Split Phase Induction Motor: Construction and Working Principle

Split phase induction motors are one of the most commonly used single-phase AC motors in domestic and light industrial applications. In the ITI Refrigeration and Air Conditioning (RAC) trade, these motors are important as they are used in devices such as fans, blowers, and small compressors. This chapter explains the construction and working principle of split phase induction motors.

Introduction

A split phase induction motor is a type of single-phase motor that uses two windings to produce a phase difference required for starting. Since a single-phase supply cannot produce a rotating magnetic field on its own, an auxiliary (starting) winding is used along with the main winding to create the necessary phase shift.

Construction of Split Phase Induction Motor

The main parts of a split phase induction motor are:

• Stator: The stationary part containing two windings:
  • Main (Running) Winding
  • Auxiliary (Starting) Winding
• Rotor: Usually a squirrel cage type rotor.
• Centrifugal Switch: Disconnects the starting winding after the motor reaches about 70–80% of its full speed.
• Shaft and Bearings: Provide smooth rotation.
• Frame (Housing): Protects internal components.

The auxiliary winding is designed with higher resistance and lower inductance compared to the main winding. This creates a phase difference between the currents in the two windings.

Working Principle

When single-phase AC supply is given to the motor, current flows through both the main and auxiliary windings. Due to the difference in resistance and inductance, the currents in the two windings are out of phase with each other.

This phase difference produces a rotating magnetic field, which generates starting torque and causes the rotor to begin rotating.

Starting Condition

• Both windings are energized.
• Phase difference creates rotating magnetic field.
• Motor starts rotating.

Running Condition

• When motor reaches about 70–80% of rated speed, the centrifugal switch opens.
• The auxiliary winding is disconnected.
• Motor continues to run on main winding.

Characteristics

• Moderate starting torque
• Simple and economical design
• Good for light load applications

Applications

• Fans and blowers
• Washing machines
• Small pumps
• Refrigeration equipment

Advantages

• Simple construction
• Low cost
• Easy maintenance

Disadvantages

• Low starting torque compared to capacitor motors
• Not suitable for heavy load applications

Conclusion

The split phase induction motor is a simple and widely used motor in domestic and RAC applications. Its construction with two windings helps in producing starting torque, while its working principle ensures smooth operation after starting.

Understanding the construction and working of this motor helps RAC students in installation, maintenance, and troubleshooting of electrical equipment used in refrigeration and air conditioning systems.