Fig. 1. Capacitor Start Induction Motor.

This capacitor is designed for short duty service. The phase displacement between the two phase currents is 90º, so the starting torque developed is more (twice that of split-phase motor). A centrifugal switch is connected in series with the capacitor and the starting winding so that they could be isolated at speeds near the full-load speeds. Because of the presence of capacitor, the starting current could be in phase with the operating supply voltage. The circuit is as shown in figure (1).

Construction of Capacitor Start Induction Motor

The construction of capacitor start induction motor is almost same as that of a split phase induction motor. In this motor capacitor is connected in series with auxiliary or starting winding and are mounted on top of the motor in any convenient external position by means of metal casing, in some cases it may be mounted inside the motor. The capacitor used in this motor provide higher starting torque and limits the starting surge of current to a lower value than developed by the split phase motor.

Working of Capacitor Start Induction Motor

The schematic diagram of capacitor start induction motor is shown in figure 2(a). In this motor an inexpensive and small A.C electrolytic type of capacitor is connected in series with the starting winding or the auxiliary winding. So that the current through the main winding, I_m lags behind the current of starting winding, I_S by an θ ( approximately equal to 90º)  as shown as figure 2(b). This results in high starting torque. The starting torque of a capacitor start induction motor, ranges between 3 to 4.5 times the full-load torque which is twice that of split phase induction motor. A centrifugal switch is connected in series with auxiliary winding and capacitor. The purpose of this switch is to disconnect the capacitor when motor attains 75% of full-load speed. At rated speed motor operates with main winding.

Figure 3.

The speed-torque characteristics of capacitor start induction motor are shown in figure (3). These motors are quite expensive than split phase induction motor because of the addition of capacitor.

Applications of Capacitor Start Induction Motor

1. Due to high starting torque, capacitor start induction motors are used for high inertia loads and also where regular starts are needed.
2. These are also used in applications like pumps, compressors, refrigerators, air-conditioners, conveyors, machine tools etc.

Q1. List out the characteristic features of single-phase capacitor start motor.

Ans: The characteristic features of single-phase capacitor start motors are as follows.

1. Capacitor start motors can be used for dual voltage ratings.
2. They can also be used in applications where starting torque requirement is high.
3. They have two windings i.e., start and run winding. When motor is started both the windings are energized but when motor attains 75% of full load speed, start winding and capacitor are disconnected from the circuit by a centrifugal switch.

Q2. How can the direction of rotation of the capacitor-start motor be reversed?

Ans: The direction of rotation of capacitor start motor can be reversed by interchanging the connections of starting winding without disturbing the capacitor connections. The direction of motor can be reversed only before the starting operation of the motor i.e., when the motor is at rest and the centrifugal switch is in closed position. Because once the motor comes in normal operating condition it will be running as single phase induction motor and at this moment the reversal of motor is attempted, then there will be no effect on the direction of rotation as centrifugal switch will be in open position and the developing torque will be in the direction of rotation.

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Fig. 1. Shaded Pole Induction Motor.

Shaded pole induction motor is the simplest and inexpensive type of motor similar to single-phase induction motor. The stator poles of this motor are wound only with main winding, so as to make them electromagnets. The pole has a slot cut at one third of one end for housing high reactance but low resistance copper bars enclosing a part of the pole. This part is called the shaded part and the other is called the unshaded part. The rotor of such motor is similar to squirrel cage rotor.

Working Principle of Shaded Pole Induction Motor

The required phase-split for making self-start is obtained through induction principle (transformer principle) when a shaded pole motor is supplied with a single-phase A.C supply. This single-phase current produces two pole alternating flux which is as explained follows.

Working of Shaded Pole Induction Motor

Consider a positive half cycle with different time instants i.e t₁, t₂, t₃ as shown in figure (2). Consider one electromagnet in which the current flowing is positive half.

Along Time 0t₁ :

During this time, as the exciting current is increasing, there will be an e.m.f generated in the shaded coil and hence large current is set-up. The current flows in a direction so as to oppose its cause and thus reduces the flux under the shaded pole. So most of the flux is concentrated under the unshaded part, thus moving the magnetic axis as shown in figure 3(1).

Along Time t₁t₂ :

During this time, there is no change in the exciting current and hence the flux will be uniform throughout the pole. This moves the magnetic axis to center of pole as shown in figure 3(ii).

