Electricity and Motors

Electricity

The word "electricity" can be used to mean an electrical charge that does not move (static electricity) or an electric charge that moves (current electricity) or a form of energy (electrical energy).

All of these depend on what is happening to the electrons in a material.

All materials are made up of atoms. Every atom consists of a nucleus containing a number of positively charged protons around which an equal number of negatively charged electrons move. The nucleus also contains neutrons, which do not have any electrical charge. The number of electrons in each atom of the material largely determines a material's physical, chemical and electrical properties.

The atoms in some materials have "free" electrons that are loosely bound to their nuclei. A free electron can easily be induced to leave its atom and move about in random directions in the spaces between the atoms. When wandering electron encounters an atom, the electron could attach itself to the atom and could force another electron to leave that atom. Electricity passes easily through these materials which are called conductors.

In other materials, most of the electrons are firmly bound to the nucleus and hence there are few "free" electrons. Electricity does not easily pass through these materials which are called non-conductors or insulators.

Electric Current

Normally, the free electrons in a conductor are moving in random directions. If an appropriate electrical force (called an electromotive force or EMF) is applied to the conductor, the free electrons can be induced to move (drift) generally in one direction. This movement of electrons is called current electricity and an electric current is said to flow.

The rate at which the electrons appear to drift through the conductor is called the drift velocity. The number of electrons per second appearing to move past any point of the conductor gives a measure of the electric current. Increasing the magnitude of the EMF applied to the conductor will increase the drift velocity of the electrons in the conductor. This increase in drift velocity would manifest itself as an increase in the electric current passing through the conductor.

The source of EMF can be a battery or a generator or a photoelectric cell. For an electric current to flow through a conductor, the EMF source must apply an electric charge to one end of the conductor and an opposite electric charge to the other end. A simple example of this is the electric current flowing in a metal wire (the conductor) connected between the "negative" terminal and the "positive" terminal of a battery (the EMF source).

All materials offer some resistance to the flow of electrons and hence work has to be done in forcing the electrons through the material. Materials with low resistance are the "conductors", the "insulators" having high resistance. The degree of resistance ranges from almost zero (for special materials called "super conductors") to very high (for the materials used to insulate powerlines).
When an electric current flows through a conductor, two effects are produced:
the electrical energy used to overcome the electrical resistance in the conductor is converted to thermal energy which increases the temperature of the conductor.

Examples:heaters, stoves and electric kettles use the heating effects (conversion of electrical energy to thermal (heat) energy); and

incandescent light bulbs emit light because their elements are raised to a high temperature (electrical to thermal energy conversion).
a magnetic field forms around the conductor.
for example:

When a current carrying conductor is placed in a magnetic field, the interaction between its magnetic field and the other magnetic field exerts a force on the conductor. In an electric motor, this interaction forces the shaft to rotate (conversion of electrical energy to mechanical energy).

Most of the ways in which electricity is used can be traced back to these two effects.

Electrical energy can therefore be easily converted to other forms of energy.

Conversely, most of the electricity in a large electricity supply system is generated by the use of magnetic fields in machines called, appropriately enough, "generators" (which can be thought of as being electrical motors driven backwards). Other forms of energy are used to produce the mechanical energy used to rotate the shafts of the generators. There are other ways in which electricity can be generated, but they all involve the conversion of a source of energy into electrical energy.

Electrical energy can therefore be easily produced by the conversion of other forms of energy.

A law of physics formulated by Isaac Newton notes that 'Energy cannot be created or destroyed but can be transferred from one form into another'. The usefulness of electricity therefore lies in its unique ability to be a convenient and easily controlled means to transport energy from one location to another location and to convert energy from one form into another form.

The Familiar Forms of Electricity

Ever walked on a carpet and been "zapped" when you touch a metal object. That is an example of static electricity. Static electricity is used to a lesser extent than current electricity in our every day lives. The ENERGEX web site has a good explanation of this form of electricity.

