Starting Methods of Single-Phase Induction Motors

 

Starting Methods of Single-Phase Induction Motors

1. Split-Phase (Resistance Start)

  • Construction: Auxiliary winding with high resistance and low inductance, placed 90° apart from the main winding.

  • Starting Torque: Moderate (about 150–200% of full-load torque).

  • Applications: Fans, blowers, small pumps.

  • Torque-Speed Curve: Starts with medium torque, then auxiliary winding is disconnected by a centrifugal switch. Curve rises smoothly to synchronous speed.

2. Capacitor Start

  • Construction: Auxiliary winding in series with a capacitor (disconnected after starting).

  • Starting Torque: High (up to 300–400% of full-load torque).

  • Applications: Compressors, refrigerators, conveyors.

  • Torque-Speed Curve: Steeper rise at low speed due to high starting torque, then smooth transition once auxiliary winding is cut off.

3. Capacitor Start–Capacitor Run

  • Construction: Two capacitors: one for starting (large electrolytic) and one for running (smaller oil-filled).

  • Starting Torque: Very high, with improved running efficiency and power factor.

  • Applications: Heavy-duty loads like air conditioners, pumps, and industrial machinery.

  • Torque-Speed Curve: High initial torque, then stable running torque with better efficiency and smoother curve.

4. Shaded Pole

  • Construction: Auxiliary winding replaced by a shading coil (copper ring) around part of each pole.

  • Starting Torque: Very low (25–75% of full-load torque).

  • Applications: Small appliances like table fans, toys, hair dryers.

  • Torque-Speed Curve: Very shallow rise; suitable only for light loads.


⚠️ Key Considerations

  • Split-phase motors are economical but unsuitable for heavy loads.

  • Capacitor start motors balance cost and performance, widely used in domestic and light industrial applications.

  • Capacitor start–capacitor run motors are the most versatile, offering high torque and efficiency but at higher cost.

  • Shaded pole motors are simple and cheap but limited to very light loads due to poor torque-speed performance.

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