Working Principles of Single-Phase and Three-Phase Induction Motors
Induction motors are widely used in the dairy industry and other sectors because they are simple, rugged, cost-effective, and require low maintenance. These motors work on the principle of electromagnetic induction, where current is induced in the rotor conductors due to the relative motion between the rotating magnetic field of the stator and the rotor.
✅ A. Working Principle of Single-Phase Induction Motor
A single-phase induction motor operates on a single-phase AC power supply (like that used in homes and small equipment).
1. Construction:
- Stator: Contains a single-phase winding supplied with AC voltage.
- Rotor: Squirrel cage type, made of aluminum or copper conductors.
2. Operating Principle:
- When AC is supplied to the stator winding, it produces a pulsating magnetic field (not a rotating field).
- This pulsating field does not create rotation in the rotor at standstill.
- Hence, no torque is produced initially, and the motor does not start on its own.
- However, if the rotor is given a manual push, it begins to rotate in that direction and continues running.
❗ Why Single-Phase Motors Are Not Self-Starting?
The main reason is that a single-phase supply creates an alternating (pulsating) magnetic field, not a rotating one.
- This pulsating field induces equal and opposite currents in the rotor.
- As a result, net torque is zero at startup, and the motor cannot start by itself.
To make them self-starting, a starting mechanism is added, such as:
✅ Starting Methods:
- Split-phase Motor: Uses an auxiliary winding with a phase difference to create a rotating magnetic field.
- Capacitor-Start Motor: Uses a capacitor in series with the auxiliary winding to improve starting torque.
- Shaded Pole Motor: Has shaded coils on one side of the pole to create a delayed magnetic field.
✅ B. Working Principle of Three-Phase Induction Motor
A three-phase induction motor operates on a three-phase AC power supply, common in industries and large equipment.
1. Construction:
- Stator: Contains three windings (R, Y, B) placed 120° apart.
- Rotor: Usually squirrel cage or wound type.
2. Operating Principle:
- When three-phase current flows through the stator windings, it generates a rotating magnetic field (RMF).
- This RMF cuts the rotor conductors, inducing an electromotive force (EMF) due to Faraday’s Law.
- The induced current in the rotor interacts with the RMF, producing torque (Lorentz Force Law).
- The rotor starts rotating in the same direction as the magnetic field, but slightly slower than the synchronous speed.
✅ Characteristics of 3-Phase Motors:
- Self-starting due to the rotating magnetic field.
- High efficiency and torque.
- Commonly used in milk chilling plants, large pumps, compressors, and centrifuges.
🔄 Comparison Between Single-Phase and Three-Phase Induction Motors
| Feature | Single-Phase Motor | Three-Phase Motor |
|---|---|---|
| Power Supply | Single-phase AC | Three-phase AC |
| Self-starting | ❌ Not self-starting | ✅ Self-starting |
| Starting Mechanism | Requires capacitor, auxiliary winding, etc. | No starting aid required |
| Cost | Lower | Higher |
| Efficiency | Lower | Higher |
| Applications | Fans, mixers, domestic machines | Industrial pumps, compressors, dairy plants |
| Torque | Low starting torque | High starting and running torque |
| Maintenance | Simple and cheap | More robust but higher cost |
✅ Conclusion
- A single-phase induction motor needs additional components to start because its magnetic field is pulsating, not rotating, at startup.
- A three-phase motor creates a rotating magnetic field by design, which makes it naturally self-starting and suitable for heavy industrial applications like those in the dairy industry.
Understanding these principles helps in selecting the right motor type based on the load requirements, phase availability, and application area.