A more useful form of electricity is "current" electricity in which the electric current "flows" in one direction only - Direct Current (DC). The batteries in our torches, toys, portable radios and cars are the most common sources of DC low voltage, low power electricity. Higher voltage, higher power DC systems are used for particular applications, such as energy storage systems associated with renewable energy systems that are not connected to an electricity supply network. High voltage, high power DC powerlines have been used successfully in special applications such as interconnectors between transmission systems and undersea power cables.

Large DC electric motors are common in certain applications, such as electric locomotives where high starting torque and variable speed are required.

The most useful type of "current" electricity is the type in which the direction of flow of the electrical current changes direction many times in each second - Alternating Current (AC). AC electricity powers the appliances in our homes, turns the electric motors of industry and energises our electric lights.

The current in an AC system does not instantaneously change direction. Rather, it gradually (in relative terms) increases in magnitude until it reaches a maximum in one direction, then gradually reduces to zero, gradually increases to a maximum in the other direction, then reduces to zero - and the whole cycle starts all over again. The number of complete cycles carried out in a second is called the frequency of the AC electricity supply. In Australia, the AC frequency is 50 cycles per second, with 60 cycles per second used in America.

If DC and AC electricity can both be used successfully, why is AC the dominant form of current electricity?
The answers lie in the consideration of:

economics - in general, AC electrical equipment is smaller and cheaper to manufacture than DC equipment of similar duty; and

voltage changes - changes in voltages can be easily carried out in an AC system, but voltage changes in DC systems are complicated and require significant equipment. This ability to change voltage is particularly important in transmission and distribution systems where line losses are reduced if the voltage is increased. The voltages used in a large electrical supply system and the importance of having various voltages in the system are discussed in the Transmission and Distribution section.

Phases

There is one further major characteristic of an AC electricity supply that requires explanation - phases.

A DC circuit has two wires through which the current in the circuit flows from a source of electricity through a load and back to the source. A single-phase AC circuit also has two wires connected to the source of electricity. However, unlike the DC circuit in which the direction of the electric current does not change, the direction of the current changes many times per second in the AC circuit. The 240 volt electricity supplied to our homes is single phase AC electricity and has two wires - an "active" and a "neutral".

The distribution line supplying your home may be single phase and have only two wires strung between the poles (we will use the overhead power lines as examples because they can be easily seen). However, the distribution line may be made up of 4 lines. What are the others? The other lines carry the currents from two other electrical circuits, making a total of three circuits or phases. The reason why there are only 4 lines is because the 3 phases have a common neutral line (i.e. 3 active lines and 1 common neutral line).

But why 3 phases? Why not 2 or 4? Because the magnitude and direction of the electricity flowing in each of the phases is slightly displaced in time from the electricity flowing in the other phases, the current flowing in the common neutral will be the sum of the neutral currents from the 3 phases. The resultant current in the common neutral is smaller in a 3 phase system than in systems with other numbers of phases. This ability to use a common neutral of relatively small capacity has large economic advantages and is the main reason why 3 phases are used.

3 phase electricity has another advantage. We mentioned above that, in Australia, the voltage between the active and neutral in the single phase, low voltage supply to our homes is 240 volts and that this phase is only one of the phases in the 3 phase system. The voltage between the phases of this 3 phase system is 415 volts (in Australia). A 415 volt, 3 phase supply is able to deliver more energy than a 240 volt, single phase supply. 3 phase supplies are normally restricted to large electrical loads, such as large electric motors.

As we travel back up the electrical network, the voltage increases and the neutral disappears! Why? The answer can be found in the consideration of why a neutral is used. A single phase supply must have a neutral, whereas a 3 phase supply does not require a neutral. More complicated reasons deal with fixing the voltage of the single phase supply relative to the earth (because domestic appliances have their metal enclosures connected to earth) and for fault protection purposes. 3 phase, medium voltage, distribution systems and high voltage transmission systems therefore use one wire for each phase and no neutral.

The above discussions focussed on active and neutral conductors (wires) as being the means to convey the electricity. One type of system uses the earth as the return path, with only the active being conveyed by a wire conductor. This type of single-phase supply system is called the Single Wire Earth Return (SWER) system and is use to supply small loads which are located far from the main distribution networks.

