Electrical Machines - Questions and Answers
Information courtesy of ALSTOM.
1. When are single-phase motors employed?
When the only available alternating-current supply is single-phase. In general, when a three-phase supply is available, three-phase motors are preferred, although exceptions are often made in the case of fractional-kilowatt drives.
2. What type of single-phase motors are in use?
- Induction motors (split-phase, capacitor and shaded-pole).
- Repulsion and repulsion-induction motors.
- Universal motors.
- Unexcited synchronous motors.
3. What is the principle of the single-phase induction motor?
A single-phase winding sets up an alternating magnetic field. Since this field does not rotate, a single-phase winding alone cannot produce torque to start the motor from rest. Once the motor has been started, however, it will continue to run provided the load is not too high.
To start the motor electrically, it is necessary to employ an auxiliary winding in parallel and make the current in this winding differ in phase from the current in the main winding. Simulating a two-phase supply in this way, the combined effect of the fields set up by the two windings is a more or less effective relating field, thus providing a starting torque.
After starting, relation of the field is carried out by currents set up in the secondary winding and the auxiliary primary winding is generally switched out of circuit.
4. How does the single-phase induction motor work?
The auxiliary winding is usually wound with about the same number of turns as the main winding but of much smaller wire. Because of the much higher resistance of the auxiliary winding, the current in it is more in phase with the supply voltage than is the current in the main winding. In practice, a phase difference of about 30 degrees is attained, which is somewhat lower than the ideal value of 90 degrees but sufficient to give ample starting torque against light load.
The auxiliary winding is usually short-time rated - it would overheat if left in circuit for more than a few seconds - so it is cut out of circuit immediately starting is accomplished.
5. What are the applications of the split-phase induction motor?
Fractional-kilowatt drives for appliances that can be brought up to speed quickly. A typical starting torque is 175 to 200 per cent of the full load torque.
6. What is a capacitor motor?
A single-phase induction motor in which the difference in phase between the main and auxiliary windings is effected by connecting a capacitor in series with the auxiliary winding. Because of the capacitor, the auxiliary-phase current approaches 90 degrees in advance of the main-phase current, giving a much higher starting torque than is possible with a split-phase motor.
Fig. 48 - Typical single-phase-motor starting connections.
Capacitor start-run motors give superior running characteristics, and high power factor.
There are three main types of capacitor motor:
- Capacitor-start induction-run (the capacitor winding being in circuit only during the starting period).
- Permanent split-capacitor motor, in which the capacitor winding is in circuit for both starting and running.
- Capacitor start-and-run motor, using two capacitors for starting, one of them being cut out for running.
7. When is the capacitor-start motor used?
When really high starting torque is necessary, as for example, motors driving refrigerator compressors. A starting torque of 300 per cent or more can be obtained.
8. What type of capacitor is employed for capacitor-start motors?
Generally a dry-type electrolytic capacitor because it has large value of capacitance in small bulk and is the cheapest. This type of capacitor is short-time rated for AC working and is therefore suitable only for starting applications where the duty is intermittent.
Fig. 49 - Windings of split-phase motor.
As the voltage across starting capacitors can be higher than the mains voltage their voltage rating must be suitable, e.g. 275 volt rating for 200-250-volt motors, when the capacitor is connected in the usual way in series with the auxiliary winding. Figure 50 shows a method of connecting a capacitor to allow its working-voltage rating to be lower than mains voltage.
9. What are the applications of the permanent split-capacitor motor?
For small motors with light starting duties, such us fan drives and coil burners. Only a small capacitor is required and this is left in circuit to avoid the complication of the extra switch. The fact that the auxiliary winding is in circuit whilst running improves the machine performance considerably in respect of power factor and quiet running. A paper-dielectric capacitor must be used, because the electrolytic capacitor is not suitable for continuous operation.
10. What is the object of using two capacitors?
A capacitor-start capacitor-run motor can be employed when the starting duty is severe and it is desired to achieve a high power-factor when running. A smaller capacitance is required for running than for starting, so two capacitors are used in parallel for starting and one capacitor is cut out for running. Special care in the design is necessary when capacitors are run in parallel, as a high surge voltage can take place on switching out.
11. How is the single-phase induction motor switched from start to run?
Switching is usually done automatically by centrifugal switch mounted on the motor shaft, but it can also be done by hand switching by responsible operators. An alternative to the use of a centrifugal switch is a specially-designed relay switch which controls the starting winding. The relay coil is in series with the running winding so that the heavy current passing through the coil when switching on with the rotor at rest causes the relay contacts to close the circuit of the starting winding. As motor speed rises, the current in the running winding and therefore in the relay coil falls and at a predetermined current value the relay contacts are released, so disconnecting the starting winding.
Fig. 50 - Capacitor-motor tee connection.
The diagram also shows connections for a reversing switch.
12. How is a split-phase or capacitor induction motor reversed?
By changing over the ends of the starting winding at the motor.
