Electrical and Controls Flashcards

A contactor coil is rated at 24V AC. If the supply voltage to the coil drops to 18V AC, what is the MOST likely result?

Answer: The contactor may chatter or fail to fully close

Insufficient coil voltage prevents full magnetic engagement, causing the contactor to chatter or fail to close completely.

What is the function of a run capacitor in a PSC (permanent split capacitor) motor?

Answer: Improve power factor and running efficiency

A run capacitor creates a phase shift in the auxiliary winding that improves power factor and running efficiency throughout operation.

What does a lockout/tagout (LOTO) procedure require before servicing electrical HVAC equipment?

Answer: Isolating and de-energizing the power source and verifying zero energy state

LOTO requires disconnecting and locking the energy source, then verifying zero energy state before any service work begins.

On a cooling thermostat, which terminals are energized when the system is calling for cooling?

Answer: R and Y

During a cooling call, the Y (compressor) terminal is energized along with R (power), activating the compressor contactor.

What does a megohmmeter (megger) measure?

Answer: Insulation resistance of motor windings

A megohmmeter applies high DC voltage to measure insulation resistance, revealing breakdown or moisture in motor windings.

Which type of transformer is commonly used to step down 240V line voltage to 24V for HVAC control circuits?

Answer: Step-down isolation transformer

A step-down isolation transformer reduces line voltage (240V) to the 24V AC required by HVAC control circuits.

A motor draws 8 amps at 240V. What is the approximate power consumption in watts?

Answer: 1,920 watts

Power (W) = Voltage × Current = 240V × 8A = 1,920 watts for a single-phase resistive load.