Shaded Pole Motor

Shaded Pole Motor

A shaded pole motor or shaded pole induction motor is a type of single-phase induction motor usually used for the lower ratings.

It is a simply self-starting single-phase induction motor with a pole shaded by a copper ring. The copper ring is known as the shaded ring.

The shaded ring acts as an auxiliary winding (secondary winding). This motor rotates only in one direction and it is not possible to rotate in a reverse direction.

The shaded pole induction motor is not used for a higher rating, because of the following reasons;

  • Low power factor
  • High losses
  • Low starting torque
  • Poor efficiency

Therefore, the shaded pole motor is only suitable for low power ratings (0.05 HP ~ 40 W).

Construction

A shaded pole motor has two or four poles. Similar to the construction of an induction motor, this motor also has a stator and rotor. The below figure shows a schematic diagram of the construction of a shaded pole motor.

Shaded Pole Motor
Shaded Pole Motor

Stator

In most cases, the stator has two or four poles and the poles are salient poles. The salient poles are the poles in which the magnet is projected towards the armature of the motor.

Each pole is excited by its own exciting coil. A portion of each pole is wrapped by a copper ring or copper coil. This ring makes a close loop across the pole and it is known as a shading ring or shading coil.

The poles are laminated with multiple layers. Hence, it increases the strength of the poles.

A slot is constructed across the laminated pole at an approximate one-third distance from the edge of the pole. The short-circuited shading ring is placed in this slot.

Part of the pole that is covered with the shading ring is known as the shaded part and the remaining part is known as the unshaded part.

Rotor

The rotor of a shaded pole motor is the same as the rotor of a squirrel cage induction motor. The rotor slots are skewed at an angle of 60˚. This type of construction is preferable for good starting torque. And also, it avoids magnetic locking between stator and rotor.

The rotor bars are placed in rotor slots. The construction of this motor is simple as it does not contain any commutator, brushes, contactor, capacitor, collector ring, or moving part. And it makes this motor cheaper and simpler.

Between stator and rotor, an air gap of 0.25 to 0.5 mm is provided. Less than this airgap may result in starting torque variations due to rotor slotting.

The construction of a shaded pole motor is rugged and reliable. And this motor does not require any centrifugal switch which reduces the chances of failure.

Working of Shaded Pole Motor

When a single-phase supply is given to the motor, an alternating flux is generated. A fraction amount of flux is linked with the shading ring.

Due to the variation in flux linking, there are some amounts of voltage are induced. As the ring is short-circuited, the circulating current flows through it.

Because of the circulating current, some flux generates in the shading ring. And the direction of this flux is opposite compared to the main flux.

Hence, the main flux is as opposed to the flux generated in the shading ring. Therefore, it creates a phase difference between the flux generated in the shaded portion and the unshaded portion.

Also, there is a space displacement (less than 90 degrees) between the shaded ring flux and the main flux.

A rotating magnetic field is produced by the space displacement that leads to a torque on the cage motor.

In this condition, the reversal of motor is not possible. To make it possible, we need to use two shading rings.

Losses and Efficiency

The magnetic flux induced in the unshaded portion increases with an increase in current flowing through its winding. Also, the magnetic flux increases in the shaded portion.

But, the magnetic flux in the shaded portion is delayed by the current induced in the winding.

From the unshaded portion to the shaded portion, the magnetic field sweeps across the pole face and develops torque in the squirrel cage.

The rotor is made up of relatively high resistance to maximize the torque.

This type of motor is used for low torque applications like fans. And generally, this motor is suitable for less than ¼ HP power rating.

Follow the below steps to find the efficiency of the shaded pole motor.

Step-1 Calculation of rotational losses

The rotational loss (friction and windage loss) is equal to the difference between the input power at no-load and the stator copper loss. Hence,

    \[ P_{fw} = P_{NL}-I_{NL}^2 R_{dc} \]

Step-2 Calculate stator copper loss at full load

At full load the stator copper loss is;

    \[ P_{scu} = I_{FL}^2 R_{dc} \]

Step-3 Calculate the slip

The slip of motor can be calculated from the below relation;

    \[ N_s = \frac{120f}{P} \]

Step-4 Calculate rotor copper loss at full load

The rotor copper loss is equal to the power transferred across the air gap multiplied by slip. The power transferred across the air gap is similar to the difference between input full-load power and the stator copper loss.

    \[ P_{rcu} = (P_{FL} - P_{scu}) \times slip \]

Step-5 Calculate total losses

Total losses = Stator copper loss + Rotor copper loss + Rotational loss

Step-6 Calculate efficiency

The efficiency is calculated by;

    \[ \eta = \frac{Input - Total \, loss}{input} \times 100 % \]

Applications

The shaded pole motor is used in;

  • Exhaust fan
  • Table fan
  • Air conditioner
  • Hair dryer
  • Tape recorder
  • Photo coping machine
  • Relays and other small devices

Advantages

The advantages of a shaded pole motor are listed below;

  • Low cost
  • Self-stating motor
  • Simple construction
  • Robust in nature
  • Reliability

Disadvantages

The disadvantages of a shaded pole motor are listed below;

  • Low power factor
  • Poor efficiency
  • Speed reversal is difficult
  • It requires a copper ring which increases the cost.
  • Low starting torque
  • High losses

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