Motion Control & Drive Systems Flashcards
7 cards from real PLC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motion Control & Drive Systems flashcards as text
What does VFD stand for in the context of industrial motor control?
Answer: Variable Frequency Drive
VFD stands for Variable Frequency Drive, a device that controls AC motor speed by varying the frequency and voltage of the power supplied to the motor.
Which type of motor is most commonly controlled by a VFD in industrial PLC applications?
Answer: Three-phase AC induction motor
Three-phase AC induction motors are the most widely used industrial motors paired with VFDs due to their robustness, efficiency, and low maintenance.
A PLC sends a 4-20mA analog signal to a VFD for speed reference. What does 4mA typically represent?
Answer: Minimum speed (0% or stop)
In the 4-20mA standard, 4mA represents minimum (or zero) speed, and 20mA represents maximum speed; a signal below 4mA indicates a broken wire, providing fault detection.
What is the purpose of an 'acceleration ramp' setting in a VFD?
Answer: To gradually increase motor speed to the setpoint over a defined time
An acceleration ramp gradually increases motor speed from zero to the commanded setpoint, preventing mechanical shock and excessive electrical inrush current at startup.
Which communication protocol is commonly used for digital control between a PLC and a VFD in modern industrial systems?
Answer: EtherNet/IP or PROFIBUS
Industrial fieldbus protocols such as EtherNet/IP, PROFIBUS, or Modbus RTU are standard for PLC-to-VFD digital communication, enabling speed commands, fault readback, and status monitoring.
Which motor parameter does a VFD NOT directly control during normal operation?
Answer: Motor winding insulation resistance
Motor winding insulation resistance is a maintenance metric measured with a megohmmeter during offline testing; VFDs control electrical parameters such as frequency, voltage, direction, and ramp rates.
In a PLC-controlled application, why is a 'dynamic braking resistor' added to a VFD system?
Answer: To dissipate regenerative energy as heat when decelerating large inertial loads
When decelerating a large inertial load, the motor acts as a generator feeding energy back to the VFD's DC bus; a braking resistor dissipates this energy as heat to prevent DC bus overvoltage faults.