Permanent Capacitor Motor – Gramin Advance Pvt. ITI Majalgaon

A Permanent Capacitor Motor (PCM) is a type of single-phase induction motor that uses a permanently connected capacitor in the motor’s auxiliary winding circuit. Unlike some capacitor motors where the capacitor is only used during starting, in a permanent capacitor motor the capacitor remains connected both during starting and running operations.

This motor is popular in applications that require quiet operation, good efficiency, and a reasonably high starting torque..

1. Basic Principle of Operation

A single-phase induction motor cannot start on its own because a single-phase supply produces only an alternating field, not a rotating one. To make it self-starting, a second phase is artificially created.

In a Permanent Capacitor Motor, a capacitor is permanently connected in series with the auxiliary winding. This creates a phase shift between the current in the main winding and the auxiliary winding, producing a rotating magnetic field that enables the rotor to start turning and continue running efficiently.

2. Construction of a Permanent Capacitor Motor

A PCM consists of the following main parts:

a) Stator

Has two windings:
- Main winding (also called the running winding)
- Auxiliary winding (also called the starting winding)
The two windings are spaced 90° electrical apart in the stator core.
The auxiliary winding is connected in series with a permanent capacitor.

b) Capacitor

The capacitor is usually of the oil-filled or polypropylene type to ensure durability.
It remains permanently connected in the circuit, unlike in capacitor start motors where it is disconnected after starting.

c) Rotor

Usually a squirrel-cage rotor, which is simple, rugged, and maintenance-free.

d) Frame and Bearings

The frame is made of cast iron or aluminum to provide strength.
Bearings are fitted to allow smooth rotation of the rotor shaft.

3. Working Principle

When single-phase AC is applied:

Current in Main Winding: Flows directly from the supply, producing a magnetic field.
Current in Auxiliary Winding: Flows through the capacitor, which causes it to lead the main winding current by nearly 90°.
The phase difference between these two currents produces a rotating magnetic field in the stator.
This rotating field induces current in the rotor bars (squirrel cage), generating torque and causing rotation.
Since the capacitor remains in the circuit during running, it improves power factor, efficiency, and torque.

4. Phasor Diagram

In a phasor diagram:

The current in the auxiliary winding (I_a) leads the voltage by a large angle due to the capacitor.
The current in the main winding (I_m) lags the voltage slightly.
The phase difference between I_a and I_m is close to 90°, creating near-ideal starting conditions.

5. Characteristics

Starting Torque: Moderate (less than capacitor start motors, but more than split-phase motors)
Running Torque: Smooth and uniform due to continuous phase shifting.
Efficiency: Higher than split-phase motors because of better phase balance.
Power Factor: Better than most single-phase motors due to the permanent capacitor.
Speed: Almost constant for a given load (slip is small, around 3–5%).
Noise: Low, because there are no centrifugal switches.

6. Advantages

Simple Design: No starting switch or relay is needed.
Low Maintenance: Permanent capacitor and squirrel-cage rotor require little upkeep.
Good Power Factor: Improves electrical efficiency and reduces line current.
Quiet Operation: Suitable for noise-sensitive environments.
Long Life: Fewer moving parts mean less wear and tear.
Continuous Duty: Suitable for applications requiring long, uninterrupted operation.

7. Disadvantages

Lower Starting Torque: Not suitable for heavy-load starting applications.
Fixed Capacitor: The same capacitor value is used for starting and running, so it is not optimized for both conditions.
Limited Power Range: Typically manufactured for small horsepower ratings (up to around 2–3 HP).
Higher Cost than Split-Phase Motors: Due to the capacitor’s expense.

8. Applications

Permanent Capacitor Motors are widely used where:

Moderate starting torque is acceptable.
Quiet and efficient running is required.

Examples:

Ceiling fans
Table fans and pedestal fans
Air conditioners
Blowers and exhaust fans
Pumps (small water pumps, aquarium pumps)
Office machines (photocopiers, printers)
Small lathes or grinders
Sewing machines
Room heaters with blowers

9. Comparison with Other Single-Phase Motors

Feature	Split-Phase Motor	Capacitor Start Motor	Permanent Capacitor Motor	Capacitor Start & Run Motor
Starting Torque	Low	High	Moderate	High
Running Torque	Low	High	Moderate	High
Power Factor	Low	Medium	High	High
Cost	Low	Medium	Medium	High
Maintenance	Moderate (switch)	Moderate (switch)	Low	Low
Applications	Fans, small tools	Compressors, pumps	Fans, blowers, appliances	Heavy-duty pumps, compressors

10. Selection Criteria

When choosing a permanent capacitor motor, consider:

Load Type: Ensure it matches the moderate starting torque capability.
Operating Environment: Suitable for quiet indoor environments.
Duty Cycle: Ideal for continuous duty.
Voltage & Frequency: Must match supply specifications.
Capacitor Quality: Use high-grade capacitors for long life.

11. Maintenance Tips

Although they are low-maintenance, a few checks can extend their lifespan:

Check capacitor regularly for signs of bulging or leakage.
Lubricate bearings if applicable (sealed bearings need no lubrication).
Ensure ventilation for cooling, as overheating can damage windings.
Inspect wiring for loose connections.

12. Summary

The Permanent Capacitor Motor is a reliable, efficient, and quiet single-phase motor design. Its key advantage is that the same capacitor is used for both starting and running, eliminating the need for a centrifugal switch and improving operational smoothness. While it cannot handle heavy starting loads, it excels in continuous, light-to-medium-load applications such as fans, blowers, and small appliances.