Question

Capacitor that is used in capacitor start motor is:

• A. Electrolytic capacitor
• B. Polymer capacitor
• C. Ceramic capacitor
• D. Carbon capacitor

Hint:

Always associate starting capacitors with AC Electrolytic types (designed for short, high-surge start periods) and running capacitors with paper, plastic film, or polymer types (designed to withstand continuous AC voltage).

Answer 1

The Correct Option is A

Step 1: Understanding the Capacitor-Start Single-Phase Motor:
Single-phase induction motors are not self-starting because they produce a pulsating magnetic field rather than a rotating one. To create a rotating field, an auxiliary (starting) winding is added in parallel with the main winding.

Step 2: Role of the Starting Capacitor:
To maximize starting torque, a large phase angle shift ($\theta \approx 90^\circ$) is needed between the currents flowing through the main and auxiliary windings. This requires a high capacitance value ($C$).

Step 3: Selecting the Capacitor Type:

• In a capacitor-start motor, the capacitor is only in the circuit during the brief starting period (typically $< 3$ seconds) until the centrifugal switch opens at approximately $75\%$ of rated speed.
• Since this is short-duty cycle operation, compact and cost-effective dry electrolytic capacitors are perfectly suited. They can provide very high capacitance values (e.g., $100\text{--}300\,\mu\text{F}$) in a small physical enclosure.
• In contrast, capacitor-run motors designed for continuous duty use non-polarized paper or polymer capacitors.

Hence, option (A) is correct.