Question

Explain the construction, working, and uses of a capacitor-start capacitor-run single-phase induction motor.

Hint:

Always remember that in this motor, the auxiliary winding and the running capacitor ($C_r$) are never disconnected; they remain energized during running to maintain a balanced magnetic field and high power factor. Only the starting capacitor ($C_s$) is switched out.

Answer 1

Step 1: Constructional Setup:
This motor is a highly efficient single-phase induction motor. It consists of the following key components:

• Stator Windings: Houses two independent windings placed $90^\circ$ electrically apart in the stator slots: the Main (Running) Winding and the Auxiliary (Starting) Winding.
• Dual Capacitor Network: Features two capacitors connected in parallel with each other, and the combined network is placed in series with the auxiliary winding:
  • Starting Capacitor ($C_s$): A high-capacity AC electrolytic capacitor designed only for short-duty starting cycles.
  • Running Capacitor ($C_r$): A lower-capacity paper or plastic-film capacitor designed for continuous-duty operation.
• Centrifugal Switch: A mechanically activated switch connected in series only with the starting capacitor ($C_s$).
• Rotor: A standard, robust squirrel cage rotor with short-circuited conducting bars.

Step 2: Working Principle:
The operation of the motor is divided into two distinct phases:

• Starting Phase (High Torque): When single-phase AC power is switched ON, both the starting capacitor ($C_s$) and running capacitor ($C_r$) are connected in the auxiliary circuit (since the centrifugal switch is closed).
  The combined capacitance ($C = C_s + C_r$) is very high. This creates a large phase angle shift (nearly $90^\circ$) between the auxiliary winding current ($I_a$) and main winding current ($I_m$): $$\phi \approx 90^\circ$$ This large phase split produces a strong, balanced rotating magnetic field (RMF) in the air gap, inducing heavy currents in the rotor bars and generating an exceptionally high starting torque.
• Running Phase (High Efficiency): As the rotor accelerates and reaches approximately $75\%\text{--}80\%$ of its synchronous speed, centrifugal force causes the centrifugal switch to open.
  This disconnects the starting capacitor $C_s$ from the auxiliary circuit. The motor continues to run with only the running capacitor $C_r$ connected in series with the auxiliary winding.
  Because $C_r$ is permanently in the circuit, it continuously corrects the phase angle under load, improving the motor's power factor, reducing magnetic hum (noise), and significantly raising operating efficiency.

Step 3: Key Applications:
Because of its high starting torque, quiet operation, and excellent running efficiency, this motor is widely used in:

• High-pressure air compressors and refrigeration units.
• Water pumps and centrifugal agricultural pumps.
• Conveyors, fans, and heavy-duty workshop machinery.