Electrical Machines - Questions and Answers
Information courtesy of ALSTOM.

INTRODUCTION

1. What are the standard AC supply voltages in South Africa?

Fifty hertz 380, 433 or 525 volts, indicating 3-phase supply of these voltages for 3-phase loads connected to the 3-phase lines. In the year 2004, voltage will change to a standard 400 volts.

2. What is the phase voltage?

The voltage between any phase wire of a 3-phase supply and the neutral wire.

3. What is the relationship between the phase voltage and the line  voltage of a 3-phase 4-wire system?

Line voltage = 1.732 phase voltage.

4. What is meant by the torque of a motor?

The turning effort developed by the motor usually expressed in so many Newton metres (Nm).  If the motor exerts a force of F(N) acting at right angles to a radius of r(m) from the centre of its pulley, the resulting torque is F x r(Nm).

Fig. 1 - The Torque Motor.
Torque is force (F) multiplied by the radius of action (r). If F is in newtons and r is in metres, the torque will be given in newton metres (Nm).

5. How is the torque of a motor produced?

The production of torque requires the interaction of two sets of magnetic fields. The usual arrangement is for these fields to be produced by two sets of windings, both carrying current derived from the supply. One set of windings is situated on the stationary outer member (stator or field poles) and the other set on the rotating member (rotor or armature).

Fig. 2 - How the torque of a motor is produced.
The magnetic-field flux interacts with the magnetic flux due to the current flowing in the conductors of the coil

The principle is illustrated in the diagram. This shows a coil free to rotate about its axis in a magnetic field. If the coil is supplied with current, or a current is induced to flow in it, as indicated, the magnetic flux associated with this current will interact with the magnetic-field flux and produce a torque on each side of the coil. The coil consequently rotates, anti-clockwise in the case shown.

Fig. 3 - Conventional signs for magnetic polarity, direction of current and direction or rotation.
The magnetic lines of force are assumed to leave the face of a north pole

Fig. 4 - The direction of rotation of a motor is obtained by the use of Fleming's left-hand rule.
Direction of rotation depends on the direction of the main-field flux and the direction of the currents in the rotor or armature conductors.

The principle is the same whether the current producing the two fields originate from AC or DC supplies. In the DC motor, the magnetic-field system is fixed and the current fed to the rotating armature coils is changed to alternating current in the coils by means of brushes and commutator.

In AC motors of the induction type, the stator windings are usually distributed in slots around the stator core. These windings ore connected to the supply and the rotating fold produced by them induces currents to flow in the rotor.

6. What torque must be considered when driving a machine?

1. The initial starting (breakaway) torque needed to overcome the static friction of the driven machine and thus start it away from standstill.
2. The accelerating torque needed to run the driven machine up to full speed.
3. The running torque when full speed is reached,

NB: Ensure

1. The run up (pull up) torque of the motor is not less than the torque required by the load.
2. The max torque has sufficient overload capacity (e.g.) presses

7. How can the initial starting torque needed for a driven, machine be found?

One method is to wrap a cord attached to a spring balance around the half-coupling or pulley of the driven machine and to give o steady pull, noting the force required to start the shaft rotating. The starting torque required by the driven machine in Newton metres is equal to - Pull (N) x radius of half coupling or pulley (m)

An alternative method is to fix a bar along the horizontal diametre of the pulley and hang weights on the bar at a known distance from the centre of the pulley until the pulley begins to turn.

8. How is the accelerating torque determined?

From the torque/speed curves of the motor and the load. The point at which the torque/speed curves intersect will be where the final running speed occurs. The motor torque available for accelerating the driven machine at a particular speed corresponds to the difference between motor torque and load torque at that speed.

Fig. 5 - Torque/speed curves of a motor and its load.
The motor curve is for a squirrel-cage induction motor. The area between the two curves indicates the torque available to accelerate the load.

9. What is meant by the power of the motor?

Its mechanical output or rate of doing work - the work it does per unit of time (per minute or second). The power rating of a motor is measured in watts, or more usually, kilowatts. Power was formerly measured in horsepower (1 hp. = 746 watts).

10. How is the torque of a motor related to its power?

The motor exerts a force 'F' acting at right-angles to the radius 'r' of its shaft or pulley. Its torque is force multiplied by radius (torque = F x r). The work the motor does in one revolution of the motor pulley is equal to the force exerted (F) multiplied by the circumference of the pulley (2πr), that is, F x 2πr.

