Which Is True About a Capacitor-Start Motor

Which Is True About a Capacitor-Start Motor

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Which Is True About a Capacitor-Start Motor

1. It Uses a Capacitor to Create a Phase Shift for Starting

• True Statement: A capacitor-start motor incorporates an electrolytic capacitor in series with its auxiliary winding. This capacitor shifts the current phase in the auxiliary winding by ~90 degrees relative to the main winding, creating a rotating magnetic field that mimics the torque generation of a starter motor.


• Mechanism: The phase shift enables the motor to produce high starting torque (3–4 times full-load torque), essential for heavy loads like compressors or pumps.

2. It Features a Centrifugal Switch to Disconnect the Capacitor During Operation

• True Statement: A centrifugal switch inside the motor disconnects the auxiliary winding and capacitor once the motor reaches ~75% of its rated speed. This prevents the capacitor from overheating and allows the motor to run efficiently on the main winding alone.


• Analogy to Starter Motors: Similar to how a starter motor disengages after an engine starts, the capacitor-start motor’s auxiliary system stops once the motor is in motion.

3. It Prioritizes High Starting Torque Over Continuous Efficiency

• True Statement: Capacitor-start motors are optimized for starting heavy loads but are less efficient during steady operation compared to capacitor-run motors. The capacitor is designed for short-term use (only during starting), making it ideal for applications that require infrequent starts with high torque demands.


• Application Example: HVAC compressors, which need strong initial force to overcome refrigerant pressure.

4. It Requires an Electrolytic Capacitor for Its Starting Circuit

• True Statement: The starter circuit relies on an electrolytic capacitor, which offers high capacitance in a compact size—critical for generating the necessary phase shift. These capacitors are polarized and must be rated for the motor’s voltage and starting current.


• Maintenance Note: Electrolytic capacitors have a limited lifespan and may dry out, requiring replacement to maintain starting performance.

5. It Is Distinct from Split-Phase and Capacitor-Run Motors

• True Statement: Unlike split-phase motors (which use resistive windings for low starting torque) or capacitor-run motors (which use non-polarized capacitors for continuous operation), capacitor-start motors balance high torque with simple design.


• Key Difference: Capacitor-run motors sacrifice starting torque for smoother, more efficient operation, while capacitor-start motors prioritize robust starting capabilities.

Common Misconceptions Debunked

• False Claim: “Capacitor-start motors use the capacitor during operation.”
  
  
  
  • Correction: The capacitor is only active during starting; the centrifugal switch disconnects it afterward.
  
  
  
  


• False Claim: “They are suitable for light loads like fans.”
  
  
  
  • Correction: Capacitor-start motors are overengineered for light loads; split-phase motors are more appropriate for such applications.