Need Help: Noise/Inteference/Grounding Problem VFD

Things like this are usually the result of poor termination and shielding practices. Describe how you have everything wired, both power and control.

For control, we are using 4-core shielded cable from SCADA, which is connected to the terminal of our panel, from there again we are using shielded cables to all VFDs.

We have connected the shield to the ground.

This was a retrofit project, so For Power, we have taken Three phase and neutral wire from the existing system, and connected the ground of all VFDs to the Panel box, which again is connected to the entire ground system.

One doubt, that all of our power and communication shield are grounded together, can this be a problem?

GA from me. In the audio world the signals are even more important to keep clear or you end up broadcasting your ground loop problems as noise, hum or static. Ground the control cables at the control box end, and leave the shield ground disconnected at the power end. That way you do not send power ground fluctuations back into the control gear.

Your problem area is earthing .Communication cable not to be taken along with power cable .communication and control cable shield earthing to be done at one end only

Problem culd be grounding loops. Check your grounding.

Dear Mr.mjain,

You have not mentioned anything about earthing in the system.

The panel Board Casing also to be earthed and the Earthing Flat should be DOUBLE the SIZE of the NORMAL Earthing Flat.

We also had a similar problem. By checking the earth and providing additional Earthing Flat for the casing, the problem what we have faced is solved.

DHAYANANDHAN.S

Thanks Everyone.

We are using the existing earthing done for the system, we don't have any idea about the existing earthing. We also suspect the earthing is flawed.

Can anyone suggest some good technical document for earthing and how can we check if the earthing is proper or not.

Manish

British Standard 7671 - it's a good bedside book, Uncle. Order one for Christmas.

Dear Mr.mjain,

The existing Earthing IS NOT AT ALL SUFFICIENT. Whether it is PERFECT to satisfy the Electrical Requirement.? For name sake - Earthing provided means it will be the problem. Supposing you need one Number of 50MMx6MM G.I. EARTH FLAT, you provide 2 Numbers of 50MMx6mm G.I FLAT and pl. do not forget to wet the Earth Pit daily with water. Also provide a seperate Earth Flat to the MCC PANEL CASING.

As already suggested by the CR4 MEMBER, provide grounding of the Power cable on one side only.

I shall try to provide details seperately as I have to locate in my Data Archives or you can google it.

Keep the Power Cable and Control cable/Data cable - a minimum distance of 1.2 Metres or use cable suitable for Protection against Induction. Last year it was a havoc for us and now totally solved.

IEEE 1100 -The IEEE Recommended Practice for Powering & Grounding Sensitive Electronic Equipment

The common mode voltage between the drives and the scada is higher that the communication voltage. Grounding may help but the basic issue is that the communication links should have isolation at each point in order to avoid this type of problems.

What kind of communication network are you using? Ethernet, RS-485...

Is the drive manufacturer saying anything about communication isolators? Are they built in? Did you bypass them by mistake?

Did you connect the control ground with the power ground? In most drives, the controls are floating with respect to the power. You ground all the controls together at the PLC (scada) independantly from the power ground. This prevents the power noise from affecting the controls by using the "built in" insulation of the drive. You might have to remove a ground jumper on the control terminal to "free" it from the power section.

Thanks All,
Even after getting new earthing done, we are still not successfull in eliminating noise.Following are our observations:(The setup includes 6 VFDs connected in a single Panel)

1. If the COM port of VFDs are disconnected and they are made to run the motor, and one of the VFD is connected with 485 port, there is no noise, IMO , it indicates that there is no Noiise because of power cables going from VFD.
2. If all the COM ports are connected and VFDs are not made run, there is still not any problem of interference.
3. As in point 2, if we make one of the VFD made run, we start getting Noise in the data.We are monitoring noise by seeing the data in Moxa Modbus RTU-TCP converter.

We are running out of options, Please help, on what can we do.
Thanks in Advance,Manish