Electrical Systems & Controls Flashcards

A hermetic compressor motor draws locked-rotor amperage (LRA) of 45A. What is the minimum wire gauge required for a 240V single-phase circuit using copper conductors at 75°C?

Answer: 10 AWG

LRA × 1.25 = 56.25A; 10 AWG copper at 75°C is rated for 35A continuous, but NEC 440.52 sizing for motors requires selecting wire rated for at least 125% of FLA, and 10 AWG handles the circuit with proper OCPD.

Which type of overload relay automatically resets after cooling down without requiring manual intervention?

Answer: Auto-reset bimetallic relay

An auto-reset bimetallic overload relay restores the circuit automatically once the bimetal strip cools and returns to its original position.

A capacitor-start, capacitor-run (CSCR) motor fails to start but hums loudly. Which component is most likely faulty?

Answer: Start capacitor or centrifugal switch

Humming without starting indicates the start winding circuit is open, typically caused by a failed start capacitor or a defective centrifugal switch that won't close.

What does a megohmmeter (megger) primarily measure in refrigeration system electrical diagnostics?

Answer: Insulation resistance of motor windings

A megohmmeter applies high DC voltage (typically 500–1000V) to measure insulation resistance in megohms, revealing winding degradation or moisture ingress.

In a refrigeration control circuit, a normally open (NO) pressure switch closes at 250 psig and opens at 300 psig. This switch is functioning as a:

Answer: High-pressure operating control

A switch that closes on pressure rise and opens at a high setpoint is a high-pressure operating control, cycling the system to maintain discharge pressure within a range.

A three-phase compressor motor is running but drawing unequal current on the three legs. The most likely cause is:

Answer: Voltage imbalance or open motor winding

Current imbalance in a three-phase motor typically indicates either a supply voltage imbalance or a partially shorted/open winding, both of which cause unequal current draw per phase.

Which NEC article specifically governs the installation of air-conditioning and refrigerating equipment?

Answer: Article 440

NEC Article 440 covers air-conditioning and refrigerating equipment, including hermetic refrigerant motor-compressors and their associated electrical circuits.