RSES Electrical Systems & Controls 3 — Questions and Answers
Question 1: A defrost termination thermostat in an electric defrost system is set to open at 55°F. What happens if this thermostat fails in the open position?
- Defrost never initiates
- Defrost runs continuously until the time clock terminates it (Correct answer)
- Compressor shuts off permanently
- The refrigerant system overcharges
Correct answer: Defrost runs continuously until the time clock terminates it
If the defrost termination thermostat fails open, it cannot provide the signal to end defrost early, so defrost runs for the full timer-set duration before the time clock cuts it off.
Question 2: What is the purpose of a time-delay relay (TDR) wired into a compressor control circuit?
- Prevent short cycling by delaying compressor restart (Correct answer)
- Increase starting torque during high-load conditions
- Reduce voltage spikes during compressor shutdown
- Monitor motor winding temperature continuously
Correct answer: Prevent short cycling by delaying compressor restart
A time-delay relay prevents short cycling by holding the compressor off for a set period (typically 3–5 minutes) after shutdown, allowing pressures to equalize before restart.
Question 3: When testing a run capacitor with a digital multimeter set to capacitance mode, a reading significantly lower than the rated µF value indicates:
- The capacitor is overcharged and needs discharge
- The capacitor has lost capacitance and should be replaced (Correct answer)
- The capacitor is functioning normally within tolerance
- The motor winding is shorted
Correct answer: The capacitor has lost capacitance and should be replaced
A capacitor reading substantially below its nameplate µF rating (more than ±6% tolerance) has lost capacitance, reducing motor efficiency and potentially causing overheating.
Question 4: In a two-speed condenser fan motor control scheme, the motor switches from high speed to low speed when outdoor ambient temperature drops. This design primarily:
- Reduces compressor discharge pressure during cold weather (Correct answer)
- Increases system capacity during low-load periods
- Prevents evaporator coil freeze-up
- Reduces the risk of motor winding overheating
Correct answer: Reduces compressor discharge pressure during cold weather
Reducing condenser fan speed in cold weather slows the rate of heat rejection, maintaining adequate condensing pressure and liquid subcooling necessary for proper system operation.
Question 5: A technician measures 120V across a contactor's coil terminals but the contactor does not pull in. The most likely cause is:
- Low supply voltage to the coil
- Open or burned-out coil (Correct answer)
- Shorted compressor motor winding
- Tripped high-pressure switch
Correct answer: Open or burned-out coil
Correct voltage across the coil terminals with no contactor pull-in means the coil itself is open or burned out and cannot generate the electromagnetic field needed to close the contacts.
Question 6: What is the function of a crankcase heater on a hermetic compressor?
- Heat the refrigerant vapor before compression
- Prevent refrigerant migration and liquid slugging during off cycles (Correct answer)
- Warm the motor windings to reduce starting current
- Maintain oil viscosity during high-load operation
Correct answer: Prevent refrigerant migration and liquid slugging during off cycles
The crankcase heater keeps the compressor oil warm during off cycles, preventing liquid refrigerant from migrating into and diluting the crankcase oil, which would cause slugging on startup.
Question 7: A SPDT (single-pole double-throw) thermostat is used in a heat-cool system. In the heating mode, which terminals are typically energized?
- R and Y
- R and W (Correct answer)
- R and G
- Y and G
Correct answer: R and W
In standard HVAC/R thermostat wiring, R is the 24V power, W is the heating output terminal; R-W energizes the heating circuit when the thermostat calls for heat.
RSES Electrical Systems & Controls 3
A defrost termination thermostat in an electric defrost system is set to open at 55°F.
What happens if this thermostat fails in the open position?