Electrical Safety

No section on electricity is complete without a short warning on the dangerous aspects of electricity. The following discussion is brief.

Electricity and the Human Body

Water makes up most of the human body, thus making the body an electrical conductor. If a person touches an electrically energised object (such as a bare wire or faulty equipment) and the person is touching the ground, electricity will pass through the person to ground. Depending on the voltage of the electricity, the frequency of the AC supply, the magnitude of the current flowing through the body and the amount of time the current is flowing, the result can range from a slight tingle to a harmful and potentially fatal shock.

The critical path of electricity through the human body is through the chest cavity. A current flowing from one hand to the other, or from a hand to the opposite foot, or from the head to either foot will pass through the chest cavity and could paralyse the respiratory or heart muscles (thus initiating ventricular fibrillation) and/or burning of vital organs. Electricity passing through any area of the body can result in burns caused by the current flowing in tissues and can be at the skin surface or in deeper layers or both.

Electrical Protection

Any piece of approved electrical equipment is designed for operation at a particular voltage and is insulated to protect its operator from coming into contact with its electrically live parts. Portable hand held electric appliances, such as electric drills, are unlikely to cause an electric shock because they are usually double insulated, with the outside of the item made from a suitable insulating material.

Protection against faulty equipment or circuits causing shocks to people in a domestic dwelling can be provided in two ways.

1. by having the circuits wired in accordance with the Multiple Earthed Neutral system, in conjunction with suitably rated circuit breakers or fuses. The circuit breakers or fuses are rated to operate just below the current limit of the wiring. When there is a fault to earth on an appliance, and the appliance is connected to the earthing circuit, the current in the circuit will greatly exceed that of the circuit breaker or fuse and cause them to operate. This disconnects the appliance from the supply. The disadvantage of the system is that, if the appliance does not have an earth connection or if the earthing circuit is faulty, the protection will not operate; or

2. for an electrical component to function, the electrical supply must have an active and a return path. The current through the circuit will be identical through both the active and neutral conductors. A protective device has been developed to sense any imbalance of current through the active and neutral conductors. This device does not provide protection unless the electricity has a path to earth, which could, in the worst case, be through a person. If this happened, the device's short detection and operating times would normally prevent severe injury to the person. This device is identified by various names, including Earth Leakage Circuit Breaker (ELCB), Residual Current Device (RCD) and Safety Switch.

Some simple ways to Avoid Electrical Accidents

install safety switches on the household power and lighting circuits;

have a qualified electrician regularly check the household's electrical systems, extension leads and appliances, particularly electric blankets and those appliances with metal cases;

make sure that a light has been switched off before changing the bulb - and do not use your finger to clean the connection;
protect power points from the probing of children;

replace worn electrical leads, particularly those on electric irons;

NEVER touch a fallen powerline unless given specific assurances of its safety by a person qualified to do so; and
think "safety" when doing anything associated with electricity.

If an Electrical Accident Occurs

Do not touch the victim unless you are sure that the victim is not still in contact with the electrical hazard or that the electricity has been switched off. If you are unsure of how do this, get qualified help without delay.

If the victim has been removed from the electrical hazard, first aid can be applied. Again if you are unsure of how do this, get qualified help without delay.

An electric motor operates on the principle of electrodynamics that states that when a current carrying conductor is placed in a magnetic field, the conductor experiences a force, when the conductor is inclined to the magnetic field.

When a current carrying loop is placed in a magnetic field so that it makes an angle with the magnetic field, the forces acting on the loop will rotate it, thus producing mechanical energy. The magnetic field can be produced by a magnet or by a current carrying coil wound on soft iron pole pieces. Electric motor use the latter method to produce a magnetic field.

In general, DC motors and small single phase AC motors (up to about 2.5 kW) are used for specific purposes.

