What Is Shaded Pole Motor?
A shaded pole induction motors are a simple single-phase induction motor that self-starts with one of the poles shaded by a copper ring. Another name for a copper ring is a shaded ring, where it serves as the motor’s secondary winding. It specifically only rotates in one direction, and it is impossible to reverse the moment. This motor has very high power inductance loss and also has a very low power factor.
The starting torque induced in the motor is very low. For these reasons, it has poor efficiency. This is its low power rating. It is also a major pole split phase motor.
It has two poles, as shown in the original construction. This motor is made up of a stator and a rotor which is of cage type. The stator has projecting poles which are also called main poles. The main winding is formed from the supply winding on the main pole.
In this motor, the poles are divided unevenly into two parts where the smaller part is the shaded part which has a copper band. A copper ring, which is a single turn, is mounted on the small part. This ring is also known as the shading coil. The shedding coil mounted on the main pole is called a shedding pole.
Shaded Pole Induction Motors:
A shaded pole induction motor is simply a self-starting single-phase induction motor whose one pole is shaded by a copper ring. A copper ring is also called a shaded ring.
These coppers ring acts as a secondary winding for the motor. The shaded pole motors rotate only in a particular direction, and the reverse motion of the motor is not possible.
Working Principle of Shaded Pole Induction Motors:
The working principle of the shaded pole induction motors is simple, the main winding carries alternating current and generates magnetic flux in the stator coil. Uniform magnetic flux cut by a shaded band of copper. This will induce EMF in the copper band.
This EMF conducts current, and this current generates its own magnetic flux in the shaded band. According to the lens law, the flux in the shade band opposes the main stator flux. This causes a slight magnetic imbalance and generates a rotating magnetic field.
Characteristics of Shaded Pole Induction Motors:
Some of the important features of shaded pole induction motors are given below.
- The speed of a shaded pole induction motor is inversely proportional to the number of poles used in the motor.
- The shaded pole induction motor produces very small starting torque, about 50% of full load torque.
- The efficiency of the shed pole motor is low due to constant power loss in the shedding coil.
- It is suitable for small equipment that requires low starting torque.
- The direction of rotation of the shaded pole motor depends on the position of the shading coil, i.e., half of the pole is wrapped around the shading coil. Therefore, the direction of rotation cannot be reversed unless the machine is constructed in such a way that the shedding coil can be transferred to the other half of the pole.
Working of Shaded Pole Induction Motor:
The operation of motors can be understood in terms of the figure, which shows one pole of motors with a shading coil. We will explain the working of a shaded-pole motor, keeping in mind the cycle of the alternating current being applied to the stator winding.
#1. During Portion OA
During the part OA of the alternating current cycle, the flux starts to increase, and an e.m.f. The shading is induced in the coil. The resultant current in the shading coil will be in such a direction (Lenz’s law) so as to oppose the change inflow.
Thus in the shaded part of the pole, the flux becomes weak, while in the unshaded part, it is strong, as shown.
#.2 During Portion AB
During part AB of the alternating current cycle, the flux has almost reached its maximum value and is not changing. As a result, the flux distribution at the pole is uniform because no current is flowing in the shading coil.
#3. During Portion BC
As the flux decreases part BC of the alternating current cycle, a current is induced in the shedding coil to oppose the decrease in current. Thus, the flux shown in the shaded part of the pole is strong, while the flux shown in the unshaded part is weak.
The effect of the shading coil causes the field current to move from the pole front to the unshaded part. This shifting flux is like a weak rotating field moving from the unshaded part to the shaded part of the pole. The rotor is of squirrel-cage type and is under the influence of this dynamic field.
As a result, a small starting torque develops. As this torque begins to spin the rotor, additional torque is generated by the single-phase induction-motor action. The motor accelerates slightly less than the synchronous speed and operates as a single-phase induction motor.
Construction of Shaded Pole Induction:
A shaded pole motor can be of two poles or four poles. Here we are considering two poles shaded pole motors. The figures in these articles also show a two-pole motor.
The stator has main poles. Usually, 2 to 4 poles are used. Each pole has its own exciting coil.
A part of each pillar is wrapped with a copper coil. The copper wire forms a closed loop at each pole. This loop is known as the shedding coil. The poles are laminated. Slots are cut across the lamination of the pole.
The slots are about a third of the distance from the edge of the pole. The short-circuit copper coil described above is placed in this slot. So we can call this part a shaded part and other parts of a pole an unshaded part. Selecting a two-pole stator gives a synchronous speed of 3000 rpm, whereas, for a 50 Hz supply, four poled stator speeds will be 1500 rpm.
The rotor of shaded pole inductions motors is a squirrel cage-type rotor. Rotors bars are provided with a 60-degree slant. This is to achieve an optimum starting torque and limit the torque dip during the run-up.
Applications of Shaded Pole Induction Motor:
Various applications of shaded pole motor are as follows:
- They are suitable for small types of equipment such as relays and fans due to their low cost and easy starting.
- They are also used in exhaust fans, hairdryers, and table fans.
- They are used in air conditioning and refrigeration equipment, and cooling fans.
- Record player, tape recorder, projector, photocopy machine.
- They are used for starting electronic clocks & single-phase synchronous timing motors.