HVAC Electrical Controls 2 — Questions and Answers
Question 1: What is the primary function of a contactor in an HVAC system?
- Measure current flow
- Switch high-voltage loads using a low-voltage control signal (Correct answer)
- Protect against short circuits
- Regulate refrigerant pressure
Correct answer: Switch high-voltage loads using a low-voltage control signal
A contactor uses a low-voltage coil (typically 24V) to magnetically close contacts that switch high-voltage loads like compressors and condenser fan motors.
Question 2: A potential relay used in a single-phase compressor motor is installed in which circuit?
- Common winding
- Start winding in parallel (Correct answer)
- Run capacitor circuit
- Discharge line
Correct answer: Start winding in parallel
The potential relay coil is wired in parallel with the start winding and its normally-closed contacts are in series with the start capacitor, opening once back-EMF rises.
Question 3: Which control device opens on a RISE in temperature to protect a heat exchanger from overheating?
- Thermostat
- High-limit switch (Correct answer)
- Low-pressure switch
- Defrost termination thermostat
Correct answer: High-limit switch
A high-limit switch is a safety device that opens its contacts when temperature rises above a set point, de-energizing the heat source before damage occurs.
Question 4: In a 24V control circuit, the transformer secondary rating is typically:
- 120V / 40 VA
- 24V / 40 VA (Correct answer)
- 24V / 240 VA
- 12V / 40 VA
Correct answer: 24V / 40 VA
Most residential HVAC systems use a 24V, 40 VA control transformer to power the thermostat and control components.
Question 5: What happens when a current-sensing overload protector in a compressor motor trips?
- The condenser fan continues but the compressor stops (Correct answer)
- The entire unit including controls loses power
- Only the capacitor is disconnected
- The reversing valve shifts to cooling mode
Correct answer: The condenser fan continues but the compressor stops
An internal overload protector opens the compressor circuit, stopping the compressor while other components like the condenser fan may remain energized.
Question 6: A defrost control board initiates defrost based on which two inputs in a time-temperature system?
- Ambient temperature and discharge pressure
- Accumulated run time and coil temperature sensor (Correct answer)
- Return air temperature and supply air temperature
- Suction pressure and superheat
Correct answer: Accumulated run time and coil temperature sensor
Time-temperature defrost boards initiate defrost after a set accumulated compressor run time AND when the coil sensor confirms frost is present.
Question 7: Which wire color is standardized for the 24V common (C) terminal in most North American HVAC low-voltage wiring?
- Red
- Green
- Blue (Correct answer)
- Yellow
Correct answer: Blue
Blue wire is the conventional color for the common (C) terminal on HVAC low-voltage wiring, though installers should always verify with a wiring diagram.
HVAC Electrical Controls 2
What is the primary function of a contactor in an HVAC system?