Larger three phase AC motors are the electric motors most used, particularly in industrial applications. The principal types of three phase motors are the induction motor and the synchronous motor.

a) A Yoke which supports poles and field windings and provides a path for the magnetic flux; and
b) An Armature with commutator and brushgear.

a) Separately excited motor - The field winding in a separately excited motor is energised from an independent source of electricity and so not affected by the load or voltage drop in the armature. Its speed remains practically constant over the entire load range;

b) Shunt motor. The speed of a shunt motor drops slightly as the load increases. However, a shunt motor is considered a constant speed motor for all practical purposes. The starting torque of a shunt motor is 100-150 % of the full load torque;

c) Series motor - The field in a series motor is connected in series with the armature winding. The speed of a series motor is high at light loads and falls off rapidly with increasing load. It cannot be run on light or no load because it may overspeed. For this reason, a series motor is used with a directly coupled load. The starting torque is of the order of 300-500% of the full load torque.

The series motor finds extensive application in traction and in other jobs where high starting and accelerating torque is required and the motor is never required to run on light load; and

d) Compound motor - A compound motor has a series as well as shunt field and its speed-torque characteristic is determined by the relative strengths of the two fields. The field windings are designed to give practically constant speed at all loads.
The starting torque about 250-300% of full load torque.
Compound motors are used where high starting torque and fine speed control are desired, such as cranes, rolling mills, and excavators.

DC motors possess the following advantages:

High starting torque;
Speed control over a wide range, both below and above the normal speed; and
Quick starting, stopping, reversing and accelerating.

The disadvantages of DC motors are:

High capital cost of the motor and the control gear; and
Increased operating and maintenance costs because of commutators and brushgear.

AC 3 Phase Induction Motors

The most commonly used motor in industrial applications is the three-phase induction motor.

Principle of operation of a three phase induction motor

The principle of operation for all three-phase motors is the rotating magnetic field.
There are three factors that cause the magnetic field to rotate.

1. The voltages of the three-phase system are 120° out of phase with each other;

2. The three voltages change polarity at regular intervals; and

3. The arrangement of the stator windings around the inside of the motor.

The speed at which the magnetic field rotates is known as the synchronous speed. The synchronous speed of a three-phase motor is determined by two factors.

1. The number of stator poles; and

2. The frequency of the AC.

The field set up by the stator windings cuts the copper bars of the rotor. The voltage induced in the squirrel cage winding produces current in the rotor bars. As a result, a field is created in the rotor core. The attraction between the stator field and the rotor field causes the rotor to follow the stator field. The rotor always turns at a speed slightly less than that of the stator field. In this way, the stator field cuts the rotor bars and induces the required rotor voltage and current in the rotor field. The torque produced by an induction motor results from the interaction between the stator flux and the rotor flux.

In the case of wound rotor induction motor, the voltage and current are induced by the rotating field similar to that in the squirrel cage induction motor. The induced current set up form a closed path from the rotor windings through the slip rings and brushes to a star connected speed controller.

At the start up, all of the resistance of the star connected speed controller is inserted in the rotor circuit. This additional resistance causes an excellent starting torque and a large percent slip.

Characteristics of Induction Motors

Some of the important characteristics of induction motors are:

i) For the same slip, the torque varies as the square of the terminal voltage;
ii) The slip is proportional to the load, and since the slip varies over a small range only, the speed of an induction motor is more or less constant with load;
iii) The torque varies directly with the slip;
iv) The slip varies inversely as the square of the terminal voltage; and
v) The efficiency of an induction motor is inversely proportional to slip. A motor with a lower value of slip will be more efficient than a motor with a higher slip because of the increased losses in the rotor of the latter. The efficiency of three phase induction motors varies with type, size and load. It ranges from 85% to 99% in the case of squirrel cage motors above 5 HP. It is about 75% for smaller motors. The efficiency is less in the case of slip-ring motors, slow speed motors and motors running at part load.

The advantages of an AC induction motor are as follows:

Simple design;
Rugged construction;
Reliable operation;
Low initial cost;
Easy operation and maintenance;
Simple control gear for starting and speed control; and
High efficiency.

