313D Electrical Systems and Controls 3 — Questions and Answers
Question 1: A residential AC system uses a dual run capacitor. Which terminals are typically labeled on this component?
- C (common), FAN, HERM (Correct answer)
- L1, L2, and ground
- Start, run, and common
- High, low, and neutral
Correct answer: C (common), FAN, HERM
A dual run capacitor has three terminals: C (common), HERM (compressor/hermetic), and FAN (condenser fan motor).
Question 2: What is the function of a hard start kit (potential relay and start capacitor) added to an existing AC compressor?
- Increase starting torque to help the compressor start under difficult conditions (Correct answer)
- Reduce running amperage during normal operation
- Replace the dual run capacitor permanently
- Provide overcurrent protection to the compressor
Correct answer: Increase starting torque to help the compressor start under difficult conditions
A hard start kit adds extra starting torque, which is helpful when supply voltage is low, refrigerant pressures are unequalized, or the compressor is aging.
Question 3: An ECM blower motor in a residential air handler is different from a PSC motor because it:
- Uses a variable-speed DC motor controlled by an integrated module (Correct answer)
- Requires a larger run capacitor
- Operates on 24V DC only
- Cannot be used with variable refrigerant flow systems
Correct answer: Uses a variable-speed DC motor controlled by an integrated module
ECM (Electronically Commutated Motor) motors use a built-in inverter and brushless DC technology to vary speed efficiently.
Question 4: When using a clamp-on ammeter to measure compressor current, you clamp around only one power conductor. Why?
- To measure the current in one leg without cancellation from opposing current flow (Correct answer)
- To avoid damaging the meter at high amperage
- Because the other conductor carries 24V control voltage
- Both conductors must be clamped for accurate readings
Correct answer: To measure the current in one leg without cancellation from opposing current flow
Clamping around both conductors simultaneously causes the opposing magnetic fields to cancel out, resulting in a near-zero reading.
Question 5: A high-pressure switch in a residential AC system is wired in series with the:
- Compressor contactor coil circuit (Correct answer)
- Condenser fan motor run capacitor
- Indoor blower relay
- Low-voltage transformer primary
Correct answer: Compressor contactor coil circuit
The high-pressure switch opens the control circuit to de-energize the contactor coil, shutting off the compressor when discharge pressure exceeds a safe limit.
Question 6: What reading would indicate a shorted run capacitor when tested with a capacitance meter?
- Reading of 0 µF or OL (overload/short) (Correct answer)
- Reading slightly above the rated µF value
- Reading significantly below the rated µF value
- Infinite resistance reading
Correct answer: Reading of 0 µF or OL (overload/short)
A shorted capacitor shows 0 µF or an OL symbol on a capacitance meter because the internal dielectric has broken down completely.
Question 7: In residential AC electrical schematics, a dashed line between two components typically indicates:
- A mechanical linkage or shared enclosure (Correct answer)
- A high-voltage conductor
- A ground wire
- A field-installed wire by the installer
Correct answer: A mechanical linkage or shared enclosure
Dashed lines in schematics represent mechanical connections or components physically housed together, not electrical conductors.
313D Electrical Systems and Controls 3
A residential AC system uses a dual run capacitor.
Which terminals are typically labeled on this component?