Motor Controls & Variable Frequency Drives Flashcards
7 cards from real Amazon Maintenance Technician practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motor Controls & Variable Frequency Drives flashcards as text
Which VFD parameter must be set to match the motor's rated speed specification?
Answer: Base (rated) frequency
The base frequency parameter (typically 60 Hz in the US) tells the VFD the frequency at which the motor reaches its nameplate rated speed.
What VFD protection feature prevents damage caused by a high DC bus voltage condition?
Answer: Overvoltage (OV) protection
Overvoltage protection monitors the DC bus and trips the drive if voltage exceeds safe limits, which can occur during rapid deceleration or regenerative conditions.
Why is an input line reactor commonly installed on a VFD?
Answer: To reduce harmonic distortion and protect the VFD from line transients
An input line reactor limits harmonic currents drawn by the VFD rectifier and protects against voltage spikes and line notching from the utility.
When performing lockout/tagout (LOTO) on a motor control center, what is the correct first step?
Answer: Identify all energy sources associated with the equipment
Per OSHA 29 CFR 1910.147, the first step in LOTO is to identify all energy sources (electrical, pneumatic, hydraulic, etc.) before any isolation or lockout occurs.
How does a soft starter differ from a Variable Frequency Drive?
Answer: A soft starter only controls acceleration and deceleration, not running speed
A soft starter reduces inrush current during starting and ramps down on stopping, but once at full speed it typically bypasses and runs the motor directly at line frequency.
What does the 'torque boost' parameter in a VFD accomplish?
Answer: Provides additional voltage at low frequencies to improve low-speed starting torque
Torque boost increases the VFD output voltage at low frequencies to compensate for stator resistance and provide adequate starting torque in V/Hz control mode.
A motor rated 460V/3Ph/60Hz running from a VFD at 30 Hz will operate at approximately what percentage of its nameplate speed?
Answer: 50%
Motor speed is proportional to frequency; at 30 Hz (half of 60 Hz), the motor runs at approximately 50% of its nameplate RPM.