- In a, split-phase induction motor the stator is provided with two ,parallel windings displaced 90 electrical degrees in space and somewhat less than 90° in time. Fig. 3 (a) shows the winding diagram of the two windings ofa split-phase induction motor.
The starting winding has fewer turns and is wound of smaller diameter copper than the running winding. The starting winding, therefore, has a high resistance and low reactance.
The running or main, winding (heavier wire of more turns) has a low resistance and high reactance. Because of its lower impedance, the current in the running winding I, is higher than the current in the starting winding, Is.
The phase relations of the lock-motor currents at the instant of starting are shown in Fig. 3 (b). The starting winding Is lags the supply voltage by about 15°, while the greater running winding current lags the single-phase voltage by about 40°. Despite the fact that the current in the two space-quadrature windings are not equal, the quadrature components are practically equal.
If the windings are displaced by 90° in space, and if their quadrature current
components, which are displaced by 90° in time, are practically equal, an equivalent two-phase rotating field is produced at starting which develops sufficient starting torque to accelerate the rotor, in the direction of the rotating field produced by the currents.
- As the motor speeds up, the torque developed increases. Above 85 per cent of synchronous speed, the torque developed by the running winding (main winding) alone is actually greater than that developed by both windings, and it might be advantageous to open the auxiliary circuit at this cross over point. To allow for individual variations among motors and switches, however, the contacts are usually designed to .open at 75 per cent of synchronous speed. This does not seriously affect the operation, because the running (or main) winding alone usually develops approximately 200 per cent of full-load torque at this speed.
- The starting winding is not designed for continuous operation, and care should be exercised that it does not remain connected to the supply after it should have been disconnected by the switch. This series switch is usually centrifugally operated, and is rather inexpensive.
In case of a hermatically sealed motor, the switch is magnetically operated, and is opened in the de-energized condition.
- Split-phase induction motors may be reversed by reversing the line connections of either the main or the auxiliary winding. If however, reversal is attempted under normal running condition, nothing will happen.
If it is necessary to reverse the motor while it is rotating, then some means must be incorporated to slow the motor down to the speed where the starting-switch contacts close, placing the starting winding across the supply lines. This may be done by incorporating a timing device which first disconnects the motor entirely from the line and then reverses one field at the proper time. A mechanical braking device which can be electrically operated may also be used.
- Speed control of split-phase windings is a relatively difficult matter since the synchronous speed of the rotating stator flux is determined by the frequency and number of poles developed in the running stator winding ( Ns =120f/p) .. By adding stator windings to change the number of poles, speed variation maybe obtained. This, however, is a stepped speed change, as in polyphase induction motor, rather than a continuous variation. It must be pointed out, however, that all speed changes must be accomplished in a range
above that at which the centrifugal switch operates.
Fig. 3 (c) shows the typical torque speed characteristics:
- The starting torque is 1.5 times to twice the full-load starting torque and starting current is 6 to 8 times full-load current.
- The speed regulation is very good.
- The percent slip is about 4-6 percent.
- Such a motor may operate with a power factor of 0.55-0.65 and efficiency of 60-65 percent.
- These motors are made in fractional kW ( 1/20 to 1/4 kW ) ratings with speed ranging from 2875 to 700 r.p.m.
Shortcomings and uses. The major objections to the motor are (1) its low starting torque; and (2) that, when heavily loaded, the slip exceeds 5 percent, reducing the e.m.f, and producing an elliptical or pulsating torque which makes the motor somewhat annoyingly noisy. For this reason, the split-phase motor is used in appliances to drive loads which are themselves noisy: oil burners, machine tools, grinders, dish washers, washing machines, air blowers and air compressors.
- Because of low starting torque these motor~ are seldom employed in sizes larger than ¼ kW.