In this system a variable-frequency drive (VFD) sends AC electrical power to an induction motor to control the speed of that motor.
The VFD receives its “command” signal in the form of a 4-20 mA DC current sent from a programmable logic controller (PLC) with an analog output card, 4 mA representing a “zero-speed” signal (no power sent to the motor) and 20 mA representing a “full speed” signal (60 Hz power sent to the motor):
Problem on PLC HMI VFD
Unfortunately, though, there is something wrong with this system. The pump does not run, regardless of what the operator commands using the touch-screen panel. When you examine the VFD faceplate, you see a few LED indicators lit, but nothing either confirming or denying that power is reaching the motor.
Supposing the only test equipment available to you is a digital multimeter (DMM), what diagnostic tests could you perform to identify the location and nature of the system fault?
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Credits: Tony R. Kuphaldt