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STANDARD SPLIT PHASE MOTORS

The stator of a standard split phase motor has two windings viz., a main winding and an auxiliary winding. These two windings have different ratios of resistance to inductive reactance. The windings are connected in parallel across the single phase ac supply. The line current is thus split into two parts; one part flowing through the main winding and the other part flowing through the auxiliary windings. Because of different ratio of resistance to inductive reactance of these two windings current flowing through them will have a time phase difference of 30 electrical degrees or more. In some motors, both the windings are energized continuously, while in most of them, the auxiliary winding is used only during the starting period along with the main winding to develop the required starting torque. When the motor reaches final speed the auxiliary winding is disconnected from the supply. The auxiliary windings can be disconnected by using a centrifugal switch in series with it. During starting, the centrifugal switch remains closed. When the motor reaches normal speed the centrifugal switch opens and disconnects the auxiliary winding. This has been shown in Figure 1(a).
Another method of disconnecting the auxiliary winding when the motor has picked up speed is by using an electromagnetic relay as has been shown in Figure 1(b).
Figure: 1 Use of (a) Centrifugal switch (b) Electro-magnetic relay for connecting an auxiliary winding in parallel with the main winding during starting
During starting the relay picks up due to high starting current and connects the auxiliary winding in the circuit. When the motor reaches normal speed, current drops to normal value and the electromagnetic relay is dropped and therefore the auxiliary winding gets disconnected from supply.

Reversal of direction of rotation of a split phase motor is obtained by interchanging the auxiliary winding terminal connections with respect to the main winding. The motor must be brought to rest or the centrifugal switch must be closed before reversal is attempted. The basic circuit for reversal of small split phase motor using a DPDT (Double Pole Double Throw) switch and contactors has been shown in Figure 2(a and b).
Figure: 2 Reversing of split phase motor
Speed control of standard split phase motors is achieved with difficulty. The usual method is to use two or more windings designed to produce different number of poles. The complications arise as both the main and the auxiliary windings have to be arranged for pole changing. The capacitor type split phase motor which will now be discussed have better possibility of speed adjustments.

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