Journeyman Electrician Exam (NEC)Control Devices and MotorsEasy
A client requests a motor control system that provides soft starting and stopping, precise speed regulation over a wide range, and improved energy efficiency for a 100 hp pump motor. Which type of motor control device best meets these requirements?
- AReversing starter
- BAcross-the-line starter
- CVariable frequency drive (VFD)
- DReduced voltage starter
Show answer & explanationAnswer & explanation
Correct answer: C. Variable frequency drive (VFD)
A Variable Frequency Drive (VFD) is specifically designed to provide soft starting and stopping, precise and continuous speed regulation, and significant energy savings, especially with variable torque loads like pumps.
Why the other options are wrong
- A. A reversing starter only changes motor direction and offers no speed control or soft start/stop.
- B. An across-the-line starter provides no soft start/stop or speed control.
- D. A reduced voltage starter provides soft starting but no speed control or soft stopping.
Variable Frequency Drive (VFD)
A Variable Frequency Drive (VFD) is an electronic device that controls the speed of an AC motor by varying the frequency and voltage of its power supply.
- Provides soft starting and stopping.
- Enables precise speed control.
- Improves energy efficiency, especially with variable torque loads.
- Requires specialized motor and cabling for optimal performance.
Memory trick: Control Choices: Simple, Gentle, Reversible, or Total Speed Command.