Dear All,

I have installed one system where S7 200 PLC is installed which is giving PTO output to drive one servo motor. One more normal (not servo) motor installed which is driven by Denfoss VFD.

My problem is, when i start that denfoss drive, PLC output (PTO) malfunctions and that servo behaves abnormal. Seems like some distortion signal is passing. Denfoss drive is connected with PLC using Modbus communication. I already tried with that communication cable removed with no success. Grounding is good. Not sure what to do next

I tried with output cable removed for Denfoss drive. No distortion observed at that time. Seems like signal is distored during running of motor only

This is being happened during Jog/Homming when speed is Low. During Auto mode - normal speed, everything runs smoother

Any Idea / Suggestion??

Hello Vithani;

I do not consider myself a drives specialist, but I have worked in many projects in various industries.. This type of problem has happened to us before, and the most likely suspect would be harmonic distorsion.

An AC drive is composed of 3 sections: an AC-DC converter, a DC bus, and a DC-AC inverter that generates the output variable frequency signal to the motor.

The converter is basically based of a diode bridge and usually uses 6-pulse technology (6 diodes in the bridge), unless you are willing to pay for 12-pulse or even 18-pulse technology (which adds phase transformers to 2 or 3 6-pulse diode bridges). The 6-pulse entry to standard VFDs is common, and relatively cheap, but produces very powerful harmonics (especially the 5th and 7th harmonic signals can be very destructive) which are multiples of the carrier frequency of the diode bridge carrier; this, through the resonant effect, will allow this harmonic power to interfere with the original power source frequency.. These harmonics are reflected back at the power source and can damage the quality of power signals in all equipment that is connected

I believe this is happening to the power circuit of the PLC in your application.

The common solution to this problem, once proper grounding of all electrical equipment has been taken care of,  is to add a harmonic filter (or line reactor) at the input of the power circuit of the drive, to lessen the power of the harmonics produced by the diode bridge of the drive. The link that follows shows a harmonic filter sold by Danfoss for its drives, look at the power spectrums shown on the second page to see how the harmonics can be attebuated.

http://www.danfoss.com/NR/rdonlyres/2B4828C1-170E-4F5C-9631-D4D93DE0AACC/0/High_AHF_MO008A02.pdf

Since I do not know the model and caracteristicss of the drive you use, I suggest you contact your Danfoss dealer to see what harmonic filter he would recommend in your case. But remember that many manufacturers provide harmonic filters, and yiou can select from any one, as long as their specifications meet your drives requirements.

Hope this helps,

Daniel Chartier

Hello Vithani,

the problem has already been explained in detail by dchartier. Nothing more to say about that.

You did not describe your model and the frequency range you use. Mostly drives will be operated with frequencies below 100Hz. If you need a low cost filter, you can create your own. You have to use a low pass with a border frequency of slightly above your max. operating frequency. All you need is a resistor in line to input and a capacitor parallel to input. Select the resistor so, that you will not have too much voltage drop and then calculate the capacitor. (fc = 1/2*pi*R*C)

You can easily experiment with different values. Normally this provides a cheap solution of the problem.

regards