13. What is a shaded-pole motor?
A type of single-phase induction motor for very small powers in which a starting torque is provided by permanently short-circuited coils displaced in position from the main stator coils. The rotor is squirrel-cage. The most usual form of a motor of this type has salient poles on the stator somewhat similar to the poles of a universal motor. Each pole is unequally divided by a slot cut in the laminations allowing one side of the pole to be surrounded by a heavy copper band, known as a shading coil or loop.
Other forms of shaded-pole mater have the normal induction motor stator, its windings consisting of single-phase winding and a shading winding, the latter being wave-wound and short-circuited inside the motor or at its terminals.
Fig. 51 - Shaded-pole motor.
14. How does the shaded-pole motor operate?
The shading loop acts as a short-circuited low-resistance coil in which is induced by transformer action a heavy current lagging in time phase with the main flux in the pole. The current in this loop produces a flux which lags in time the main flux in the unshaded portion of the pole. The resultant effect is a movement of flux across the pole face and air gap, thereby producing a magnetic field which cuts the rotor conductors.
15. What are the applications of the shaded-pole motor?
This type of motor requires no automatic centrifugal or other type of starting switch since there is no starting winding to be switched out of circuit. Its simple construction makes it a particularly robust machine suitable for long-hour duty. It is not, however, as efficient electrically as other single-phase induction motors mainly on account of the fairly-heavy copper-losses in the pole-shading loops. Usual sizes of such metres are up to about 25 W and they are used where efficiency is of little importance, such as for driving small fans, motorised valves, recording instruments, record players, etc.
16. Can the shaded-pole motor be reversed?
The motor is normally not reversible since this would involve mechanical dismantling and reassembly. Special machines are made comprising two rotors on a common shaft, each having its own stator assembled for opposite directions of rotation.
Fig. 52 - Reversal of universal motor.
The connections to the armature windings are changed over to reverse the motor.
17. What is a universal motor?
A motor in the fractional kilowatt range of about 10 to 400W constructed on similar lines to a series-type DC motor adapted to AC by having a completely-laminated field core. They cannot satisfactorily be made to run at less than about 2000 r/min.
If similar performance on AC and DC is required from motor running at less than about 3000 r/min, a tapped field winding is desirable.
18. How are universal motors started?
By switching straight on to the line. Starting torques of up to about five times full-load torque with starting currents of about three to four times full-load current, depending upon the size and speed of the motor, are usual.
19. What are the applications of universal motors?
Universal motors have a series characteristic so that they run at their rated speed only on the rated load. If the load is reduced the speed will rise. Such motors are suitable for driving fans, vacuum cleaners, domestic sewing machines, portable tools, etc., where the load is constant, or where a steadily-maintained speed is not important.
As brush wear takes place more rapidly in universal motors they are not generally considered suitable for long-hour duty. Speed control and reversing can be arranged as for DC motors. Three-lead reversing, which greatly simplifies the control gear, can be arranged, two separate fields being provided, one for each direction of rotation.
20. What are repulsion motors?
Single-phase machines made up to about 4kW having a stator wound with a single-phase winding and a rotor very similar to a DC armature with commutator. The brushes are permanently short-circuited. Currents induced in the rotor by the magnetic field from the stator give the rotor a magnetic polarity that, with suitable brush position on the commutator causes repulsion to take place between like poles of the stator and rater.
No rotation results if the brush axis corresponds with the axis of the stator winding, called the neutral position, the magnetic polarity of the rotor then being the same as that of the stator. In this form, the machine is a plain repulsion motor having series characteristics, with which speed rises as the load on the machine is reduced.
Fig. 53 - Repulsion-motor connections.
The stator is wound with a single phase winding. The rotor is very similar to a DC motor armature with commutator.
The machine is often converted into an induction motor during the period of running by arranging that all the commutator segments are short-circuited by a centrifugally-operated device when the motor is up to speed. The brushes are also lifted in same cases to reduce wear.
To avoid the complication of the short-circuiting device, the rotor may be arranged with a squirrel-cage winding at the bottom of the slots. This takes over at speed and gives induction-motor characteristics.
Fig. 54 - Two methods of reversing repulsion-motor direction of rotation by switching.
21. How are repulsion motors reversed?
To get reversed rotation, the brushes are moved round to a corresponding point on the other side of the neutral position. When it is desired to avoid having to move the brushes, two sets of brushes may be used, one set for each rotation and to short-circuit them as required. Another method of avoiding brush movement is by tapping the stator winding and changing the direction of rotation by shifting the axis of the stator poles. This is usually done by one reversing winding in addition to the main winding and a change-over switch, as shown in the diagram.
What are the application of repulsion motors?
The repulsion motor is suitable for drives requiring very high starting torques, although it has been replaced to a large extent by the capacitor motor. Where variable speed is required, a plain repulsion motor (without the short-circuiting and brush-lifting mechanism) can be used, the speed control being obtainable by racking the brushes which may be connected to a hand-wheel or lever on the motor end-bracket.
Fig. 55 - Connections of shunt, series and compound-wound motors.
C.P., commutating poles, or interpoles, are not fitted on the smallest motors.