313A Electrical Controls & Wiring 1 — Questions and Answers
Question 1: What is the purpose of a contactor in an HVAC electrical circuit?
- To measure and display voltage and current to the compressor
- To act as an electrically operated switch that uses a low-voltage control coil to open and close high-voltage power contacts to the compressor or fan motor (Correct answer)
- To protect against ground faults in the power supply
- To convert single-phase power to three-phase power for the compressor
Correct answer: To act as an electrically operated switch that uses a low-voltage control coil to open and close high-voltage power contacts to the compressor or fan motor
A contactor uses a low-voltage coil (typically 24V) energized by the control circuit to magnetically close heavy-duty power contacts, allowing full line voltage to energize loads like compressors and fan motors.
Question 2: What does a thermal overload relay do in a motor starter circuit?
- It measures motor winding temperature directly and shuts down on overtemperature
- It senses motor current and opens the control circuit if sustained overcurrent is detected, protecting the motor from overheating due to overload (Correct answer)
- It provides power factor correction for the motor
- It reverses motor rotation if the motor starts in the wrong direction
Correct answer: It senses motor current and opens the control circuit if sustained overcurrent is detected, protecting the motor from overheating due to overload
A thermal overload relay uses a bimetallic element or electronic sensor to monitor motor current; if the motor draws excessive current for a sustained period (indicating mechanical overload or locked rotor), the relay trips and removes the motor from the circuit.
Question 3: In Canadian HVAC wiring, what voltage is most commonly used for low-voltage control circuits?
- 12 V DC
- 24 V AC (Correct answer)
- 48 V AC
- 120 V AC
Correct answer: 24 V AC
24 V AC is the standard low-voltage control circuit voltage in Canadian and North American HVAC systems, provided by a step-down transformer from 120 V or 240 V.
Question 4: What is the function of a high-pressure safety control (high-pressure cutout) in a refrigeration system?
- To prevent the condenser fan from operating when ambient temperature is too high
- To shut down the compressor if discharge pressure exceeds a safe limit, preventing equipment damage and refrigerant venting (Correct answer)
- To limit the maximum current draw of the compressor motor
- To signal a low refrigerant charge condition to the building management system
Correct answer: To shut down the compressor if discharge pressure exceeds a safe limit, preventing equipment damage and refrigerant venting
A high-pressure cutout is a safety switch that opens the compressor control circuit when discharge pressure rises above the set point, protecting the system from excessive pressure that could damage components or cause refrigerant leaks.
Question 5: What does a phase-loss relay protect against in a three-phase compressor installation?
- Protecting the compressor from reverse rotation due to miswired phases
- Detecting the loss of one phase of the three-phase supply, which would cause the motor to overheat and potentially burn out (Correct answer)
- Preventing the compressor from starting when the power supply frequency is out of range
- Protecting against voltage spikes from lightning or utility switching
Correct answer: Detecting the loss of one phase of the three-phase supply, which would cause the motor to overheat and potentially burn out
A phase-loss relay monitors all three phases of the power supply; if one phase is lost, the motor attempts to run on two phases (single phasing), drawing excessive current and overheating, so the relay shuts it down immediately.
Question 6: What is the purpose of a lockout-tagout (LOTO) procedure when servicing HVAC equipment in Canada?
- To lock the thermostat at a fixed temperature to prevent tampering during service
- To isolate and lock all energy sources (electrical, refrigerant pressure, etc.) to prevent unexpected re-energization during maintenance, protecting the technician from injury (Correct answer)
- To seal the refrigerant circuit to prevent leaks during transport
- To record service work completed on a tag attached to the equipment
Correct answer: To isolate and lock all energy sources (electrical, refrigerant pressure, etc.) to prevent unexpected re-energization during maintenance, protecting the technician from injury
LOTO procedures, required under provincial occupational health and safety (OHS) regulations across Canada, ensure that electrical power and other energy sources are isolated and locked before service work begins, preventing accidental startup.
313A Electrical Controls & Wiring 1
What is the purpose of a contactor in an HVAC electrical circuit?