What is a Shaded Pole Motor : Working & Its Applications

A large number of motors are manufactured with small ratings for the operation of a single-phase AC supply. The motors that are built with fractional horsepower sizes are called small motors. They are most useful in home, offices, business, and many more. Since the requirements of the applications differ widely and manufacturing industries develop several types of motors. In such a way that their operating characters meet demands. Single-phase AC motors are cheaper in cost, more reliable, simple in construction, and very easy to repair. These are widely used in fans, cleaners, refrigerators, blowers, washing machines, etc. Single-phase AC motors are divided into three types. They are single-phase induction motor, commutator type single phase motor, single-phase synchronous motor. Single-phase induction motors are further divided into four types- split-phase motor, shaded pole induction motor, reluctance start induction motor, and repulsion starts induction motor. This article describes a shaded pole motor and its working.

What is Shaded Pole Motor?

Definition: A shaded pole induction motor is a simple single-phase induction motor, which is self-starting with one of the poles shaded by the copper ring. The other name of the copper ring is a shaded ring, where it acts as a secondary winding motor. It rotates only in one direction particularly and reverses moment is impossible. This motor has very high power induction losses and also has a very low power factor. Starting torque induced in the motor is very low. Due to these reasons, it has poor efficiency. This, it has low power ratings. It is also a salient pole split phase motor.

It has two poles as shown in the basic construction. This motor is made up of stator and a rotor which is cage type. The stator has projected poles also called main poles in it. The supply winding on the main poles forms the main winding. The poles in this motor are unequally divided into two halves where the smaller portion is shaded portion that carries a copper band. Copper ring, which is a single turn is fitted on the smaller part. This ring is also known as a shading coil. The shading coil fitted on the main pole is called shading pole.

Working Principle of Shaded Pole Motor

When power is supplied to the stator, flux is induced in the main part of the pole. This flux induces a voltage in the shading coil. This acts as a secondary winding. By Lenz’s law, the current direction should be in such a way that, is opposing the flux entering into the coil. This acts as a secondary winding of a transformer.

Working of Shaded Pole Motor

In the core, when a single phase is applied an alternating flux is generated. This flux links with the shaded coil in fraction amounts. Then voltage gets induced in the coil due to the variation in the flux linking. Hence, the shaded portion is short-circuited due to which it produces the circulating current in it. In such a way, the direction is opposing the main flux.

The main core flux is opposed by the flux in the ring that is developed by the circulating current. Hence, flux is induced in the shaded portion of the motor along with the unshaded portion with a phase difference, which is lagging behind the unshaded pole flux. There is also a space displacement that is less than 90 degrees between a shaded ring flux and the main

motor flux. Due to this space displacement, a rotating magnetic field is produced which leads to a torque on the cage motor. In order to obtain reversal in the direction of rotation, we have to provide two shading coils.

The Efficiency of Shaded Pole Motor

This motor develops a rotating field with a delay of magnetic flux through the structure of the pole. To isolate the shaded portion of the pole, a copper conductor is used to the rest of the pole which forms a single turn around the pole. As the current increases, the magnetic flux in the unshaded portion increases through the winding. This results in an increase in the magnetic flux of the shaded portion and delayed by the current induced in the copper.

The torque of this motor is very low and mostly less than 1/4HP. In order to increase the torque, a rotor with high resistance is used. Since the efficiency of this motor is very low and used in applications where it operates in very short periods.

Steps to Calculate the Losses and Efficiency of 4-pole Shaded Pole Motor

Step 1:  Calculation of the Rotational Losses

Here, the rotational losses of the friction and windage losses are equal to the input power, which is less than the stator and copper losses. The resistance of the stator is measured in DC. Only a 10-30 percent difference between the increase of resistance to AC and DC.
The rotational losses at full load are,

P_fw= P_NL – I_NL²(R_dc)

Where Rdc = Correction factor of ac to dc

Step 2:  Calculation of Stator and  Copper Loss at Full Load

The stator-copper loss at full load is,

P_scu = I_FL²(R_dc)

Step 3: Calculation of Slip

Consider the synchronous speed of 4-pole shaded pole motor,

n= 120 f/p

Where f= frequency in Hertz

P= no.of poles.

Step 4: Calculation of Rotor and Copper Loss at Full Load

In this motor, the rotor-copper loss can be obtained by multiplying the slip with the power transferred across the air gap

Power transferred across the air gap At full load = input power – stator and copper loss

Step 5: Calculate the Full-load Losses

Total losses = stator-copper loss + Rotor-copper loss + Friction and windage losses

Step 6: Calculation of Efficiency

The efficiency of the shaded pole motor can be obtained by,

η = (input –total losses)(100%)/input

Characteristics

The shaded pole motor characteristics include the following.

• It produces starting torque that equals to half of the torque on full load
• Efficiency is low due to power loss in the shading coil.
• Used in small devices like fans
• Depending on the shaded coil position, the direction of rotation depends.

Advantages of Shaded Pole Motor

• Low cost,
• Capable of self-starting
• Simple in construction
• Robust in nature
• Reliability

Disadvantages of Shaded Pole Motor

• Very low starting torque
• Low power factor
• High losses
• Less efficiency
• Difficult in speed reversal as it requires expensive copper rings

Applications

The applications of the shaded pole motor include the following.

• Relays, Fans and other small devices due to its low cost
• Exhaust fans
• Hairdryers
• Table fans
• Cooling fans
• Refrigerators
• Air conditioners
• Projectors
• Record players
• Tape recorders
• Photocopying machines and many more.

FAQs

1). How do I identify a shaded pole motor?

This motor can be identified by using an auxiliary winding which is made up of a copper ring called a shading coil.

2). Does a shaded pole motor use a capacitor?

They don’t use capacitors as it has a small part of the wire around each pole.

3). Can a shaded pole motor be reversed?

Yes, by the field reversing, the reversal in rotation can be obtained.

4). What is the difference between shaded pole and PSC motors?

The shaded type has old design and old technology whereas PSC motor uses new technology.

5). How do you control the speed of a shaded pole motor?

By varying the voltage using dimmer, the speed of this motor can be controlled.

Thus, this is all about the shaded pole motor, working, diagram, working principle, characteristics, efficiency, advantages, disadvantages, and applications. Here is a question for you, ” What is the working principle of PSC motor?”