Construction of Induction Motors

Broadly there are two types of three-phase induction motors:

1. Squirrel-cage induction motor; and

2. Wound-rotor or slip-ring induction motor.

Each of these two types of three phase induction motors consist of: The Stator; and The Rotor.
Stator: The squirrel-cage and the wound-rotor induction motors have nearly the same stator construction and winding arrangement.The stator is a three-phase winding placed in the slots of a laminated steel core and formed of three single-phase windings spaced 120 electrical degrees apart. The three single-phase windings are connected in star or delta formation. The three line leads from the three windings are brought out to a terminal box mounted on the frame of the motor. The laminations of the steel core are insulated by varnish or oxide coating, and are slotted in their inner periphery.

Squirrel cage rotor:

The rotor is constructed of laminated steel sheets assembled around a shaft. The rotor winding consists of copper or aluminium bars. The copper bars are soldered to two copper end rings. In the case of rotors with aluminium windings, the bars and end rings are all die cast in position without soldering at the ends. The aluminium conductor rotors are therefore more rugged.

The slots of the rotor are not always parallel to the slots on the stator. Skewed rotors are twisted (skewed). Skew effectively reduces noise, eliminates the magnetic locking of the rotor and increases starting torque.

In very small motors, the rotor is sometimes made up of solid steel without any winding. Such motors operate by virtue of the eddy currents established in the rotor.

The speed performance of a squirrel cage motor is measured in terms of slip. Slip is usually expressed as the percentage by which the speed of the rotor falls behind the speed of the rotating synchronous speed of the stator field.

Wound-rotor (or slip-ring motor): The cylindrical core of the rotor is made up of steel laminations, slotted to hold the formed coils of the three single-phase windings. These windings are placed 120 electrical degrees apart. The insulated coils of the rotor winding are grouped to form the same number of poles as in the stator windings. The three single-phase rotor windings are connected in star. The three leads from these windings terminate at three slip rings mounted on the rotor shaft. Carbon brushes press against these slip rings and are held securely by adjustable springs mounted in the brush holders. The brush holders are fixed rigidly. Leads from the carbon brushes are connected to an external speed controller.

The advantages of a slip-ring motor are:

Its susceptibility to speed control by regulating rotor resistance;

High starting torque of 200 - 250% of full load torque; and

Relatively low starting current (250 to 350% of the full load current) compared to a squirrel-cage motor, which may have a starting current in the order of 600% of its full load current.

Power Factor of Induction Motors

The power factor of an induction motor depends on its type, size, rotational speed and load. Slip-ring motors have lower power factors than squirrel cage motors of the same size. A motor running at high speed and near full load has a better power factor than when it operates at part load and low speed.

The power factor at no load is approximately 0.15 lagging. The no load current consists mainly of magnetising current. This current produces the magnetomotive force (mmf) required to send the stator flux across the air gap and through the magnetic circuit. The in-phase component of the no load current is low. Hence the power factor at no load is low. As the load on the motor incr4eases, the in-phase current component supplied to the motor increases and hence the power factor increases. In practice, the power factor of the inductive motor at the rated load is between 0.85 and 0.90 lagging.

Induction Motor Losses and Efficiency

The losses in an induction motor consist of stray losses and the copper losses. The stray power losses include mechanical friction losses, windage losses and iron losses. These losses are nearly constant at all loads and are often called fixed losses.

The copper losses consist of the I2R losses in the windings of the motor. An increase in load increases the current in the motor windings and hence the I2R losses. At light loads, the percent efficiency is low because the fixed losses form a large part of the input power. As the load increases, the fixed losses become a smaller part of the input power. Thus, the efficiency increases with load. However, as the rated capacity of the motor is exceeded, the copper losses become excessive and the efficiency decreases.

A synchronous motor is started as an induction motor or by a separate induction motor. When the motor comes up to speed, the DC excitation is supplied to the field winding and the motor pulls into synchronism. No voltage is generated in the auxiliary rotor winding during synchronous operation.

The DC excitation is provided by an exciter driven either from the motor's shaft or by a separate motor. To reduce maintenance, brushless synchronous motors are now being manufactured. An alternator mounted on the motor shaft replaces the exciter. The AC from the alternator is converted to DC by bridge connected silicon diodes and then supplied directly to the field winding. This arrangement eliminates the exciter, commutator and the field slip rings.

