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Wednesday, 13 November 2019

Split Phase Motor

SPLIT PHASE MOTOR


1. Capacitor Start Capacitor Run Motor :

In this type the stator is wound with two sets of a coils displaced by 900 electrical. The main or running winding has low resistance and high inductance and is connected across the supply. The starting or auxiliary winding is connected across the supply through a capacitor as shown in Fig. I f the capacitor is connected even while running then it is called capacitor start capacitor run motor and if during running it is cut off with the help of a centrifugal switch, it is called capacitor start induction run motor. Almost all ceiling fans are capacitor start capacitor run motors and most of the single phase pump are capacitor start induction run motors.

Since the running winding is inductive and the starting winding is capacitive, the two currents have a phase difference between them as shown in the vector diagram (See Figure). These two currents Is and lm produce a revolving flux as in the case of 3 phase induction motor due to which the torque produced in the rotor gives the direction. Hence the motor is self starting. Since the capacitor remains permanently in the circuit, motor, is sometimes referred as Permanent split capacitor run motor behaves practically like a unbalanced 2 phase motor. The capacitance usually used are 2 to 20 micro farads because of continuous duty rating which results in small starting torque of about 50 to 100 % of the rated value. Refer to Fig which gives the relation between percentage speed and percentage full load torque. These motors are used where starting torque is low such as fans, voltage regulators and oil burners. These motors are quite in operation and trouble free.


The advantage of keeping the capacitor continuously in the circuit are :

a. It improves the overload capacity of the motor
b. works on high power factor
c. has higher efficiency
d. runs smoothly

2. Capacitor Start Induction Run Motors :

In the capacitor start induction run motor, the capacitor serves short duty' service and gets cut-off when the motor reaches 73% of full load speed. For this purpose a centrifugal switch is incorporated in series with the starting winding. Usually electrolytic capacitors are used. In such type the main winding is designed to carry WI load current. Fig shows the connection diagram of a capacitor start induction run motor which uses a switch for disconnecting the capacitor from the circuit while Fig (b) shows the diagram of the centrifugal switch.
The necessary phase difference between the two torques lire and Is is provided by connecting a capacitor in series with the starting winding. The capacitor is so designed that it is capable of operating 20 operations per hour not exceed 3 seconds per operation. The phasor diagram of the currents is shown in Fig.

The current due to the main winding lags the voltage V by a large angle whereas the current due to starting winding leads the V by certain angle. Thus the phase difference between the two currents is large of about 80o thus increasing the value of torque produced.

TYPES OF CAPACITOR START MOTORS

Some important types of capacitor start motors which are manufactured are dealt below.

1. Single Voltage External Reversible Type:

In this type of motor, four leads are brought out, two from each of the main and Starting winding. The starting winding is connected in series with the electrolyte capacitor and a centrifugal switch and for reversing the direction of rotation, the leads of the starting winding is reversed with respect to the running winding with the help of a switch.

2. Single Voltage Non-Reversible with Thermostat :

Thermostat provides protection against overload and short circuits. When overload occurs, the bi-metallic contacts of the thermostat opens and thus disconnects the motor from the supply. The contacts gets reconnect again when the bi-metal cools.

3. Single Voltage Non Reversible with Magnetic Switch :

Where the use of centrifugal switch is not possible, and if direction of rotation is required, a plunger type magnetic switch is used.

4. Two Voltage Non Reversible Type:

These motors are designed to operate on two voltages such as 110V and 220V . Two main windings are provided separately or one main winding in two sections are provided. One starting winding with suitable number of leads are brought out to permit change over from one voltage to another. When the motor is to operate from lower voltage, the two main windings are connected in parallel and for higher voltage, the main windings are connected in series.

5. Two Voltage Reversible Type :

In this type, two additional leads are brought out from the starting winding for reversing the direction of rotation externally.


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