Which capacitor has a higher value?

Question

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Option A: Starting Capacitor

Explanation & Concepts:
In a single-phase capacitor-start induction motor, the starting capacitor has a significantly higher value of capacitance compared to the running capacitor. The starting capacitor is designed to produce a large phase shift (nearly $90^\circ$) between the main and auxiliary winding currents to generate a high starting torque.

Working Principle: Single-phase induction motors lack a rotating magnetic field at standstill. By introducing a high-capacitance electrolytic capacitor in series with the starting winding, a high quadrature current is produced. Once the motor reaches approximately $75\%$ to $80\%$ of its synchronous speed, a centrifugal switch disconnects the starting capacitor to prevent overheating and winding damage.

Key Formulas: $X_C = \frac{1}{2\pi f C}$ — Capacitive reactance inversely proportional to capacitance; $T_{start} \propto I_m I_a \sin	heta$ — Starting torque dependent on phase angle between currents

Key Points: Starting capacitor value is typically 60 µF to 300 µF, providing high starting torque.; Running capacitor value is typically 1.5 µF to 50 µF, optimized for running power factor and efficiency.; Starting capacitors are short-time rated (electrolytic type) while running capacitors are continuously rated (oil-filled or polypropylene film type).

Advantages: High value of starting capacitor provides extremely high starting torque (up to 300% of full load torque).; Reduces starting current draw from the main supply lines.

Disadvantages: Electrolytic starting capacitors cannot handle continuous operation without overheating.; Requires a reliable centrifugal switch mechanism to disconnect it.

Applications: Compressors and refrigeration units; Air conditioners and heavy-duty pumps; Conveyors and machine tools

Additional Info: Typical ratio: The starting capacitor capacitance is usually 5 to 10 times higher than that of the running capacitor in a capacitor-start capacitor-run motor.. Option B (Running Capacitor) has a smaller capacitance because a large capacitance continuously connected would cause excessive current and losses in the auxiliary winding.. Option C and Option D are incorrect as their values are distinct and non-zero.

Answer: A is correct — The starting capacitor has a higher value of capacitance to lower capacitive reactance $X_C$, allowing maximum auxiliary winding current and high starting torque.

Exam Tip: Remember for exams: $C_{start} > C_{run}$, but voltage rating of $C_{run}$ is often higher than $C_{start}$ because $C_{run}$ continuously withstands the vectorially added supply voltage and induced EMF across the auxiliary winding.

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Option B: Running Capacitor

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Option C: Both Starting and Running Capacitor

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Option D: neither starting nor running

Feature	Starting Capacitor	Running Capacitor
Capacitance Value	High (60 µF to 300 µF)	Low (1.5 µF to 50 µF)
Construction Type	Non-polarized AC Electrolytic	Oil-filled / Polypropylene Film
Operating Duty	Short-time / Intermittent	Continuous
Primary Purpose	Provide high starting torque	Improve running power factor & efficiency

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