313A Electrical Controls 2

Question 1

What is a potential relay and what does it do in a compressor motor circuit?

Accepted Answer

A relay that disconnects the start capacitor and/or start winding once the compressor motor reaches approximately 75% of its running speed, using the back-EMF voltage of the start winding

Answer

A potential relay monitors the back-EMF (counter-electromotive force) voltage generated by the start winding as the motor accelerates. When this voltage reaches a threshold (typically at 75% of running speed), the relay opens its normally closed contacts, disconnecting the start capacitor from the circuit to prevent overheating and damage.

Question 2

What is the function of a low-pressure safety switch in a refrigeration system?

Accepted Answer

To shut off the compressor if suction pressure drops too low, protecting against loss of charge, restricted airflow, or blocked metering device

Answer

The low-pressure cutout protects the compressor by shutting it off when suction pressure drops below a safe minimum. Low suction pressure can result from refrigerant loss, restricted airflow (dirty filter/iced coil), blocked metering device, or faulty indoor fan. Operating at very low suction pressure can cause motor overheating and oil return problems.

Question 3

What is a defrost timer and how does it function in a heat pump system?

Accepted Answer

A control that initiates and terminates defrost cycles at set intervals or based on demand, reversing the system to melt frost accumulated on the outdoor coil

Answer

A defrost control (timer-based or demand-based) manages the defrost cycle when frost accumulates on the outdoor coil during heating mode. Time-initiated defrost runs at set intervals (e.g., every 90 minutes). Demand defrost uses sensors (temperature differential, air pressure) to initiate defrost only when needed, improving efficiency.

Question 4

What is a current sensing relay and where is it used?

Accepted Answer

A relay that monitors compressor motor current draw to disconnect the start winding; used primarily in fractional-horsepower compressors

Answer

A current sensing relay uses the compressor motor's running current to control the start winding circuit. At startup, high locked-rotor current keeps the relay's contacts closed, energizing the start winding. As the motor reaches speed and current drops, the relay opens, disconnecting the start circuit. It is used in small, fractional-HP compressors.

Question 5

What is the purpose of a crankcase pressure regulator (CPR) valve?

Accepted Answer

To limit the suction pressure entering the compressor during pulldown or hot gas defrost to prevent motor overload

Answer

A CPR valve is installed in the suction line to throttle suction gas and prevent the compressor from seeing excessive suction pressure during pulldown after defrost or startup of a warm system. High suction pressure causes high current draw that can trip overloads or damage the motor. The CPR gradually allows full pressure as the system cools down.

Red Seal HVAC 313A Practice Test

Red Seal HVAC 313A Practice Test

313A Electrical Controls 2