313A Electrical Controls — Questions and Answers
Question 1: What is the function of a contactor in an HVAC system?
- To measure voltage
- An electrically operated switch that uses a low-voltage control signal to engage high-voltage power to the compressor and condenser fan motors (Correct answer)
- To convert AC power to DC
- To protect against lightning strikes
Correct answer: An electrically operated switch that uses a low-voltage control signal to engage high-voltage power to the compressor and condenser fan motors
A contactor is an electrically operated switch controlled by the thermostat's low-voltage signal (typically 24V AC). When the thermostat calls for cooling or heating, the contactor coil energizes, pulling in the contacts to connect high-voltage power (240V) to the compressor and outdoor fan motor.
Question 2: What is the purpose of a high-pressure safety switch (cutout) on a refrigeration system?
- To regulate the normal operating pressure
- To shut off the compressor if discharge pressure exceeds a safe limit, preventing equipment damage or rupture (Correct answer)
- To increase the pressure for faster cooling
- To control condenser fan speed
Correct answer: To shut off the compressor if discharge pressure exceeds a safe limit, preventing equipment damage or rupture
The high-pressure cutout is a safety device that opens the compressor control circuit if the discharge (head) pressure exceeds a predetermined setpoint. This protects against conditions such as condenser blockage, overcharge, non-condensable gases, or condenser fan failure that could cause dangerous pressures or compressor damage.
Question 3: What is the purpose of a run capacitor on a compressor motor?
- To store backup power for outages
- To improve the efficiency and power factor of the motor by creating a phase shift in the run winding during normal operation (Correct answer)
- To start the motor spinning
- To regulate voltage to the motor
Correct answer: To improve the efficiency and power factor of the motor by creating a phase shift in the run winding during normal operation
A run capacitor remains in the circuit during normal motor operation. It creates a phase shift between the start and run windings, improving motor efficiency, power factor, and torque. A failed run capacitor causes the motor to run inefficiently, draw higher amperage, overheat, and eventually fail.
Question 4: What is the difference between a start capacitor and a run capacitor?
- They are identical and interchangeable
- A start capacitor provides high torque during motor startup and disconnects via a relay; a run capacitor stays in the circuit continuously to improve efficiency (Correct answer)
- A start capacitor is larger; a run capacitor is smaller
- Start capacitors are used only in heating; run capacitors only in cooling
Correct answer: A start capacitor provides high torque during motor startup and disconnects via a relay; a run capacitor stays in the circuit continuously to improve efficiency
A start capacitor provides a large capacitance boost (typically 100-400 µF) for high starting torque and is disconnected by a potential relay or current relay within seconds of starting. A run capacitor provides lower capacitance (typically 5-45 µF) and remains in the circuit during operation to maintain motor efficiency and power factor.
Question 5: What is a thermostat and how does a programmable thermostat save energy?
- A switch that only turns on heating
- A temperature-sensing control that activates heating or cooling; a programmable thermostat saves energy by automatically adjusting setpoints based on schedules (setback during unoccupied periods) (Correct answer)
- A device that measures humidity only
- A safety device that prevents overheating
Correct answer: A temperature-sensing control that activates heating or cooling; a programmable thermostat saves energy by automatically adjusting setpoints based on schedules (setback during unoccupied periods)
A thermostat senses room temperature and signals the HVAC equipment to cycle on or off to maintain the setpoint. Programmable thermostats save energy by automatically reducing heating or increasing cooling setpoints during scheduled unoccupied or sleeping periods (setback/setup), reducing the temperature difference the system must maintain.
Question 6: What is a transformer in an HVAC control circuit and what voltage does it typically provide?
- It converts 24V to 240V for the compressor
- It steps down line voltage (240V or 120V) to 24V AC to power the thermostat and control circuit safely (Correct answer)
- It converts AC to DC for electronic controls
- It boosts voltage during brown-out conditions
Correct answer: It steps down line voltage (240V or 120V) to 24V AC to power the thermostat and control circuit safely
The control transformer in an HVAC system steps down the line voltage (typically 240V or 120V) to 24V AC. This low voltage safely powers the thermostat, control relays, and safety switches. The transformer is usually located in the air handler or furnace and is protected by a fuse on the secondary (24V) side.
313A Electrical Controls
What is the function of a contactor in an HVAC system?