Electrical Systems and Controls Flashcards
7 cards from real 313D 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 Systems and Controls flashcards as text
A residential heat pump defrost control board initiates defrost based on:
Answer: Time and temperature, or time and pressure, depending on board design
Most defrost boards use a time-and-temperature or time-and-pressure strategy to determine when frost buildup warrants a defrost cycle.
On a residential AC system, what is the role of the low-voltage common wire (C-wire) connected to the thermostat?
Answer: To complete the 24V circuit by providing a return path to the transformer secondary
The C-wire is the common terminal of the 24V transformer secondary, completing the circuit for thermostat operation and smart thermostat power.
What type of electrical fault would cause a ground fault circuit interrupter (GFCI) to trip on an AC circuit?
Answer: Current leaking from a hot conductor to ground exceeding ~5 mA
A GFCI detects an imbalance between hot and neutral (or ground path) current as small as 4–6 mA, indicating current is leaking to ground through an unintended path.
A residential condenser fan motor runs in the wrong direction after replacement. The MOST likely cause is:
Answer: The start and run capacitor leads were swapped on the new motor
Swapping the start winding leads relative to the run winding reverses the phase shift and changes motor rotation direction.
What does it mean when a contactor shows pitted or burned contact surfaces?
Answer: The contacts have experienced arcing and should be replaced to prevent unreliable operation
Pitted contacts result from electrical arcing during switching, which degrades contact surface quality and increases resistance, leading to heat buildup and potential failures.
An AC technician measures 127V at L1-to-ground and 121V at L2-to-ground on a 240V circuit. The voltage imbalance concern is because:
Answer: Voltage imbalance between phases causes disproportionately higher current imbalance, stressing compressor windings
A small voltage imbalance creates a much larger current imbalance (roughly 6–10× the percentage), causing uneven heating in motor windings and premature failure.
Which safety device protects an ECM blower motor module from power surges and lightning-induced voltage spikes?
Answer: Transient voltage surge suppressor (TVSS) or MOV installed on the circuit
ECM motor modules contain sensitive electronics vulnerable to voltage transients; a TVSS or metal oxide varistor (MOV) clamps surge voltages before they reach the module.