of the transformer is filled with a layer of nitrogen above the oil in which the windings and the core are immersed. Nitrogen, being an inert gas, reduces the risk of a fire hazard. It also protects the transformer oil and the transformer insulation from reacting with oxygen in the air and deteriorating.  

In transformers containing nitrogen, the nitrogen is led into the space above the conservator through a pressure reducing valve. When the oil level in the transformer rises and the transformer breathes out, the nitrogen inside is released into the atmosphere. When the oil levels fall due to a reduction in ambient temperature or the load, fresh nitrogen is released into the conservator through the cylinder and the valves. 

Distribution Transformers

Distribution Transformers play a vital role in the system which delivers electricity to the end user.  It is the final part of the transmission system from the power plant to the consumer. Distribution Transformers step down the MV power, usually 11kv into the domestic LV, 440 V supply.

Distribution Transformers are a critical part of the distribution network. These transformers are always online throughout the year. Hence, design of the distribution transformer is made considering the high iron losses. Besides, the transformer is sized to have high efficiency at 70% of the load as the power output varies through the day as per the load cycle.

Distribution transformer are protected by fuses in the HV side. They are also designed to withstand unbalanced loading. They have ONAN cooling (Oil Natural, Air Natural ).

Distribution Transformer are usually of the vector Dyn11. While, designers are not particular about any particular vector group , most systems will standardize on one particular vector group, usually the Dyn11. Some systems also use the Dyn1. These vector groups have a difference of 30 degrees between the primary and secondary vectors which is unavoidable in delta to star conversion.

Single Phase Pole Mounted Distribution Transformers

Single Phase Pole Mounted Transformers are usually used in rural areas where three phase power may not be required. These Transformers reduce the voltage from the line voltage 11kV to a single phase voltage usually 230V.

The secondary of the Pole mounted Distribution is usually connected between the two phases of a MV line. The secondary voltage is a single phase voltage which is fed to the house. These transformers can be easily installed and do not require extensive mounting Structures.
Pole mounted Transformers come in sizes up to 500 kVA.

These transformers have a fuse to protect against faults inside the transformer. They are also equipped with an interrupting device. These Transformers are tested to withstand the impulse of lightning. In the US, The secondary of these transformers has a centre tapping and therefore has three terminals.

The voltage between the end terminal and the centre tapping will provide 120V while the voltage between two end terminals will be 220 volts. These Single Phase Pole Mounted transformers can also be used to provide three phase LV supply.

Three pole mounted transformers are connected in wye or delta to get the desired connection. Besides, small size three phase transformers which can be mounted on poles are also available. Video Showing the Manufacture of Single Phase Transformers

Amorphous Metal Transformers

The chief losses in a transformer are the losses that occur in the core. These are the the eddy current loss and the hysteresis losses. About 1 to 4 percent of the power which passes through a transformer is lost due to these losses.

The losses in the distribution transformers constitute nearly 20% of the total losses in the distribution system. Since transformers are online continually, the no load losses of the transformers is constant throughout the day regardless of the load.

The Amorphous Metal Transformer is fast emerging as an efficient alternative to the conventional transformer. The Amorphous Metal transformer has a core which is made of ferromagnetic materials such as Iron or Cobalt in a glass former such as phosphorous, silicon or boron.

Metglas, as this substance is known, has high susceptibility, low coercivity and high resistance. The low coercivity reduces the hysteresis losses while the high resistance greatly reduces the eddy current losses.

By using Amorphous Metal Transformers, it is estimated that many millions of units of electricity can be saved. Amorphous Metal Transformers are widely used in developing economies such as India and China in an effort to bring down the distribution losses.

Some of the other advantages of Amorphous Metal Transformers are the lower operating temperature, higher overloading capability, slower ageing of the winding insulation and better performance when subjected to harmonics.

Grounding Transformers

Grounding Transformers are used in Ungrounded systems to provide a earth point. Grounding Transformers are classified into two types

1) Zig Zag Transformers and

2) The Star-Delta Grounding Transformer with secondary unloaded.

We had looked at the Zig-zag Transformer in an earlier Post (Click here).

Let us now look at the Star-Delta Grounding Transformer

The Star-delta grounding Transformer has a primary which is star connected and a delta secondary. The phases of the star primary are connected to the busbar while the neutral is grounded. The secondary of the transformer which is delta connected is usually left unloaded, though it can also be used to supply power. The delta serves to provide a return flux path for unbalanced loads. 

During an earth fault, the zero sequence currents can flow through the grounded neutral of the transformer. If the current is to be limited, a resistor can be added in series to the neutral of the transformer primary.

What is a Rectifier Transformer

Rectifiers Transformers are Transformers which have an in-built Rectifier in the secondary side. The Diodes are mounted inside the Transformer Assembly itself.   The Rectifier Transformer is used in applications which require high DC power such as in DC Traction Systems and in processes such as electrolysis and smelting.

Voltage regulation is achieved using Tap Changers in the HV side.

Rectifier Transformers are sometimes also known as Rectiformers.

Rectifier Transformers are usually connected to a bank. Each Rectifier-Transformer is designed with an inherent phase shift such that the transformers attain the peak voltage one after another. This ensures a steady supply at peak voltage. Thus a system with five banks of rectifier transformers with twelve pulses at a phase shift of +12°, – 6°, 0°, + 6° and +12° will result in a system with 60 pulses .

Furnace Transformers

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.
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.

Generator Step Up Transformers

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.

