CET Electrical & Control Systems 3 — Questions and Answers
Question 1: What is the function of an 'up-direction' contactor in a traditional relay-logic elevator controller?
- Energizes the motor for counterclockwise rotation to move the car downward
- Reverses motor phase sequence to drive the car in the up direction (Correct answer)
- Controls the speed of the motor during upward travel only
- Applies holding current to the electromagnetic brake
Correct answer: Reverses motor phase sequence to drive the car in the up direction
The up-direction contactor reverses the phase sequence of the motor supply, causing the motor to rotate in the direction that moves the car upward.
Question 2: In a microprocessor-based elevator control system, what is a 'serial link' typically used for?
- Carrying high-voltage power to the car top junction box
- Communicating data between the machine room controller and the car operating panel (COP) (Correct answer)
- Providing a dedicated safety circuit path for the door contacts
- Supplying DC power to the motor encoder
Correct answer: Communicating data between the machine room controller and the car operating panel (COP)
A serial communication link (e.g., CAN bus or RS-485) transmits button calls, status data, and commands between the controller and the COP, reducing the number of traveling cables needed.
Question 3: What causes 'nuisance tripping' of the elevator main line circuit breaker?
- Excessive motor starting current exceeding the breaker's instantaneous trip rating (Correct answer)
- Low insulation resistance in the traveling cable
- Incorrect phase sequence at the main disconnect
- A failed encoder sending false speed signals
Correct answer: Excessive motor starting current exceeding the breaker's instantaneous trip rating
High inrush current during motor starting can exceed the instantaneous trip threshold of an incorrectly sized breaker, causing nuisance trips.
Question 4: What is the purpose of a 'motor thermistor' or thermal overload device embedded in the elevator motor windings?
- Measure motor speed for the VFD feedback loop
- Detect excessive motor winding temperature and signal the controller to shut down (Correct answer)
- Balance current between motor phases
- Provide a reference signal for torque control
Correct answer: Detect excessive motor winding temperature and signal the controller to shut down
Embedded thermistors change resistance with temperature; when winding temperature exceeds safe limits, the controller detects the resistance change and shuts down the motor.
Question 5: In elevator electrical work, what does 'megger testing' verify?
- Motor shaft alignment and bearing condition
- Insulation resistance of wiring, motors, and cables (Correct answer)
- Correct phase sequence of the main power supply
- Continuity of the safety circuit chain
Correct answer: Insulation resistance of wiring, motors, and cables
A megohmmeter (megger) applies high DC voltage to measure insulation resistance, verifying that insulation has not degraded due to moisture, heat, or age.
Question 6: What is the effect of connecting a 230V elevator control transformer primary to a 208V supply?
- The secondary output voltage will be proportionally lower than design, potentially causing control malfunctions (Correct answer)
- The transformer will overheat and fail immediately
- The secondary voltage will remain at the rated value due to voltage regulation
- There will be no effect because both voltages are within 10% tolerance
Correct answer: The secondary output voltage will be proportionally lower than design, potentially causing control malfunctions
A lower-than-rated primary voltage produces a proportionally lower secondary voltage, which can cause contactors to drop out or PLC inputs to fall below their minimum threshold.
Question 7: What is the role of an 'encoder' on the elevator hoist motor in a closed-loop VFD system?
- Count the number of motor starts for maintenance scheduling
- Provide real-time speed and position feedback to the VFD for precise motor control (Correct answer)
- Monitor motor winding temperature
- Detect the direction of motor rotation for direction contactor logic
Correct answer: Provide real-time speed and position feedback to the VFD for precise motor control
The encoder generates pulse signals proportional to shaft rotation, giving the VFD accurate speed and position data to maintain precise control over acceleration, deceleration, and leveling.
CET Electrical & Control Systems 3
What is the function of an 'up-direction' contactor in a traditional relay-logic elevator controller?