A single-phase motor is one of the most commonly used types of electric motors, especially in homes, shops, and small industries where only single-phase AC power supply (230V) is available. It converts electrical energy into mechanical energy to drive various machines such as fans, pumps, washing machines, mixers, and small compressors.
1. What is a Single Phase Motor?
A single-phase motor is an AC motor that operates on a single-phase power supply. It uses alternating current (AC) to produce rotating motion. Unlike a three-phase motor, it does not produce a self-starting rotating magnetic field on its own — hence, it usually needs an auxiliary means (like a capacitor or auxiliary winding) to start.
2. Principle of Operation
The working principle of a single-phase motor is based on Faraday’s Law of Electromagnetic Induction and Lorentz Force.
When AC current flows through the stator winding, it produces an alternating magnetic field. This field induces a current in the rotor (in case of an induction motor), creating a magnetic interaction between the stator and rotor, producing torque.
However, the problem is that the magnetic field created by a single-phase supply pulsates rather than rotates — so the motor is not self-starting. To overcome this, various starting methods (like split-phase, capacitor, or shaded pole) are used.
⚙️ 3. Construction of a Single Phase Motor
A single-phase induction motor mainly has two parts:
(A) Stator (Stationary Part)
- Contains main winding and auxiliary (starting) winding.
- Connected to the single-phase power supply.
- Creates an alternating magnetic field.
(B) Rotor (Rotating Part)
- Usually of squirrel cage type (conductors short-circuited at both ends).
- The rotor rotates when torque is produced by the interaction of magnetic fields.
4. Working of a Single Phase Motor
- When power is supplied, the stator winding produces an alternating magnetic field.
- This magnetic field can be considered as two rotating fields rotating in opposite directions (called double field revolving theory).
- At standstill, the torques produced by both fields are equal and opposite — net torque = zero, so the motor does not start.
- Once the rotor is given a small push, the torque in one direction becomes greater, and the motor continues to rotate in that direction.
To make the motor self-starting, auxiliary components are added (capacitors, additional windings, etc.).
5. Types of Single Phase Motors
(1) Split Phase Induction Motor
- Has two windings: main and auxiliary (starting).
- The starting winding is displaced by 90° electrically.
- A centrifugal switch disconnects the starting winding once the motor reaches ~75% of rated speed.
- Applications: Fans, washing machines, small tools.
(2) Capacitor Start Motor
- Uses a capacitor in series with the auxiliary winding.
- Produces a phase shift and a stronger starting torque.
- Capacitor is disconnected after startup by a centrifugal switch.
- Applications: Compressors, pumps, refrigerators.
(3) Capacitor Start and Run Motor
- Uses two capacitors: one for starting and one for running.
- Provides smooth running and high efficiency.
- Applications: Air conditioners, refrigerators, and high-torque devices.
(4) Permanent Split Capacitor (PSC) Motor
- Has a single capacitor permanently connected.
- No centrifugal switch — quiet and reliable.
- Applications: Ceiling fans, blowers, and exhaust fans.
(5) Shaded Pole Motor
- Simple and cheap design.
- Uses copper shading rings on part of the stator pole to create a rotating magnetic field.
- Low starting torque.
- Applications: Table fans, record players, small ventilators.
6. Advantages of Single Phase Motors
- Simple in design and easy to repair.
- Compact and lightweight.
- Requires only single-phase power.
- Suitable for domestic and light industrial use.
- Economical for low power applications.
7. Disadvantages of Single Phase Motors
- Not self-starting (except shaded pole type).
- Lower efficiency compared to three-phase motors.
- Vibrations and noise at high load.
- Limited to small power applications (typically up to 3 HP).
8. Applications of Single Phase Motors
| Appliance / Machine | Motor Type Used |
|---|---|
| Ceiling Fan | PSC Motor |
| Refrigerator | Capacitor Start Motor |
| Washing Machine | Split Phase Motor |
| Air Conditioner | Capacitor Start and Run Motor |
| Table Fan | Shaded Pole Motor |
| Mixer/Grinder | Capacitor Start Motor |
| Water Pump | Capacitor Start Motor |
9. Difference Between Single Phase and Three Phase Motor
| Feature | Single Phase Motor | Three Phase Motor |
|---|---|---|
| Power Supply | Single-phase (230V) | Three-phase (415V) |
| Starting | Not self-starting | Self-starting |
| Efficiency | Lower | Higher |
| Torque | Low starting torque | High starting torque |
| Cost | Cheaper | More expensive |
| Application | Domestic | Industrial |
10. Summary
A single-phase motor is an essential electrical machine widely used where three-phase supply is not available. It is simple, economical, and reliable for light-duty work. Although not self-starting, with the help of capacitors or shading coils, it provides satisfactory operation in countless everyday devices.
