ECN Forum Forums General Topics of the Trade General Discussion Area Phase converter question

Some things to be aware of with Variable Speed Drives.

1) The motors may need higher voltage rated insulation due to the harmonics present on the output of the VFD.

2) The distance between the VFD and the motor is usually limited to less than 100 feet.

3) Depending on the design of the VFD, you MAY need inductors either on the input to the VFD or between the VFD and the motor.

4) The motor may need to have insulated bearings installed to stop the induced current on the shaft from arcing thru the bearings to frame ground.

5) You MAY need special cabling between the VFD and the motor.

I am not trying to discourage you from using VFDs, I am just trying to make you aware of possible pitfalls.

Larry C

Your "keyword" is (may). Everything you mention is a possibility. He "may" need to address some of these possible issues. But he "may" not.
Good post. Everything you mention is possible.

You WILL need screened cable between the output of the VSD and the motor.

I've seen this time and again with RFI problems where the installing sparky just used Romex-type cable, do it once, do it properly.

We have a type of cable over here that is called Varolex, it is made specifically for VSD's, I'm not sure if it is available in the US, but using anything less is rather silly.

With regard to the list of things above, the problems with harmonics, bearing burn, etc, got ironed out a couple of years ago, there were a few VSD manufacturers that nearly got sent to the wall, after these things were picked up.

I mean, a factory that has a heap of these things installed, wants action NOW, not wait until the next best thing comes off the production line.
Thankfully, a lot of motors and VSD's were replaced under warranty ( some were out of it too).

But, if you get a brand-new VSD these days, I seriously doubt that (if installed to the manufacturers specs) you'd have any issues.

One caveat though, these devices are certainly not a plug and play thing, there is usually a pretty thick manual that comes with them, this should be read thoroughly (avoid the usual safety warnings garb at the front).

Most of the VSD faults I have ever been sent to have been caused by either the thing not being installed properly or it had not been set up appropriately for it's application (ie: the settings for start up or ramp-down were too slow or too fast).
Just bear in mind, at the end of the day, even with all that flash control system on the front end, you are still using a motor at the other end, VSD's cannot perform miracles.

I know the drive manufacturers and wire manufacturers are making a killing on these new output cables. I am sure they work very well. But, I have installed and repaired hundreds of VFD's in my career. THHN with line and load reactors always did the trick for me. You really do not need the load reactor unless the distance from the drive to the motor is very long. Like over 100'.
Never mix line and load conductors in the same raceway. Always run control logic cables separately in there own raceway. Always program the carrier frequency as low as possible. Buy quality drives. Price is not always the indicator.

In many parts of the world, it is a common practice to use cables (think SO cord) instead of 'pipe and wire'. For all but a very few instances grounded metallic conduit serves the same purpose as screened cable.

Yes, but it (the conduit) is more expensive.