Question

The starting torque of a capacitor-start single-phase induction motor is:

• A. Zero
• B. Low
• C. High
• D. Same as a shaded-pole motor

Hint:

Capacitor-start motors exhibit the highest starting torque among common single-phase induction motors.
They are ideal for heavy-starting duty applications such as compressors, refrigerators, and pumps.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks about the magnitude of the starting torque of a capacitor-start single-phase induction motor relative to other types of single-phase motors.

Step 2: Key Formula or Approach:
The starting torque ($T_{st}$) of a split-phase induction motor is proportional to the sine of the phase angle ($\alpha$) between the currents in the main winding ($I_m$) and the auxiliary winding ($I_a$):
\[ T_{st} \propto I_m I_a \sin\alpha \]

Step 3: Detailed Explanation:

• A single-phase induction motor is not self-starting because a single-phase winding produces a pulsating stator magnetic field rather than a rotating one.

• To make the motor self-starting, a second auxiliary winding is placed in parallel with the main winding, physically shifted by 90 electrical degrees.

• In a capacitor-start motor, a high-value electrolytic capacitor is connected in series with the auxiliary winding during starting.

• The capacitor causes the current in the auxiliary winding ($I_a$) to lead the applied voltage.

• The current in the main winding ($I_m$), being highly inductive, lags the applied voltage.

• This phase difference ($\alpha$) between the two currents is brought very close to $90^\circ$.

• Since $\sin(90^\circ) = 1$, the starting torque reaches a very high value, typically in the range of 200% to 300% of the full-load torque.

Step 4: Final Answer
Therefore, the starting torque of a capacitor-start single-phase induction motor is high.