NATE Electrical and Controls 2 — Questions and Answers
Question 1: A contactor coil is rated at 24V AC. If the supply voltage to the coil drops to 18V AC, what is the MOST likely result?
- The contactor operates normally
- The contactor may chatter or fail to fully close (Correct answer)
- The contactor coil burns out immediately
- The contactor opens the circuit
Correct answer: The contactor may chatter or fail to fully close
Insufficient coil voltage prevents full magnetic engagement, causing the contactor to chatter or fail to close completely.
Question 2: What is the function of a run capacitor in a PSC (permanent split capacitor) motor?
- Provide starting torque only
- Improve power factor and running efficiency (Correct answer)
- Protect against overcurrent
- Store energy during power outages
Correct answer: Improve power factor and running efficiency
A run capacitor creates a phase shift in the auxiliary winding that improves power factor and running efficiency throughout operation.
Question 3: What does a lockout/tagout (LOTO) procedure require before servicing electrical HVAC equipment?
- Lowering thermostat setpoint to off
- Isolating and de-energizing the power source and verifying zero energy state (Correct answer)
- Wearing rubber-soled shoes
- Switching the system to cooling mode
Correct answer: Isolating and de-energizing the power source and verifying zero energy state
LOTO requires disconnecting and locking the energy source, then verifying zero energy state before any service work begins.
Question 4: On a cooling thermostat, which terminals are energized when the system is calling for cooling?
- R and W
- R and Y (Correct answer)
- R and G
- R and O
Correct answer: R and Y
During a cooling call, the Y (compressor) terminal is energized along with R (power), activating the compressor contactor.
Question 5: What does a megohmmeter (megger) measure?
- AC line frequency
- Insulation resistance of motor windings (Correct answer)
- Capacitor microfarad rating
- Contactor coil impedance
Correct answer: Insulation resistance of motor windings
A megohmmeter applies high DC voltage to measure insulation resistance, revealing breakdown or moisture in motor windings.
Question 6: Which type of transformer is commonly used to step down 240V line voltage to 24V for HVAC control circuits?
- Autotransformer
- Current transformer
- Step-down isolation transformer (Correct answer)
- Step-up transformer
Correct answer: Step-down isolation transformer
A step-down isolation transformer reduces line voltage (240V) to the 24V AC required by HVAC control circuits.
Question 7: A motor draws 8 amps at 240V. What is the approximate power consumption in watts?
- 1,200 watts
- 1,920 watts (Correct answer)
- 960 watts
- 2,400 watts
Correct answer: 1,920 watts
Power (W) = Voltage × Current = 240V × 8A = 1,920 watts for a single-phase resistive load.
NATE Electrical and Controls 2
A contactor coil is rated at 24V AC.
If the supply voltage to the coil drops to 18V AC, what is the MOST likely result?