313A Electrical Controls & Wiring 2 — Questions and Answers
Question 1: What is the purpose of a defrost timer in a refrigeration system?
- To limit compressor run time per hour to prevent overheating
- To initiate defrost cycles at preset time intervals and terminate them after a set duration or when a termination thermostat trips (Correct answer)
- To delay compressor startup after a power interruption to prevent short cycling
- To record the number of hours the compressor has operated for maintenance scheduling
Correct answer: To initiate defrost cycles at preset time intervals and terminate them after a set duration or when a termination thermostat trips
A defrost timer initiates defrost at programmed intervals (e.g., every 6 or 8 hours) and terminates the defrost cycle when a time limit or defrost termination thermostat setpoint is reached, then returns the system to normal cooling.
Question 2: What does a dual-pressure control combine into a single device?
- A high-pressure cutout and a low-pressure cutout in one housing, with separate adjustable setpoints for each (Correct answer)
- A temperature controller and a pressure controller for simultaneous monitoring
- A pressure transducer and a flow switch for pump protection
- A low-pressure cutout and a differential pressure switch for filter monitoring
Correct answer: A high-pressure cutout and a low-pressure cutout in one housing, with separate adjustable setpoints for each
A dual-pressure control combines a high-pressure safety cutout and a low-pressure cutout (or low-pressure pump-down control) in one device, allowing both functions with a single installation.
Question 3: In an HVAC electrical schematic, what does a normally open (NO) contact symbol indicate?
- A contact that is always electrically connected and cannot be opened
- A contact that is open (non-conducting) in its de-energized (resting) state and closes when the associated coil or mechanism is actuated (Correct answer)
- A contact that opens automatically on power failure for safety
- A contact used exclusively in three-phase circuits
Correct answer: A contact that is open (non-conducting) in its de-energized (resting) state and closes when the associated coil or mechanism is actuated
A normally open contact is open (circuit broken) when the relay or switch is in its rest state, and closes (conducts) when the coil is energized or the switch is actuated, completing the circuit.
Question 4: What is the function of a motor start capacitor in a single-phase PSC or CSIR compressor?
- To improve the power factor of the motor during continuous running
- To provide a phase-shifted current to the start winding to create a rotating magnetic field and initiate motor rotation (Correct answer)
- To store electrical charge for emergency compressor operation during power outages
- To protect the motor windings from voltage spikes during starting
Correct answer: To provide a phase-shifted current to the start winding to create a rotating magnetic field and initiate motor rotation
A start capacitor shifts the phase of the current in the start winding relative to the run winding, creating a rotating magnetic field that produces starting torque; it is switched out of the circuit once the motor reaches operating speed.
Question 5: What does megohm testing (megging) a compressor motor winding indicate?
- The capacitance of the motor run capacitor
- The insulation resistance of the motor windings to ground, indicating whether insulation breakdown or moisture contamination has occurred (Correct answer)
- The resistance of the motor windings between terminals to check for open or shorted windings
- The inductance of the motor windings to verify correct winding configuration
Correct answer: The insulation resistance of the motor windings to ground, indicating whether insulation breakdown or moisture contamination has occurred
A megohmmeter applies a high DC voltage (typically 500 V) to the motor winding and measures the resistance to ground; low megohm readings indicate degraded insulation, moisture ingress, or early winding failure.
Question 6: What is the purpose of a time-delay relay (anti-short-cycle timer) in a compressor control circuit?
- To delay the opening of the liquid line solenoid valve after the compressor starts
- To prevent the compressor from restarting immediately after shutdown, allowing pressures to equalize and protecting the motor from high-torque restarts (Correct answer)
- To time the duration of each defrost cycle precisely
- To sequence the startup of multiple refrigeration circuits to reduce electrical demand peaks
Correct answer: To prevent the compressor from restarting immediately after shutdown, allowing pressures to equalize and protecting the motor from high-torque restarts
An anti-short-cycle timer prevents the compressor from restarting for a preset period (typically 3–5 minutes) after shutdown, allowing system pressures to equalize and protecting the motor from the strain of starting against high differential pressure.
313A Electrical Controls & Wiring 2
What is the purpose of a defrost timer in a refrigeration system?