RSES Electrical Systems & Controls 5 — Questions and Answers
Question 1: A dual-element time-delay fuse is preferred over a standard fast-blow fuse for protecting motor circuits because it:
- Provides lower cost replacement
- Withstands high inrush current during motor starting without nuisance blowing (Correct answer)
- Responds faster to short circuits
- Provides ground fault protection
Correct answer: Withstands high inrush current during motor starting without nuisance blowing
Dual-element fuses have a separate time-delay element that tolerates the brief high inrush current during motor starting while still providing fast protection against short circuits.
Question 2: A solid-state motor protector monitors which parameters to provide comprehensive motor protection?
- Only current draw
- Current, voltage, and phase sequence (Correct answer)
- Only winding temperature
- Voltage only
Correct answer: Current, voltage, and phase sequence
Solid-state motor protectors monitor current, voltage, phase loss, phase imbalance, and often phase sequence to provide complete protection against all common motor failure modes.
Question 3: When replacing a contactor in a refrigeration system, the technician should verify that the replacement contactor has the correct:
- Wire color coding matching the original
- Coil voltage and contact ampere rating (Correct answer)
- Physical size only
- Number of auxiliary contacts only
Correct answer: Coil voltage and contact ampere rating
The replacement contactor must match the original's coil voltage (24V, 120V, 208V, etc.) and contact ampere rating to ensure proper operation and adequate current-carrying capacity.
Question 4: A head pressure control valve in a refrigeration system's electrical controls is used to:
- Protect the compressor motor from overcurrent
- Maintain minimum condensing pressure during low ambient conditions (Correct answer)
- Control the evaporator fan speed based on suction pressure
- Regulate the defrost termination temperature
Correct answer: Maintain minimum condensing pressure during low ambient conditions
Head pressure control valves (or fan cycling controls) maintain minimum condensing pressure during cold ambient operation, ensuring adequate liquid pressure for proper expansion valve operation.
Question 5: An oil pressure safety control (oil differential switch) on a refrigeration compressor will shut down the unit if oil pressure does not build within a certain time after startup. This time delay is typically:
- 5–10 seconds
- 30–90 seconds (Correct answer)
- 5–10 minutes
- 20–30 minutes
Correct answer: 30–90 seconds
Oil pressure differential switches typically allow 30–90 seconds after startup for oil pressure to build; if pressure remains insufficient within that window, the safety control opens the circuit.
Question 6: What is the purpose of a phase-loss monitor in a three-phase refrigeration system?
- Measure power factor of the motor circuit
- Detect loss of one phase and shut down the compressor before winding damage occurs (Correct answer)
- Balance current across all three phases automatically
- Reduce starting current during motor startup
Correct answer: Detect loss of one phase and shut down the compressor before winding damage occurs
A phase-loss monitor detects when one of the three supply phases is lost, immediately shutting down the compressor to prevent single-phasing, which causes the motor to draw excessive current and overheat rapidly.
Question 7: A technician is troubleshooting a refrigeration unit and finds the compressor contactor chattering (rapidly opening and closing). The most likely cause is:
- Undersized compressor motor winding
- Low or fluctuating control circuit voltage to the contactor coil (Correct answer)
- High ambient temperature around the control panel
- Oversized overcurrent protection device
Correct answer: Low or fluctuating control circuit voltage to the contactor coil
Contactor chattering occurs when the coil voltage is too low or fluctuating to maintain the magnetic pull needed to hold the contacts firmly closed, causing rapid opening and closing.
RSES Electrical Systems & Controls 5
A dual-element time-delay fuse is preferred over a standard fast-blow fuse for protecting motor circuits because it: