CPM CPM - Certified Plant Mechanic Electrical Systems and Controls 2 — Questions and Answers
Question 1: What is the primary function of a contactor in a motor control starter circuit?
- Measure motor speed
- Switch high-voltage power to the motor coil (Correct answer)
- Step down control voltage
- Monitor motor temperature
Correct answer: Switch high-voltage power to the motor coil
A contactor is an electromechanically operated switch that connects and disconnects the main power supply to the motor based on control circuit signals.
Question 2: What does the abbreviation DOL stand for in motor starting methods?
- Direct Output Load
- Direct On Line (Correct answer)
- Differential Operating Level
- Dynamic Operating Loop
Correct answer: Direct On Line
DOL (Direct On Line) starting connects the motor directly to full supply voltage, resulting in high inrush current at startup.
Question 3: A star-delta (Y-Δ) starter reduces motor starting current to approximately what fraction of the DOL value?
- 1/2
- 2/3
- 1/3 (Correct answer)
- 3/4
Correct answer: 1/3
Star-delta starting reduces inrush current to approximately 1/3 of the DOL starting current by initially connecting windings in star configuration.
Question 4: What is the purpose of a control transformer in a motor starter panel?
- Convert DC to AC for the motor
- Step down voltage to a safe level for the control circuit (Correct answer)
- Boost motor starting torque
- Filter noise from power lines
Correct answer: Step down voltage to a safe level for the control circuit
A control transformer steps line voltage down (typically to 120 V or 24 V) to power the control components such as relay coils and pilot devices.
Question 5: Which type of motor control device allows a motor to run in both forward and reverse directions?
- Single-phase starter
- Reversing starter (Correct answer)
- Autotransformer starter
- Soft starter
Correct answer: Reversing starter
A reversing starter uses two contactors wired so that energizing the second contactor swaps two of the three phase leads, reversing motor rotation.
Question 6: What is the main purpose of a motor circuit disconnect switch?
- Adjust motor speed
- Isolate the motor from power for safe maintenance (Correct answer)
- Limit starting current
- Balance phase voltages
Correct answer: Isolate the motor from power for safe maintenance
A disconnect switch provides a visible means to de-energize and lock out the motor circuit before performing maintenance, per OSHA LOTO requirements.
CPM CPM - Certified Plant Mechanic Electrical Systems and Controls 2
What is the primary function of a contactor in a motor control starter circuit?