Electrical & Control Systems Flashcards
7 cards from real CET practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical & Control Systems flashcards as text
What is the function of an 'up-direction' contactor in a traditional relay-logic elevator controller?
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.
In a microprocessor-based elevator control system, what is a 'serial link' typically used for?
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.
What causes 'nuisance tripping' of the elevator main line circuit breaker?
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.
What is the purpose of a 'motor thermistor' or thermal overload device embedded in the elevator motor windings?
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.
In elevator electrical work, what does 'megger testing' verify?
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.
What is the effect of connecting a 230V elevator control transformer primary to a 208V supply?
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.
What is the role of an 'encoder' on the elevator hoist motor in a closed-loop VFD system?
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.