Along Time t₂t₃ :

During this time, the exciting current is decreasing thus, inducing e.m.f in the shaded coil. The currents flowing in the shading coils are in a direction so as to oppose the exciting current and the flux produced will be adding with the main field flux and thus the magnetic axis moves under the shaded pole as shown in figure 3(iii). This motion of magnetic axis from unshaded part to the shaded part (this effect) is similar to that as if the real poles are actually sweeping in space. So, the rotor starts rotating in the direction of magnetic axis (unshaded part to shaded part). As the torque is very small, this type of motors are generally used in toys, hair driers, desk fans, electric clocks etc.

Torque-Speed Characteristics of Shaded Pole Induction Motor

A shaded pole motor is a self start single-phase induction motor. This property of self start is achieved by splitting the phase by induction principle using shaded ring. So, this motor is thereby named as shaded pole motor. From torque-speed characteristics, it is known that with the increase in speed initially the torque increases and then decrease.

Disadvantages of Shaded Pole Induction Motor

The following are the limitations of shaded pole motors.

1. Compact size
2. Less power rating
3. Poor starting torque
4. Less power factor
5. Less efficiency due to I²R and copper losses in the shading ring
6. Complexity in speed reversal.

Applications of Shaded Pole Induction Motor

Shaded pole motors have the characteristics of low starting torque, low power factor, high losses and hence low efficiency. These are preferred for applications requiring low power ratings in the order of 40 W. Some of the applications of shaded pole motor are, relays, table fans, exhaust fans, hair driers, fans for refrigerators, air conditioning equipments, toys, film projectors, photo copying machines, record players, tape recorders, single-phase synchronous timing motors etc.

Q1. What will be the direction of rotation of a shaded pole single phase induction motor?

Whenever the exciting winding of a single phase induction motor is excited by a single phase A.C supply, the magnetic axis will shift from the unshaded part of the pole to the shaded part of the pole. This shifting is similar to the actual rotation of poles. So, the rotor starts rotating in the direction of magnetic axis. Hence, the direction of rotation of a shaded pole single phase induction motor will be from unshaded part to the shaded part.

Q2. State the method of reversal of rotation of shaded pole motor.

There are two common methods of reversing the rotation of shaded pole motor. They are,

1. By using two shaded poles and one main winding
2. By using one shaded pole and two main windings.

Figure shows reversing the direction of rotation of a shaded pole meter.  In the first method, the switch is moved to the other position, connecting the alternate winding and the previous winding which was being used is disconnected. Due to this other side of the main winding is connected to the shaded pole which causes the rotor to move in opposite direction.

In the second method the two main windings are wounded in the slots such that shaded pole is positioned on the opposite side of each winding. As rotor will move towards the shaded pole, this will cause the rotor to rotate in reverse direction.

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Fig. 1. Split phase motor.

The electrical angular displacement between these two windings is 90° electrical as shown in Fig. 1. The currents of both windings differ in value and phase. The current of starting winding I_s is near to the voltage vector because of more resistance and current of main winding I_m is towards lagging side because of more inductance. The main winding is connected across the line where the starting winding only for the starting period. The starting winding is disconnected from the mains by the centrifugal switch.

Fig. 2. Vector diagram.

Working of Split Phase Induction Motor

When the motor is connected across the supply the starting winding because of more resistance draws current at a phase difference near to the voltage vector. The main winding because of more inductance draws current on the lagging side, away from the voltage vector as shown in Fig. 2. There are two currents, resulting two magnetic fields which produces the magnetic field of rotating in nature. Hence the motor starts. As the motor attains its 75% of the synchronous speed, the centrifugal switch disconnect the starting winding and now motor is running on main winding only.

Characteristics of Split Phase Induction Motor

Torque

The starting torque is low. It is about 1.3 to 1.7 the full load torque. The torque increases with the increasing of load as shown in Fig. 21.6.

Current

The starting current is approximately five times the full load current. Without load, current is minimum and increases with the increasing of load.

Speed

The speed at no load is approximately equal to the Synchronous speed and slightly falls down with the increasing of road.

Power factor

The power factor is low i.e. 0.6 to 0.8 lagging.

Efficiency

The efficiency of the motor is low as compared with the three phase induction motors.

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