Electrical Controls Flashcards
7 cards from real HVAC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical Controls flashcards as text
What is the primary function of a contactor in an HVAC system?
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.
A potential relay used in a single-phase compressor motor is installed in which circuit?
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.
Which control device opens on a RISE in temperature to protect a heat exchanger from overheating?
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.
In a 24V control circuit, the transformer secondary rating is typically:
Answer: 24V / 40 VA
Most residential HVAC systems use a 24V, 40 VA control transformer to power the thermostat and control components.
What happens when a current-sensing overload protector in a compressor motor trips?
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.
A defrost control board initiates defrost based on which two inputs in a time-temperature system?
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.
Which wire color is standardized for the 24V common (C) terminal in most North American HVAC low-voltage wiring?
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.