313A Electrical Controls 2 — Questions and Answers
Question 1: What is a potential relay and what does it do in a compressor motor circuit?
- A relay that detects power outages
- 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 (Correct answer)
- A relay that controls the reversing valve
- A relay that switches between heating and cooling modes
Correct 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
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?
- To maintain minimum suction pressure for efficiency
- To shut off the compressor if suction pressure drops too low, protecting against loss of charge, restricted airflow, or blocked metering device (Correct answer)
- To start the compressor when pressure builds up
- To control the expansion valve opening
Correct answer: To shut off the compressor if suction pressure drops too low, protecting against loss of charge, restricted airflow, or blocked metering device
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?
- A timer that shuts the system off after a set period
- 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 (Correct answer)
- A timer that controls the indoor fan speed
- A timer that schedules maintenance reminders
Correct 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
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?
- A relay that senses the current in the thermostat wire
- A relay that monitors compressor motor current draw to disconnect the start winding; used primarily in fractional-horsepower compressors (Correct answer)
- A relay used in variable-speed drives
- A relay that measures current in the defrost heater
Correct answer: A relay that monitors compressor motor current draw to disconnect the start winding; used primarily in fractional-horsepower compressors
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?
- To regulate oil pressure in the crankcase
- To limit the suction pressure entering the compressor during pulldown or hot gas defrost to prevent motor overload (Correct answer)
- To control the crankcase heater
- To regulate discharge pressure
Correct answer: To limit the suction pressure entering the compressor during pulldown or hot gas defrost to prevent motor overload
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.
Question 6: What is a hard lockout versus a soft lockout on an HVAC safety control?
- Hard lockout uses a physical lock; soft lockout uses a digital code
- A hard lockout requires manual reset after a safety trip; a soft lockout automatically resets after the fault condition clears or after a timed delay (Correct answer)
- Hard lockout is for commercial; soft lockout is for residential
- They refer to the same type of reset
Correct answer: A hard lockout requires manual reset after a safety trip; a soft lockout automatically resets after the fault condition clears or after a timed delay
A hard lockout safety control requires a technician to manually reset it (by pressing a button or cycling power) after it trips — ensuring a qualified person investigates the cause. A soft lockout automatically resets when the fault condition clears or after a timed delay. High-pressure switches are often hard lockout; low-pressure switches may be soft lockout.
313A Electrical Controls 2
What is a potential relay and what does it do in a compressor motor circuit?