The work done by the motor in one minute is its mechanical output or power and this is equal to work done per revolution x r/min, that is, - F x 2πr x r/min.

As we have already seen, F x r is the Torque, so work done per minute = mechanical output = 2πr x r/min x torque.

If torque is in Nm, the power in watts is obtained by dividing by 60, which gives Power of motor = {2π x torque (Nm) x r/min} / 60

11. What is meant by the time rating of a motor?

Some motors are required to carry full load all day, others to run only for a series of short periods on full load, say few minutes at a time. The less time a motor is actually in operation over a period, the more time it has to cool down between spells of operation and the smaller its frame size needs to be. It is economical in practice therefore to classify motors as to their time rating.

Various time ratings have become standardized: maximum continuous rating, short time rating or some special rating based on a particular duty cycle. The manufacturer guarantees that the motor will not exceed a certain temperature after being run on full load for a certain length of time.

12. What is maximum continuous rating?

This is a statement of the load and conditions assigned to the machine by the manufacturer at which the machine may be operated for an unlimited period.

13. What is short time rating?

This is again a statement of the load and conditions under which the machine may be operated but this time for a limited period only, starting at ambient temperature. There are several preferred periods for being 10, 30, 60 and 90 minutes.

14. For what type of applications does a short time rating apply?

Applications with fairly intermittent periods of operation, such as cranes, hoists, lifts and certain machine tools that are operated only infrequently.

15. What is meant by the temperature rating of a motor?

The rated output of a motor is controlled by a temperature that the winding insulation is capable of withstanding. The maximum-permissible-operating-temperature depends on the type of insulation and the classification. The more general classifications are:-

Class 'B' - Polyester film, impregnated rayon, treated leatheroid. 
Class 'F' - Polyamide paper polyester film laminate treated terylene. 
Class 'H' - Polyamide paper, treated glass fibre.

16. How is the power required to drive an individual machine decided?

The power required is best obtained from the maker of the machine to be driven. In many applications the power required depends upon the efficiency of the driven machine and this can only be estimated by its manufacturer. This particularly applies to pumps, compressors, generators, centrifugal separators and fans.

For driving machine tools, the required power is increased for heavy duty and rapid production and reduced when only light work is the rule.

17. What calculation is commonly used to determine the minimum power required when the loads to be driven vary over a particular work cycle?

The minimum power required to avoid overheating is determined approximately by the root-mean-square method: 

Power =

Where P1, P2, P3, etc. are the powers required during the duty cycle and t1, t2, t3, etc. are the periods of time in minutes corresponding to the above power demands.

18. How is power decided for driving a line-shaft belt to a group of machines?

If there is a sufficient number of machines, say six or more, and these are hand controlled so that there are pauses during which no work is being done, such as for setting up, the motor speed will seldom exceed half the sum of the requirements of the driven machines. A more powerful motor will be necessary when the driven machines are on full-capacity work all the time.

19. What is the power required for driving cranes, hoists and winches?

Power (in watts) =	9.81 x kg lifted x lifting speed in m/s
	Efficiency of mechanical parts

An efficiency of about 0.70 may be assumed for worm and spur gearing or 0.65 if a stage of friction gearing is incorporated as in a frictional hoist. These are average figures that may be exceeded in certain favourable cases. On the other hand they may easily be less, requiring more power if the gear train consists of many stages or if badly cut or worn or if cast gears are employed.

20. How is current required to supply a 3-phase motor at full-load calculated?

From details of power, efficiency and power factor. The current is equal to:-

Current =	Watts x 100
	1.732 x line voltage x % efficiency x power factor

21. How is the full-load current of a single-phase motor arrived at?

From the formula:-

Currents =	Watts x 100
	Voltage x %FFF x PF

22. How is the full-load current of a DC motor calculated?

From the formula:-

Current =	Watts x 1 00
	Voltage x % efficiency

23. What is power factor?

The factor or percentage of the current in an AC circuit that is supplied in the form of energy, the remaining current being idle.

This idle current is termed reactive current or wattless current. The power factor of a circuit is calculated from:-

pf =	kW
	kVA

24. If an induction motor is described as having a power factor of 80 percent or 0.8, what does this mean?

That 80 percent of the motor current at full load is power current, doing work, and 20 percent is idle. A kVA demand metre or an ammetre will register the full 100 percent of current but a wattmetre will take into account only the true energy component of the current, namely 80 per cent.

