RSES Electrical Systems & Controls 4 — Questions and Answers
Question 1: An ECM (electronically commutated motor) used in a refrigeration system fails to respond to speed control signals. The technician should first check:
- The supply voltage and control input signal at the module (Correct answer)
- The start capacitor micro-farad rating
- The contactor coil resistance
- The compressor's internal overload
Correct answer: The supply voltage and control input signal at the module
ECM motors require both proper supply voltage and valid control signals (0–10V DC or PWM); verifying these inputs at the control module is the first diagnostic step.
Question 2: Ground fault protection is different from overload protection because ground fault protection:
- Responds to sustained overcurrent in motor windings
- Detects current leaking to ground and trips the circuit very quickly (Correct answer)
- Monitors three-phase voltage balance
- Measures motor insulation resistance continuously
Correct answer: Detects current leaking to ground and trips the circuit very quickly
Ground fault protection detects small amounts of current flowing to ground (typically as low as 5mA for personnel protection) and interrupts the circuit almost instantaneously to prevent electrocution or fire.
Question 3: When using a clamp-on ammeter to measure current in a two-wire single-phase circuit, the technician clamps both conductors simultaneously. The reading will be:
- Doubled compared to single conductor measurement
- Equal to the sum of both conductors' current
- Zero, because the currents cancel each other out (Correct answer)
- Equal to the ground fault current only
Correct answer: Zero, because the currents cancel each other out
When both conductors of a balanced single-phase circuit are clamped together, the equal and opposite currents cancel magnetically, producing a zero reading on the ammeter.
Question 4: A potential relay used for starting a single-phase compressor motor opens the start circuit based on:
- Motor current decreasing to a set level
- Back-EMF voltage across the start winding reaching a set threshold (Correct answer)
- A timed delay after power application
- Centrifugal force from the rotor reaching operating speed
Correct answer: Back-EMF voltage across the start winding reaching a set threshold
A potential relay senses the back-electromotive force (back-EMF) generated by the start winding; when the motor reaches approximately 75–80% of synchronous speed, this voltage rises enough to open the relay coil and disconnect the start capacitor.
Question 5: What color is the grounding conductor required to be in a 120/240V single-phase electrical system per NEC standards?
- White or gray
- Black
- Green or bare copper (Correct answer)
- Red
Correct answer: Green or bare copper
NEC Article 250 requires equipment grounding conductors to be green, green with yellow stripe, or bare copper; white/gray identifies the neutral (grounded) conductor.
Question 6: A refrigeration system's low-pressure control is set to cut out at 10 psig and cut in at 25 psig. The differential of this control is:
- 10 psig
- 15 psig (Correct answer)
- 25 psig
- 35 psig
Correct answer: 15 psig
The differential is the difference between cut-in and cut-out pressures: 25 psig − 10 psig = 15 psig.
Question 7: In a three-phase electrical system, voltage imbalance exceeding what percentage is generally considered harmful to motor operation?
- 1%
- 2%
- 5% (Correct answer)
- 10%
Correct answer: 5%
NEMA standards specify that voltage imbalance should not exceed 1% for optimal motor operation, and imbalance above 2% will cause current imbalance roughly 6–10 times greater, leading to overheating; 5% is the practical threshold where damage becomes significant.
RSES Electrical Systems & Controls 4
An ECM (electronically commutated motor) used in a refrigeration system fails to respond to speed control signals.
The technician should first check: