GATE Electrical Engineering - Complete Master Library

 

📘 Single Phase Induction Motor – Starting Methods

Single phase induction motor is not self starting. Special starting methods are used to create rotating magnetic field.

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🔹 1️⃣ Why Single Phase IM is Not Self Starting?

Single phase supply produces:

• Pulsating magnetic field

This can be resolved into:

• Forward rotating field
• Backward rotating field

At starting: Torque produced by both fields is equal and opposite. Net starting torque = 0. Therefore motor cannot start by itself.

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🔹 2️⃣ Split Phase Motor (Resistance Start)

Construction:

• Main winding
• Auxiliary winding (high resistance)

Working: Auxiliary winding creates phase difference. Small starting torque produced. Applications: Fans, washing machines.

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🔹 3️⃣ Capacitor Start Motor

Capacitor connected in series with auxiliary winding. Creates larger phase difference (~90°). High starting torque. After starting: Centrifugal switch disconnects capacitor. Used in: Refrigerators, compressors.

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🔹 4️⃣ Capacitor Start – Capacitor Run Motor

Two capacitors used:

• Starting capacitor
• Running capacitor

Better power factor. Smooth operation. Most widely used.

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🔹 5️⃣ Shaded Pole Motor

Copper ring placed on part of pole. Creates delayed magnetic field. Very low starting torque. Used in: Small fans, toys.

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🔹 6️⃣ Comparison

• Split phase → Medium starting torque
• Capacitor start → High starting torque
• Capacitor run → Good efficiency
• Shaded pole → Very low torque

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🔹 7️⃣ Important Observations

• Capacitor start gives highest starting torque
• Shaded pole simplest but weak
• All methods create phase difference

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🎯 GATE Important Points

• Not self starting due to pulsating field
• Capacitor start most common
• High torque applications use capacitor start
• Shaded pole for small power ratings

Single Phase IM Needs Artificial Phase Shift to Start