CET Electrical & Control Systems 2 — Questions and Answers
Question 1: What is the primary function of a phase-loss relay in a three-phase elevator motor circuit?
- Regulate motor speed during acceleration
- Disconnect the motor if one phase is lost (Correct answer)
- Balance the load across all three phases
- Provide overcurrent protection for the motor
Correct answer: Disconnect the motor if one phase is lost
A phase-loss relay monitors all three phases and disconnects the motor if any single phase is lost, preventing single-phasing damage.
Question 2: In an elevator control circuit, what does a 'slowdown' switch typically do?
- Stops the car at the terminal floor immediately
- Signals the controller to begin deceleration before reaching a landing (Correct answer)
- Engages the emergency brake system
- Switches the motor from high to low speed winding
Correct answer: Signals the controller to begin deceleration before reaching a landing
A slowdown switch triggers the controller to initiate deceleration so the car arrives at a landing at proper leveling speed.
Question 3: Which type of relay is commonly used in elevator circuits to provide a time delay before re-acceleration after a brief power interruption?
- Latching relay
- Time-delay relay (TDR) (Correct answer)
- Solid-state relay (SSR)
- Differential relay
Correct answer: Time-delay relay (TDR)
A time-delay relay (TDR) holds the circuit open for a preset period, preventing immediate re-acceleration that could cause passenger discomfort or mechanical stress.
Question 4: What is the purpose of a 'door zone' signal in a microprocessor elevator controller?
- Enables the car to level within a defined zone of the floor
- Activates hall lantern indicators
- Triggers the door open command only when the car is within ±3 inches of a landing (Correct answer)
- Inhibits the safety circuit when the car is near a floor
Correct answer: Triggers the door open command only when the car is within ±3 inches of a landing
The door zone signal confirms the car is within approximately ±3 inches of a landing, enabling the door operator to open safely.
Question 5: In a variable-frequency drive (VFD) controlled elevator, what determines the motor's rotational speed?
- The DC bus voltage level
- The output frequency supplied by the VFD (Correct answer)
- The number of motor poles only
- The gate signal to the IGBT modules
Correct answer: The output frequency supplied by the VFD
Motor speed in a VFD system is directly proportional to the output frequency, allowing smooth acceleration and deceleration profiles.
Question 6: What is 'regenerative braking' in an elevator VFD system?
- Using motor friction to stop the car
- Converting kinetic energy back to electrical energy during deceleration or descent of a loaded car (Correct answer)
- Applying the mechanical brake to slow the motor
- Bypassing the drive to use line power during descent
Correct answer: Converting kinetic energy back to electrical energy during deceleration or descent of a loaded car
Regenerative braking converts the motor's kinetic or gravitational energy back into electricity that is fed back to the power grid or a resistor bank.
Question 7: Which ASME A17.1 code requirement applies to the elevator car lighting circuit?
- Must operate on 480V three-phase power
- Must be supplied from a dedicated circuit separate from the driving machine circuit (Correct answer)
- Must be connected downstream of the main line disconnect
- Must automatically switch to battery backup after 30 seconds
Correct answer: Must be supplied from a dedicated circuit separate from the driving machine circuit
ASME A17.1 requires the car lighting circuit to be on a dedicated branch circuit independent of the driving machine to ensure lighting remains on during motor faults.
CET Electrical & Control Systems 2
What is the primary function of a phase-loss relay in a three-phase elevator motor circuit?