Test meters have trouble reading VFD output voltage because an AC motor prefers sinusoidal power, but a variable frequency drive’s output voltage is NOT sinusoidal. As shown below, a VFD’s output voltage is a series square pulses which range from wider to thinner to emulate a sinewave. The following image represents typical voltage output from a VFD.
VFD Voltage Output is the Culprit
These square pulses can cause many testers/multimeters to give illogical results. The peak of the pulses is always the same. Certain meters will show little change in voltage as the motor speeds up. The average voltage to the motor is increasing but the meter is reacting mostly to the peak of the pulses.
To eliminate the problem of test meters having trouble reading VFD output voltage, use only meters with built-in low-pass filters. Such meters can read VFD output voltage properly.The motor’s strength is a function of the fundamental voltage applied and special meters are required to read the that value properly.
Note: A variable frequency drive’s output pulses can, in some instances, can cause damage to a motor, especially the bearings. This condition is sometimes called a “compatibility issue.” and will be covered in a forthcoming blog.
To learn more facts about variable frequency drives, download our 5 Basic VFD Fact Guide.