Electrical Systems and Controls Flashcards
7 cards from real Air Conditioning Repair practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical Systems and Controls flashcards as text
A variable-speed ECM blower motor does not respond to speed change commands from the control board. The motor runs at one fixed speed only. What is the most likely fault?
Answer: Loss of PWM or DC control signal from the control board
ECM motors rely on a PWM or 0–10V DC control signal from the board to vary speed; loss of this signal causes the motor to default to a fixed speed.
What is the purpose of a crankcase heater on a hermetic compressor?
Answer: Prevent refrigerant migration into the compressor oil during the off cycle
A crankcase heater keeps the compressor oil warm during the off cycle, preventing liquid refrigerant from migrating into and diluting the oil.
An HVAC technician measures 0V AC between the R and C terminals at the air handler control board with the disconnect closed. What does this indicate?
Answer: The 24V transformer has failed or is not receiving primary voltage
No voltage between R and C with power applied means the 24V transformer is not producing output, either due to a failed transformer or no primary voltage reaching it.
Which type of motor protection device senses both motor current and winding temperature to prevent compressor burnout?
Answer: Internal motor protector (IMP)
An internal motor protector monitors both winding temperature and current, opening the circuit if either exceeds safe limits to protect the compressor.
A technician is testing a potential relay used in a hard start kit. At what point should the potential relay open the start capacitor circuit?
Answer: When the motor reaches approximately 75% of its synchronous speed
The potential relay senses back-EMF from the start winding; when the motor reaches about 75% of synchronous speed, back-EMF is high enough to open the relay and drop the start capacitor out of the circuit.
A three-phase scroll compressor fails to start and the contactor hums but doesn't pull in fully. Which condition is most likely?
Answer: Loss of one phase (single phasing) on the supply voltage
Single phasing causes the three-phase contactor to chatter or fail to fully pull in, and the compressor motor cannot develop starting torque on only two legs.
When using a clamp-on ammeter to measure current on a two-wire 240V single-phase circuit, the technician clamps around both conductors and reads zero amps. Why?
Answer: The currents in the two conductors cancel each other because they flow in opposite directions
Clamping around both conductors of a 240V single-phase circuit captures equal and opposite currents that cancel to zero; only one conductor should be clamped at a time.