The ability of a synchronous motor to operate at leading power factor makes it suitable to be used for power factor improvement. When a synchronous motor is used exclusively for power factor improvement and not for driving any mechanical load, it is called a synchronous condenser.

AC Single Phase Motors

Single phase motors are usually used in domestic appliances because they are suitable for low power ratings. The magnetic effect of a single phase winding results in a pulsating magnetic field which may not be able to start the rotor turning if the rotor is in certain positions. In order to make a single phase motor self-starting, a second (starting) winding is installed in the stator slots at 90 degrees, or half a pole pitch, from the main winding. As the motor is only supplied by a single phase source of electricity, further variations are required to provide a phase difference for starting purposes. This is done by either:

placing a non-inductive resistance in series with the starting winding. This does not produce an exact 90 degree phase difference, but is enough to start the motor; or

connecting a condenser in series with the starting winding. This will provide a 90 degree phase difference.

It is usual to fit the single phase motor with a centrifugal switch which will take the starting winding out of service as soon as the motor comes up to speed. If a condenser is provided, the starting winding and condenser are left in the circuit.

The torque of the induction motor is dependent on the magnetic field strength (flux per pole), the rotor current and the rotor power factor. On starting with the rotor at standstill, the rotor frequency will be less than the stator or supply frequency. As a laminated iron frame surrounds the rotor conductors, there is considerable inductance inherent in the rotor circuit. Due to this large inductance, the reactance of the rotor is much higher than the resistance when the motor starts, subjecting the rotor current to a low power factor. The starting torque of the induction motor is therefore low.

As the motor comes up to speed, the inductive effect decreases and the power factor improves. The resistance is constant and the torque improves to a maximum at approximately 80% nominal speed. Starting torque can be improved by adding resistance to the rotor circuit. Maximum torque is reached when the value of rotor reactance reaches the value of rotor resistance. Above this speed, the power factor decreases and the torque rapidly reduces to become zero at synchronous speed. The induction motor will therefore never reach synchronous speed due to its inherent induction and loss factors.

Maintenance of Electric Motors

Under normal operation, motors should be checked on a regular basis. The environment in which the motors are operating and the importance of the motors' continued operation should dictate the regularity of the checks to be made. If the environment is hostile (e.g. wet, dirty and hot), the checks should be at least on a daily basis.

Visual checks should include:

Cleanliness of the surroundings, ensuring that cooling vanes are clear of extraneous matter;

Signs of grease or oil leakage from the bearings;

The motor frame and bearing plates (if accessible) are not unduly hot;

The motor is not showing signs of abnormal vibration or noise;

Protection against the weather, sun, dust, heat, etc is in place;

There is no evident damage to incoming cables and or terminal boxes; and

Air intakes and filters (if applicable) are not clogged.

It is normal to carry out maintenance on motors when the item being driven is to be serviced. The maintenance on a motor can include the following:

Disassemble the motor in a workshop;

Clean the windings and the rotor. This may involve as little as dusting the parts down with a clean dry cloth. It can be as much as washing the parts including the windings thoroughly with a spray using water and a solvent. Under these circumstances, the stator and rotor will have to be placed in an oven or at least heated to thoroughly dry the insulation. After this process, the windings are given an appropriate insulation test;

The insulation needs to be checked to ensure there has been no movement, no cracking or signs of deterioration. In some cases the insulation may need to be sprayed with an insulating varnish or even re-dipped;

The slot wedges should be checked for tightness and or cracking;

Checks are to be made for any signs of abrasion on the rotor or the iron core of the stator;

The condition of the fan and fan cover is to be checked;

An insulation check is to be made on the stator prior to assembly and a further test given when the motor is being reinstalled;
The condition of any internal connections is to be made and the connections into the terminal boxes. This includes the connections to any heaters, RTDs or thermocouples;

The motor frame/housing is to be checked for any damage. This applies particularly to the feet of the motor;

The coupling of the motor is to be checked for wear and any sign of looseness during operation;

Bearings must be checked for wear and deterioration. If there is any sign of deterioration, the bearings must be changed, ensuring that the correct type is used, and, on reassembly, the correct recommended grease or oil is used;

Where possible, the motor should be given a no load run in the workshop prior to reinstallation in the field;

On reinstallation in the field, it is normal to further check the state of the coupling or other drive mechanism associated with the motor;
Terminations are checked and made in accordance with maintenance instructions for tightness and phasing;

Care is taken to ensure rotation direction is correct prior to final connection to the driven equipment; and

The cables being connected to the motor are checked for condition and to ensure they are connected in the correct sequence on the motor terminals.