Hermetically Sealed Transformers

Hermetically Sealed Transformers are transformers which are airtight i.e. they do not allow the air from the atmosphere to enter the transformer. These transformers are totally filled with oil. Preventing the entry of air protects the oil from oxidation and consequent deterioration.
Since the transformer is totally sealed, water cannot enter the transformer. This protects the insulation from moisture.
The transformer has low maintenance as there is no need for testing the oil or the insulation for a period of 10 years. This reduces the life cycle cost of the Transformer.
Since, there is no ingress of water or air into the transformer, the aging of the transformer insulation and oil are considerably reduced.
Hermetically sealed Transformers are used applications where it is difficult to conduct maintenance such as Offshore Platforms. Hermetically sealed transformer are more expensive to purchase. However, since they have lower maintenance costs and downtime, the life cycle cost of these transformer is cost effective as compared to conventional transformers.

Converter Transformers

Converter Transformers are equipment which convert the voltage and the frequency of the system voltage. Converter Transformers are used in applications such as Variable Frequency Drives, Electric Traction, copper refining and even in HVDC Transmission.

Converter Transformers have a converter unit on the primary or secondary side. The voltage is stepped up or down in the transformer and then converted into the desired frequency in the converter circuit. Alternatively, the frequency can be changed in the converter prior to stepping the voltage up or down.
Converters are used to couple two AC power systems asynchronously.
Converter Transformers are designed to withstand high harmonic content.

Medical Isolation Transformers

Medical Isolation Transformers are transformer used in medical facilities to power medical equipment. These transformers are designed to provide isolated AC power with noise suppression and are designed to withstand AC transients.
Noise can disturb sensitive medical equipment and interfere with measurements. Medical Transformers are designed with withstand short-circuits and are explosion proof.
They are also designed to have very low leakage currents. Excess leakage currents can cause unwanted and sometimes dangerous physiological effects in the patient.
Medical Isolation Transformer come in a box shaped enclosure to which the equipment can be connected.
Transformers are designed to withstand a very high hipot voltage.

Traction Transformers-An Overview

Traction Transformers are used in locomotives to step down the voltage from the overhead power lines. These transformers are similar to power transformers. However, they can be designed to function at different frequencies from 16 2/3 Hz to 50 Hz. The size of the core will vary depending on the frequency the transformer is designed for. At lower frequencies, a higher cross section of the core is required.
Cooling is carried out by different means such as through the use of silicone or mineral oil.
These transformers have a heater winding if they are operated at low temperatures. Traction Transformers will also have multiple traction windings. They may also have auxiliary windings for other circuits.
Traction Transformers are generally roof mounted. Some locomotives have transformers below the chassis as well.

Transformer Oil Deterioration

The Oil inside power transformers have a vital role to play in the transformer's functioning. The function of the transformer oil is two-fold, to provide cooling to the transformer windings and to provide insulation. However, over a period of many years, the transformer oil deteriorate owing to many factors. This deterioration causes a change in the physical and chemical properties of the oil.

Some of the reasons for transformer oil deterioration are

Oxidation of the oil.

The transformer breather permits the entry of air into the transformer, although it filters the moisture. The air which flows inside the transformer oxidizes the oil and forms a sludge of hydrocarbons. This process, though, usually occurs gradually over a period of many years. The sludge thus formed hinders the cooling of the transformer and causes heating. The sludge, sometimes, blocks the cooling ducts of the transformer. Higher temperatures inside the transformers, in turn, cause further sludge formation.

Thermal Decomposition

At high temperatures, the organic compounds in the transformer oil break down due to a phenomenon known as pyrolysis. This results in the formation of unwanted carbon compounds, sludge, etc.

Moisture contamination

Under ideal conditions, the oil in a transformer is protected against the entry of moisture by means of the silica gel filter in the breather. The silica gel changes color from blue to pink when it gets saturated with moisture. If the silica gel is not renewed in time, moisture may pass through the filter contaminating the oil.

Transformer Oil Regeneration

Transformer oil regeneration refers to the treatment of old transformer oil. Oil Gets contaminated due to the entry of moisture and the formation of sludge. The contaminated oil in the transformer Regenerated. Regeneration involved degasification, dehydration and filtration. The old transformer oil is regenerated by passing it through columns containing fuller's earth. Fuller's earth is a type of clay which removes the impurities in oil when it is passed through it.
The impurities are thus removed without the use of any chemical. The oil which is purified can be reused in the tank. After a certain number of times of cleaning, the Fuller's Earth can be reactivated.
The oil is drawn from the transformer, purified and sent back to the tank. Thus the transformer need not be taken offline.

Effects of sludge in Transformer Oil 

Sludge in Transformer is formed as the oil breaks down. Sludge attacks the cellulose of the transformer windings causing it to deteriorate. It reduces the oil circulation inside the transformer.
Sludge also forms a layer on the transformer winding and reduces the heat transfer. These lead to higher transformer winding temperatures.
Sludge thus has to be removed. The transformer oil can be replaced or filtered.

Transformer Temperature Rise Ratings

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

Reduction in Transformer Capacity due to Aging of Core

The core of a Transformer is made of a number of steel sheets which are placed one on top of another.  These sheets are laminated to prevent losses due to eddy current.

During the operation of the transformer, these sheets get deformed temporarily due to the magnetic flux.  This phenomenon is called magnetostriction.  Over long periods of time, due to repeated movement, gaps form between the surface of the sheets.

These gaps affect the magnetic circuit of the transformer circuit.  This causes a reduction in the flux of mutual inductance.  The transformer current increases by about 10 percent.

The gaps in the core sheets also distort the flux lines and cause an increase in the leakage flux.  This leakage flux causes eddy current heating in metallic components of the transformer such as the tank and other fixtures.  This causes the current to increase by another 5 percent.

This causes an overall reduction in transformer capacity.