The Capacitor Start Induction Motor (CSIM) is a type of single-phase motor that uses a capacitor in series with the auxiliary (starting) winding to produce a high starting torque. The capacitor introduces a necessary phase shift between the current in the main and auxiliary windings, simulating a two-phase motor condition and thus providing better starting characteristics compared to split-phase motors.
🔧 Construction of CSIM:
- It has a squirrel cage rotor.
- Two stator windings: Main winding and Starting (Auxiliary) winding.
- An electrolytic capacitor is connected in series with the starting winding.
- A centrifugal switch is mounted on the shaft to disconnect the capacitor and auxiliary winding once the motor reaches 70%–80% of rated speed.
⚙️ Working Principle:
When power is supplied, the capacitor introduces a phase difference of around 80° between the main winding current (Im) and the starting winding current (Is). This produces a rotating magnetic field that helps generate high starting torque. Once the motor attains around 75% of its rated speed, the centrifugal switch opens, disconnecting the capacitor and auxiliary winding.
📐 Phasor Diagram:
The phasor diagram shows a significant angular displacement (≈80°) between Is and Im, resulting in higher torque production at startup. This mimics a balanced two-phase motor behavior.
📊 Characteristics of Capacitor Start Motor:
- High starting torque: 3 to 4.5 times the full-load torque.
- Low starting current as compared to split-phase motors.
- Capacitor is usually an electrolytic type rated between 150µF to 300µF for short duty cycles.
- Capacitor is mounted externally for better heat dissipation and safety.
- Capacitor and auxiliary winding are not used in continuous operation – disconnected after startup.
🔁 Reversing Rotation:
To reverse the direction of rotation, you must first bring the motor to a complete stop and then reverse the connections of either the main or auxiliary winding – just like in a split-phase motor.
🛠 Applications of Capacitor Start Induction Motor:
- Refrigeration and air-conditioning compressors (RAC)
- Water pumps and jet pumps
- Conveyor belts
- Washing machines and domestic appliances
- Desert coolers and blowers
- Lathe machines and other small machine tools requiring high starting torque
📌 Additional Notes:
- The capacitor start motor is more expensive than the split-phase motor but offers better performance at startup.
- Not suitable for continuous high-frequency start-stop operations as the capacitor is designed for limited duty cycles (usually 20 starts/hour, 3 seconds each).
- To improve reliability and performance, some modern designs use a dual-capacitor setup (start and run capacitors), evolving into a Capacitor Start-Capacitor Run Motor.