Minimum Earth Cable Size for Testing

As you have indicated that the motor cables are not screened, adding an earth wire from the VSD out to the motor is not really going to accomplish anything. The earth is only being used as a protective device, and as such you will (likely) only have a problem with your existing motor earth if the earth resistance between the motor and earth is too high. You may run into problems (or already have them) with electrical noise being generated by the new VSD, especially since no screening is being used. This may or may not require you to replace the cable with a screened version (this will really depend on the new drive, local electrical regulations, susceptibility of nearby electrical equipment, et).

What is the length of motor cable between the VSD and motor?

I understand that having screened cable will be the way to go, unfortunately, there are simply too much legacy issues with the wirings. I am trying to breakdown individual problem into sizable bits so I can rectify them. I understand that the grounding is mainly used as a protective device in case of earth leakage and it should be of lowest resistance possible to allow maximum take away of a ground fault. But as you mentioned, the VSD may have generated electrical noise that affected other electrical instruments, which by the way, is the case now. The option of separating the ground cable is simply to isolate the motor from the instrument. In doing that, I can only hope to see a slight improvement which should sufficiently give me the backing of asking for funds to do the real work.

The length of cable between VSD and Motor is currently about 120m, considering the run through cable tray.

So what do you think? Assuming there is no ground fault present, would a 2.5mm2 cable serve its purpose as a test?

150m of 2.5mm2 earth wire is going to give you well over 1 ohm of earth resistance. Its just too high, and in the event of a fault the earth wire will most likely blow apart. Even changing to a larger size of earth wire isn't really going to help anything in this particular application.

Lets go back to basics, as there are a number of things you can try first.

First consult your VSD manual. If you don't have it, get it off the companys web site.

Second, look carefully thru the manual for recommended practices and tips for setting up your system (more information should be available on the web site).

Third, look in the programming settings sections for the VSD. There should be some settings with regard to the switching frequency which you can change which should hopefully help or fix the electrical noise issues (remember the VSD chops up the ac waveform before sending it to the motor, hence the generated noise).

These are your best options, and depending on your overall system and VSD, may fix the problem. Other options may include the need for an output choke between the drive and the motor (150m is a long cable run). Consult your manual and the companys website. If your lucky, you may only need to add the output choke and alter the output frequency setting of the drive.

Good luck, and let us know how you get on.

Check the earth cable sizing. In the absence of a detailed calculation, 2.5mm² at first glance looks too small for a prospective live-to-earth current of 360A, and in the event of a fault occurring may work very effectively as a fuse! Any earth cable, as a rough-and-ready guide, should have a cross-sectional area that is at least the same, and possibly larger, than the phase conductors.

Thanks for all the comments and solutions. I have considered reducing the switching frequency but unfortunately, the drive is running at its lowest limit, i.e. 3KHz. I have however, not considered putting in an output choke so that might be a cheaper option to see if it improves the situation. Will post again after some additional testings. Thanks guys

Any chance of moving the VFD closer to the motor, the control wiring won't generate significant noise.

Unfortunately, moving the VSD is not an option. I have got 12 motors in that area and each has got its own drive. The funny thing about the set up is only specific drive in operation cause this woe.

Another thing I noticed (just today) the grounding cables on the motors are tied together before heading back to the switchroom rather than individual grounding leads. The leads are also not grounded back to the drive but rather straight to the building earth strip. I don't think this standard is acceptable anywhere.

Sounds like your well on your way to a solution. Getting the instalation to code shouldn't be too hard to sell, mumble stuff about liability. Are the bad effects close to the drive or the motor? Interesting the old drives don't produce issues. What brands are involved?

Well, I certainly hope the non compliance to standards is enough to get some money coming. The 'bad effects' is closer to the motor though like I said, only when the said drive is in operation.

The drive that is giving me the pain is Dan Foss VLT 5302 while the existing drive is the Delairco (can't remember the model).