Electric Motor Standards and Tests

When purchasing an electric motor, the specification will list a number of Engineering Standards that the motor will have to meet. These will determine the details of the motor type, size, speed, winding insulation, cooling system and required temperature rise limits, mounting and the tests that the motor must be subjected to during manufacture. Other items will detail the starting characteristics, the vibration and noise levels, the inbuilt thermal protection devices required, use of anti-condensation heaters, requirement for special lubrication systems and bearings, use of porous plugs and type of painting. The protection required during transport and lifting facilities can also be specified.

Generally the larger electric motors would be expected to comply with at least the following Australian Engineering Standards or their equivalent:

-AS 1359 parts 4; 50; 60; 69 - Rotating Electrical Machines;
-AS 1081 & AS 1469 - Noise Levels;
-IEC 34-6: 1991 - Methods of Cooling; and
-AS 1939 - Protection Degree Ratings of Enclosures.

The performance tests required during manufacture are specified to be in accordance with AS 1359 part 60 or its equivalent. It is normal to require that the first motor of a design be subjected to a more stringent set of tests than subsequent motors of the same design. This series of tests are called 'Type Tests'. The tests normally required for type tests are as follows:

1. Resistance of windings;
2. No-load losses and current;
3. Locked rotor test (see description below);
4. Temperature rise (see description below);
5. Power factor;
6. Efficiency;
7. Momentary overload;
8. Medium voltage insulation (as per AS 1359 Pt. 60);
9. Vibration (as per the requirements of AS 1359 Pt. 50 or equivalent);
10. Noise (as per AS 1081 with the level as described in AS 1469); and
11. Determination of run-up speed/torque characteristic.

If the proposed manufacturer has previously produced this type of motor and has carried out the required type tests, less stringent test may be allowed. These less stringent tests are known as 'duplicate' tests and include the following:

1. Resistance of windings;

2. No-load losses and current;

3. Locked rotor (see description below);

4. Medium voltage insulation;

5. Vibration.

Test Certificates are required for all tests and should show the results and description of all tests.

Locked Rotor Tests are performed with the rotor locked to establish starting torques and starting currents. Starting torque can be evaluated using a torque arm which locks the rotor. The voltage is slowly increased until full load current circulates in the stator winding. It is preferable to carry out this test at 100% rated voltage but if this is not possible (due to the potential of damaging the motor and/or the limitations of the test facilities), the voltage can be raised in equal increments up to 50% rated voltage. A curve is then drawn through the plotted values and extrapolated to the 100% rated voltage value.

1. A no-load test which provides full voltage iron loss for the motor - the motor is uncoupled and run at full voltage until thermal equilibrium is reached; and

2. A full load test at full copper loss - the motor is coupled to a load as close to full load as the test facility will allow, the voltage reduced until full load current is reached and the motor run until thermal equilibrium is reached.

A record of all readings, including temperature, voltage and current, are to be supplied as part of the test results. Note that, if anti-condensation heaters are fitted, then these are to be energised while the test is in progress. To satisfy the specified requirements, the motor temperature rise should not exceed that specified.

The Insulation Class on the windings, the maximum Temperature Rise under full load conditions and the maximum ambient temperature are usually specified together. For example, the specification could require a motor whose windings are to have insulation of 'F' Class and a maximum temperature rise of 800° C when operating at an ambient temperature of 400° C.

Terminals are usually described to suit the cabling requirement of size and direction of location of the cables on the side or top of the motor.

DIAGRAM OF A LARGE SQIRREL CAGE INDUCTION MOTOR

This motor is similar to that in the above photo. Note the air cooler mounted on the top of the motor assembly.