An additional point about an induction motor is that the idle or wattless current is lagging.

25. How does lagging wattless current arise?

Every induction motor (or transformer or other electric apparatus comprising coils of wire embedded in or surrounding an iron core) constitutes an inductive reactance. If the motor is switched on to an AC supply, whether it does useful work on not, a current is taken from the system to excite it. This current is 90 degree lagging in phase on the voltage and is reactive current or so-called idle or wattless current. The only energy in this current is that required to overcome the losses and is but a small fraction of the total.

When the motor is put to work, it will take in addition to its excitation current, a power or energy current according to the amount of work to be done and the efficiency loss in the motor. The proportion of the two currents varies according to the percentage load on the motor. Consequently, the nearer the motor runs to full power the greater will be the proportion of power current to idle current, i.e. the higher the power factor.

26. What is meant by power factor correction?

Excitation current is an essential feature of AC induction motors (and other AC inductive apparatus). The demand for excitation current exists whether the motor runs loaded or light.

If this current circulates back and forth in the supply system, the supply cables, alternators and other equipment have to be designed to carry it and the additional expense involved is passed on to the consumer in higher price per kW.

A low power factor on the supply involves the locking up of capital by the supply authority in order to carry heavy idle current. However, the user can arrange to reduce the excitation currents carried by the supply by improving the power factor of his installation.

To enable supply authorities to turn idle capital into revenue-producing capital, they offer a tariff that in effect gives a substantial bonus to those consumers who improve the power factor of their load.

27. How can power factor be improved?

By the installation of static capacitors, by the provision of DC exciters as used in synchronous motors, or by the use of compensated motors on rotary phase-advancers. These provide the necessary excitation current for the motors and thus relieve the supply system of a portion for the whole of the wattless lagging current (see Fig. 6 below).

28. Why can a capacitor be used for improving the power factor?

Introducing inductance into an AC circuit causes the current to reach its maximum value later than the voltage. Introducing capacitance into an AC circuit causes the current to attain its maximum value earlier than the voltage. Therefore, by adding a suitable capacitor to an inductive circuit the time lag can be reduced by any desired amount.

29. Where are capacitors placed in an installation?

The installation of a capacitor has the effect of decreasing the current taken from the supply but does not decrease the excitation current actively circulating round the capacitor-motor circuit.

Theoretically therefore, the best position for a capacitor is as near as possible to the motor, that is directly across the motor terminals, thus allowing the use of a smaller size feeder owing to the reduced current taken from the supply. However, it is not generally an economic arrangement to correct the power factor of each motor in an installation individually, unless they are large motors.

This is because of the relatively high cost per kVA of capacitors in small sizes. It is often desirable to group small motors together for correction with a moderately-sized capacitor whose cost would be much less than a number of small capacitors of the same total kVA. The connective equipment in such cases is installed either in groups distributed about the internal supply system or, in small compact installations in a block connected at on near the point of supply.

When a capacitor is individually connected to a motor it is controlled by the same switch as the motor so that it is always brought into service when required. Group or block connection of capacitors may require some method of control as capacitance must be taken out of service when not required. The best type of control is automatic control.

NB: If capacitors are connected directly across the motor terminals, then the capacitors KVAR must not exceed 85% of the no load KVA of the motor, without reference to the motor manufacturer.

Fig. 6 - Capacitor connections for individual power-factor control of induction motors.
Connection of the capacitors through fuses is preferred for protection and isolation. Fuse rating approximately 50 percent higher than capacitor current. Connections shown across the terminals of the star-delta starter allow power-factor improvement during starting as well as running.

30. How is the insulation resistance of a motor winding checked?

With a 500 volt insulation-resistance tester. The insulation resistance in megohms should not be less than:-

Resistance =	Motor rated voltage
	1000 + rated kVA

31. If insulation resistance is low, what is the most likely cause?

The presence of water in the windings.

32. How can motor windings be dried out?

By placing the armature or stator concerned (not the complete machine) in an oven for about 12 hours at a temperature of 100-120°C. After the windings have cooled, the insulation should be rechecked.

33. What is the purpose of the motor enclosure?

To protect the windings and bearings from the abrasive, destructive or corrosive effects of dust, liquid or gas in the surrounding atmosphere. A motor with inadequate protection will have its life greatly curtailed.

34. What is the effect of dust on the windings?

The ventilation of the motor may be impeded and will tend to overheat. If the dust is abrasive it will become embedded in the insulation and result in short circuits.