313D Electrical Components and Wiring 2 — Questions and Answers
Question 1: What is the purpose of a start capacitor in a single-phase AC compressor motor?
- Reduce running current draw
- Provide a phase shift to boost starting torque (Correct answer)
- Protect the motor from voltage spikes
- Regulate motor speed during operation
Correct answer: Provide a phase shift to boost starting torque
A start capacitor creates a phase shift in the start winding current, producing the torque needed to overcome inertia and start the motor.
Question 2: Which wire color is designated as the common conductor in a standard residential thermostat wiring scheme?
- Red (R)
- Yellow (Y)
- Green (G)
- Blue or Black (C) (Correct answer)
Correct answer: Blue or Black (C)
The C (common) wire, typically blue or black, completes the 24V control circuit by providing the return path to the transformer secondary.
Question 3: A residential air conditioning system uses a 240V/24V step-down transformer. If the primary side shows 240V but the secondary shows 0V, the MOST likely cause is:
- Shorted secondary winding
- Open primary winding or blown fuse (Correct answer)
- Overloaded control circuit
- Incorrect transformer sizing
Correct answer: Open primary winding or blown fuse
An open primary winding or a blown fuse on the primary side will prevent current flow and result in zero secondary voltage output.
Question 4: What is the standard voltage range for the low-voltage control circuit in residential HVAC equipment?
- 12V AC
- 24V AC (Correct answer)
- 48V DC
- 120V AC
Correct answer: 24V AC
Residential HVAC control circuits operate at 24V AC, supplied by the system transformer, to power thermostats, relays, and contactors.
Question 5: A contactor coil measures infinite resistance with an ohmmeter. This indicates the coil is:
- Shorted and drawing excess current
- Open and will not energize (Correct answer)
- Functioning normally
- Grounded to the cabinet
Correct answer: Open and will not energize
Infinite resistance on a coil indicates an open circuit, meaning no current can flow through the coil and the contactor will not pull in.
Question 6: When sizing wire for a residential condensing unit, which document provides the minimum circuit ampacity and maximum overcurrent protection?
- Unit installation manual and nameplate data (Correct answer)
- Local building department standard tables
- NEC Article 210 branch circuit rules only
- Manufacturer's suggested retail guidelines
Correct answer: Unit installation manual and nameplate data
The unit nameplate and installation manual list MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protection), which must be used for wire and breaker sizing.
Question 7: A run capacitor rated 45+5 MFD is used with a condenser fan motor and compressor. What do the two capacitance values represent?
- Startup and shutdown capacitance
- Compressor and fan motor capacitance respectively (Correct answer)
- Primary and secondary winding values
- Capacitance at high and low ambient temperatures
Correct answer: Compressor and fan motor capacitance respectively
In a dual-run capacitor, the larger value (45 MFD) serves the compressor and the smaller value (5 MFD) serves the condenser fan motor.
313D Electrical Components and Wiring 2
What is the purpose of a start capacitor in a single-phase AC compressor motor?