Air Conditioning Repair Electrical Systems and Controls 5 — Questions and Answers
Question 1: 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?
- Failed run capacitor on the ECM motor
- Loss of PWM or DC control signal from the control board (Correct answer)
- Low-voltage supply to the motor module
- Open start relay in the motor
Correct 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.
Question 2: What is the purpose of a crankcase heater on a hermetic compressor?
- Pre-heat refrigerant before compression to improve efficiency
- Prevent refrigerant migration into the compressor oil during the off cycle (Correct answer)
- Reduce starting current by warming motor windings
- Control suction superheat during low-ambient operation
Correct 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.
Question 3: 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?
- The thermostat is in standby mode
- The 24V transformer has failed or is not receiving primary voltage (Correct answer)
- The contactor coil is open
- The low-pressure switch has opened the circuit
Correct 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.
Question 4: Which type of motor protection device senses both motor current and winding temperature to prevent compressor burnout?
- Current transformer
- Internal motor protector (IMP) (Correct answer)
- Phase monitor relay
- Potential relay
Correct 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.
Question 5: 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?
- When line voltage is applied to the compressor
- When the motor reaches approximately 75% of its synchronous speed (Correct answer)
- When the run capacitor reaches full charge
- When discharge pressure exceeds the high-pressure switch set point
Correct 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.
Question 6: A three-phase scroll compressor fails to start and the contactor hums but doesn't pull in fully. Which condition is most likely?
- Loss of one phase (single phasing) on the supply voltage (Correct answer)
- Open start capacitor
- Failed run capacitor
- Low 24V control voltage at the coil
Correct 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.
Question 7: 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?
- The circuit has no load connected
- The currents in the two conductors cancel each other because they flow in opposite directions (Correct answer)
- The meter is set to DC instead of AC
- The wire gauge is too large for the clamp jaws
Correct 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.
Air Conditioning Repair Electrical Systems and Controls 5
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?