Air Conditioning Repair Electrical Systems and Controls 5

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?

Accepted Answer

Loss of PWM or DC control signal from the control board

Answer

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?

Accepted Answer

Prevent refrigerant migration into the compressor oil during the off cycle

Answer

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?

Accepted Answer

The 24V transformer has failed or is not receiving primary voltage

Answer

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?

Accepted Answer

Internal motor protector (IMP)

Answer

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?

Accepted Answer

When the motor reaches approximately 75% of its synchronous speed

Answer

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.

Air Conditioning Repair Practice Test

Air Conditioning Repair Practice Test

Air Conditioning Repair Electrical Systems and Controls 5