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Problem on PLC, HMI, VFD, and Motor Circuit

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

Problem on PLC, HMI, VFD, and Motor Circuit

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?

Share Your ideas on solving the problem with us through comments.

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Credits: Tony R. Kuphaldt

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8 comments

Sunderasen Thangarajan February 12, 2020 at 5:08 pm

Hi, may I suggest that d whole power is to b shut down including its main switch, wait for 30seconds then restart by switching d main switch to on to enable d system restart

Reply
P Maruthi Prasad February 12, 2020 at 5:35 pm

here is the steps to find out the cause

1. Need to Measure 3 phase supply is coming to VFD or not.

2. If 3 phase supply is available to VFD, need to turn on the motor through HMI , check 3 phase output is reaching to motor or not

3. If 3 phase output is reaching to motor then VFD is fine and something wrong with motor

4.If 3 phase output not reaching to motor then something wrong with VFD or power cables between VFD output terminal & motor input terminals

5. After tun the motor ON from HMI Check whether VFD output is giving output or not

6. If the VFD output is not giving output then something wrong wth VFD parameters settings

7. If VFD output is giving output & power not reaching it to motor then need to check power cables between VFD & motor.

Reply
Ernest Buckner February 12, 2020 at 7:40 pm

First I would use DMM to check if the VFD is getting it’s start signal from HMI, probably 120v from PLC digital output card.

If that is good I would then use the DMM to check VFD input for 4 to 20 ma. Need to make sure the 4 to 20 ma analog output card on PLC is good. Then I would use the DMM to check the fail to start relay to see if it is tripped.

If that is good I would use the DMM to check for 120 V at the motor starter relay. If the motor starter relay has voltage I would use DMM to check to see if 480 V is being applied to motor.

Reply
Muhammad Farhan Baluch February 12, 2020 at 9:10 pm

Inspect for any error on VFD or HMI. If no alarm present.

Do following step.

1. Check incoming AC voltage using DMM at VFD panel.

2. Verify control loop for start and stop signal and speed input to VFD. If it is normal and all signal are recieving at VFD termination point then only thing to check is VFD settings and parameters.

Reply
Mohamed said February 13, 2020 at 2:24 am

1- 1st of all we must know if the enable signal reach the VFD or not either by DO from the PLC or by enable key in the VFD (according to your wiring).

2- For the 4-20ma you can check the VFD LCD you must find the equivalent HZ to this analog signal.

3- you must check VFD configurations such as the enable signal, run soure fixed speed or 4-20ma.

4- By the DMM you can check the 3 phase input voltage to the VFD if you found it and not found any voltage in the output of the VFD to the motor. So the problem in the VFD, and you must check by following the A/M points.

Reply
Ranga Saravana February 13, 2020 at 10:47 pm

The output voltage and input voltages can be measured using the multimeter.we can know the faults with the power supply of there is any else there might be a problem with the output of the PLC it can be measured using the multimeter in DC voltage mode.
If any of these fails there might be a problem with PLC or the VFD itself.

Reply
singi February 19, 2020 at 4:58 pm

Make sure polarity of 4-20 is correct. simple stuff first. then you can start trouble shooting the higher voltage.

Reply
Gopi February 21, 2020 at 10:26 am

First check analog o/p dcs coming 25% check mA, Note amps and check on command ofter all are ok main supply off wait some time after-on test all, check all interlocks healthy and control supply check.

Reply

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