Shaded Pole Motor: Construction, Working Principle, Characteristics and Applications

What is a Shaded Pole Motor?

A shaded pole motor is a simple, self-starting, single-phase induction motor. It consists of a stator with salient poles, each having a copper ring called a shading coil around a small portion of the pole. The shading coil acts as a short-circuited secondary winding. It produces a phase difference between the magnetic flux in the shaded and unshaded portions of the pole. This creates a weak rotating magnetic field that produces starting torque. A shaded pole motor generally rotates in one direction. Its starting torque, power factor and efficiency are relatively low. Therefore, it is mainly used in small electrical appliances and low-power applications.

Construction of a Shaded Pole Motor

A shaded pole motor generally consists of a stator and a squirrel-cage rotor. The stator has salient poles, also known as projected poles, which carry the main winding.

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Each main pole is divided into two unequal sections:

• Unshaded Portion: The larger section of the pole that does not contain a shading coil.
• Shaded Portion: The smaller section of the pole surrounded by a single-turn copper ring called the shading coil.

The shading coil is made of copper and is short-circuited. It acts like a secondary winding of a transformer and helps produce the phase difference required for motor starting. The rotor is generally of the squirrel-cage type and consists of conducting bars connected by end rings.

Working Principle of a Shaded Pole Motor

A shaded pole motor works on the principle of electromagnetic induction. When a single-phase AC supply is applied to the stator winding, an alternating magnetic flux is produced in the pole. This changing magnetic flux links with the copper shading coil and induces an electromotive force (EMF) in it. Since the shading coil is short-circuited, a circulating current flows through it.

According to Lenz’s law, the induced current produces a magnetic flux that opposes the change in the original magnetic flux. As a result, the magnetic flux in the shaded portion lags behind the flux in the unshaded portion. This creates a phase difference between the two fluxes. The magnetic field appears to move across the pole face from the unshaded portion towards the shaded portion. This produces a weak rotating magnetic field that induces current in the squirrel-cage rotor and develops torque. Once the rotor starts rotating, it continues to operate in the same direction under normal conditions.

Working of a Shaded Pole Motor

The working of a shaded pole motor can be explained in the following steps:

1. When the AC supply is applied to the stator winding, an alternating magnetic flux is produced.
2. A portion of this magnetic flux links with the shading coil.
3. The changing flux induces an EMF in the short-circuited shading coil.
4. A circulating current flows through the shading coil.
5. This current produces a magnetic flux that causes the flux in the shaded portion to lag behind the unshaded portion.
6. The resulting phase difference produces a sweeping magnetic field across the pole face.
7. This field induces current in the rotor and develops starting torque.

The direction of rotation depends on the position of the shading coil. In a conventional shaded pole motor, reversing the direction is difficult because the shading coil is fixed around the pole.

Efficiency of a Shaded Pole Motor

The efficiency of a shaded pole motor is generally low compared with other types of induction motors. This is mainly due to the losses in the shading coil, stator winding and rotor. The shading coil produces the phase difference required for starting, but it also generates additional copper losses. The motor also experiences friction, windage and core losses.

The efficiency of a motor can be calculated using:

η = (Output Power / Input Power) × 100%

Alternatively, when total losses are known:

η = [(Input Power – Total Losses) / Input Power] × 100%

Calculation of Losses and Efficiency of a 4-Pole Shaded Pole Motor

The losses and efficiency of a four-pole shaded pole motor can be estimated by following these steps.

Step 1: Calculation of Rotational Losses

Rotational losses mainly include friction and windage losses. At no load, these losses can be estimated by subtracting the stator copper loss from the no-load input power.

Pfw = PNL – INL²R

Where:

• Pfw = Friction and windage losses
• PNL = No-load input power
• INL = No-load current
• R = Corrected stator winding resistance

The resistance used should account for the difference between DC resistance and the effective AC resistance at the operating frequency.

Step 2: Calculation of Stator Copper Loss at Full Load

The stator copper loss at full load is calculated using:

Pscu = IFL²R

Where:

• Pscu = Stator copper loss
• IFL = Full-load current
• R = Effective stator winding resistance

Step 3: Calculation of Slip

The synchronous speed of a four-pole shaded pole motor is calculated using:

Ns = 120f/P

Where:

• Ns = Synchronous speed in RPM
• f = Supply frequency in hertz (Hz)
• P = Number of poles

For a four-pole motor, P = 4.

The slip is calculated using:

s = (Ns – Nr)/Ns

Where Nr is the actual rotor speed in RPM.

Step 4: Calculation of Rotor Copper Loss at Full Load

The rotor copper loss is calculated using the slip and air-gap power.

Rotor Copper Loss = s × Air-Gap Power

The air-gap power is the power transferred from the stator to the rotor through the magnetic field.

Step 5: Calculation of Total Full-Load Losses

The total losses are calculated by adding the major losses in the motor:

Total Losses = Stator Copper Loss + Rotor Copper Loss + Friction and Windage Losses + Other Losses

Other losses may include core losses and additional stray-load losses, depending on the available test data.

Step 6: Calculation of Efficiency

The efficiency of the shaded pole motor is calculated using:

η = [(Input Power – Total Losses) / Input Power] × 100%

Characteristics of a Shaded Pole Motor

The main characteristics of a shaded pole motor are:

• It is a self-starting single-phase induction motor.
• It has low starting torque.
• Its efficiency and power factor are relatively low.
• It generally has a low power rating.
• It uses a squirrel-cage rotor.
• Its direction of rotation is determined by the position of the shading coil.
• It is suitable for small appliances and light-duty applications.

Advantages of a Shaded Pole Motor

• Low manufacturing cost.
• Simple and compact construction.
• Self-starting operation.
• Reliable performance.
• Requires very little maintenance.
• Does not require a separate starting winding or capacitor.

Disadvantages of a Shaded Pole Motor

• Low starting torque.
• Low power factor.
• Low efficiency due to additional losses.
• Limited power output.
• Direction reversal is difficult.
• Not suitable for heavy-load applications.

Applications of a Shaded Pole Motor

Shaded pole motors are commonly used in small appliances and equipment that require low starting torque and low power. Some important applications include:

• Exhaust fans
• Table fans
• Cooling fans
• Small blowers
• Hair dryers
• Refrigerator cooling fans
• Projectors
• Record players
• Tape players
• Photocopiers
• Small ventilation systems

Conclusion

A shaded pole motor is a simple and economical single-phase induction motor. It uses a copper shading coil to create a phase difference between the magnetic fluxes in the shaded and unshaded portions of the stator pole. This phase difference produces a weak rotating magnetic field that provides the starting torque. Although the motor has low efficiency and starting torque, its simple construction, low cost and reliable operation make it suitable for small electrical appliances such as fans, blowers and cooling equipment.