CEA CEA Motor Controls and Control Circuits Questions and Answers 2 — Questions and Answers
Question 1: In a forward/reverse motor control circuit, what must be used to prevent both forward and reverse contactors from energizing simultaneously?
- A time-delay relay
- Mechanical and/or electrical interlocking (Correct answer)
- A phase monitor relay
- A selector switch only
Correct answer: Mechanical and/or electrical interlocking
Interlocking (both mechanical between the contactor frames and electrical using normally closed auxiliary contacts) prevents both contactors from closing at the same time, which would cause a short circuit.
Question 2: What does a time-delay relay (TDR) set for 'on-delay' do?
- It closes its contacts instantly when energized, then opens them after a set time
- It waits a preset time after being energized before changing contact states (Correct answer)
- It opens its contacts instantly when de-energized
- It delays turning off the coil
Correct answer: It waits a preset time after being energized before changing contact states
An on-delay TDR energizes its coil immediately but waits the set time interval before its timed contacts change state.
Question 3: Which device in a motor control circuit is used to stop the motor automatically when a conveyor reaches a certain position?
- Overload relay
- Limit switch (Correct answer)
- Phase failure relay
- Control transformer
Correct answer: Limit switch
A limit switch is a mechanical position sensor wired into the control circuit that automatically opens or closes contacts based on physical position.
Question 4: What is the typical control circuit voltage supplied by a control transformer in a magnetic starter panel?
- 480V
- 208V
- 120V (Correct answer)
- 24V DC
Correct answer: 120V
Most industrial motor control circuits use a control transformer to step the line voltage down to 120V for safer and more standardized control wiring.
Question 5: A two-wire control circuit uses which type of pilot device?
- Momentary-contact pushbutton
- Maintained-contact device such as a toggle switch or thermostat (Correct answer)
- Selector switch with spring return
- Emergency stop button only
Correct answer: Maintained-contact device such as a toggle switch or thermostat
Two-wire control uses a maintained-contact device that holds the circuit open or closed without operator presence, providing low-voltage release and automatic restart.
Question 6: What is the purpose of a phase monitor relay in a three-phase motor installation?
- To balance loads across circuits
- To protect the motor by tripping if a phase is lost or severely unbalanced (Correct answer)
- To convert three-phase power to single-phase
- To reduce starting current
Correct answer: To protect the motor by tripping if a phase is lost or severely unbalanced
A phase monitor relay detects phase loss, phase reversal, or severe voltage imbalance and opens the control circuit to prevent motor damage.
CEA CEA Motor Controls and Control Circuits Questions and Answers 2
In a forward/reverse motor control circuit, what must be used to prevent both forward and reverse contactors from energizing simultaneously?