313A Electrical Controls

Question 1

What is the function of a contactor in an HVAC system?

Accepted Answer

An electrically operated switch that uses a low-voltage control signal to engage high-voltage power to the compressor and condenser fan motors

Answer

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?

Accepted Answer

To shut off the compressor if discharge pressure exceeds a safe limit, preventing equipment damage or rupture

Answer

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?

Accepted Answer

To improve the efficiency and power factor of the motor by creating a phase shift in the run winding during normal operation

Answer

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?

Accepted 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

Answer

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?

Accepted 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)

Answer

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.

Red Seal HVAC 313A Practice Test

Red Seal HVAC 313A Practice Test

313A Electrical Controls