313D Electrical Systems and Controls 2 — Questions and Answers
Question 1: What is the purpose of a time-delay fuse in a residential AC disconnect box?
- To allow momentary inrush current during compressor startup without blowing (Correct answer)
- To slow down the circuit breaker response for safety
- To reduce voltage spikes during normal operation
- To prevent nuisance tripping during brownout conditions
Correct answer: To allow momentary inrush current during compressor startup without blowing
Time-delay fuses tolerate the high inrush current drawn when the compressor motor starts without opening the circuit.
Question 2: A residential condenser fan motor has three leads labeled common, start, and run. Which winding has the highest resistance?
- Start winding (Correct answer)
- Run winding
- Common lead
- Both windings are equal
Correct answer: Start winding
The start winding is made of thinner wire with more turns, giving it higher resistance than the run winding.
Question 3: When measuring amperage on a dual-voltage (208/240V) compressor operating at 240V, the current draw is 18A. What would you expect if the same compressor runs at 208V?
- Higher amperage (Correct answer)
- Lower amperage
- Same amperage
- The compressor cannot run at 208V
Correct answer: Higher amperage
At lower voltage with the same load, current must increase to deliver equivalent power (P = V × I).
Question 4: Which component in a residential AC control circuit prevents the compressor from restarting immediately after it shuts off?
- Time-delay relay (anti-short cycle timer) (Correct answer)
- High-pressure switch
- Capacitor
- Contactor coil
Correct answer: Time-delay relay (anti-short cycle timer)
An anti-short cycle timer enforces a minimum off time (typically 3–5 minutes) to allow refrigerant pressures to equalize before restart.
Question 5: A homeowner reports the AC runs but does not cool. You measure 240V at the contactor load side and hear the compressor hum briefly then shut off. The MOST likely cause is:
- Open start capacitor (Correct answer)
- Shorted run capacitor
- Faulty thermostat
- Low refrigerant charge
Correct answer: Open start capacitor
A failed start capacitor prevents the start winding from producing the torque needed to start the compressor, causing it to hum and trip on thermal overload.
Question 6: What does a megohmmeter (megger) test on a compressor motor measure?
- Insulation resistance between windings and ground (Correct answer)
- Winding resistance in ohms
- Capacitor microfarad rating
- Supply voltage quality
Correct answer: Insulation resistance between windings and ground
A megohmmeter applies high DC voltage to measure insulation resistance, revealing moisture or degradation in motor windings.
Question 7: On a residential split system, the 24V control transformer secondary is protected by an inline fuse. If the fuse blows repeatedly, the FIRST diagnostic step should be:
- Check for a short circuit in the low-voltage wiring (Correct answer)
- Replace the transformer with a higher VA rating
- Install a larger fuse
- Check refrigerant charge
Correct answer: Check for a short circuit in the low-voltage wiring
A repeatedly blown control fuse indicates an overcurrent condition caused by a short or ground fault in the 24V control wiring or components.
313D Electrical Systems and Controls 2
What is the purpose of a time-delay fuse in a residential AC disconnect box?