CWEA Electrical/Instrumentation (E/I) Grade 1 Practice Test

A Variable Frequency Drive is a device that controls how fast an AC motor runs by changing the frequency of the electrical supply, measured in hertz. For AC motors, the speed is tied to the supply frequency through the synchronous speed formula; lowering or raising the frequency directly changes RPM, and the VFD adjusts torque accordingly. Internally, the VFD converts incoming AC to DC and then back to AC at the desired frequency using modulation techniques, giving smooth, controllable motor speed.

The option describing a drive used to vary frequency for AC motors matches this purpose exactly. The other choices refer to unrelated technologies: a method for DC motor speed control (which typically uses voltage or other means, not frequency), a vacuum fluorescent display (a display device), and a valve frequency diagram (not a standard term in motor control).