View Full Version : VFD's...Good or Bad

heavy metal machine
04-26-2007, 10:03 AM
Any of you guys have VFD's on any of your equipment? Do you like them?

The reason that I am asking is because I read an article in the April 2007 issue of Motion System Design magazine about VFD's causing damage to the AC motors that are being used to control by them. The claim is that that the VFD induce voltage that builds up on the motor shaft until it can find a path to discharge through the motor frame. Guess where this discharge path goes through?...yep through the bearings. The conscept is referred to as being similar to a EDM machine at work. The long term effect of this is that the discharge going through the motor bearings is eventually erroding the bearing race and causes bearing failure and yep that means eventually motor failure. The claim is that an AC motor that has a 100,000 hour life expectancy could be reduced to mere 720 hours.:eek: But dont worry because the article does go on to explain a few ways of preventing this damage from happening. I just thought maybe I would make mention of the article to the guys that are using VFD's

04-26-2007, 10:20 AM
The bearing problem was discussed in this thread;


While it is not widespread, it might occasionally present a problem . If it does occur, there are realtively simple fixes for it.

I would not let that keep me from using a VFD, as the benefits outweigh the possible disadvantages. Three phase motors are dead simple to repair, and bearings are cheap (relatively) such that if a problem does arise, it is easily corrected and repaired.

J. R. Williams
04-26-2007, 06:15 PM
For my part a good VFD is the only way to go in a shop where true three phase power is not available. My mill came with a VFD installed and I installed a VFD on my small surface grinder and my lathe. The lathe had a single phase 3 hp motor that I replaced with an inverter rated motor and the improvement in surface fininsh was quite noticeable. I built a static converter for my old milling machine and used rotary converter on my new mill for a while. The mill runs fine on single phase source power with the internal VFD unit. Bearing life should not be a problem with a good motor.


04-26-2007, 06:43 PM
The lathe had a single phase 3 hp motor that I replaced with an inverter rated motor and the improvement in surface finish was quite noticeable.

Same here. I've upgraded several pieces of machinery from single-phase to 3-phase, and there was a marked reduction in vibration, and improvement in surface finish.

The other big advantage with using a VFD is that you get nearly infinite variable speed control.

04-26-2007, 09:21 PM
I just put a VFD on my milling machine, and really wished I would have done it long ago. The motor runs great, and surprisingly cooler than it did with the phase converter.

As far as the bearings being eaten up by currents flowing through them, I think as the others have stated that the benefits far exceed the risk of premature bearing failure. And if you are using it in a home shop, odds are it will last for a very long time before that happens, anyway.

Just be sure to ground your machine well, and you should be fine.

04-27-2007, 05:27 AM
I use Cofer Robotics Equipment. All functions Perfectly.

04-27-2007, 11:50 AM
I'm not worried about it. From the description, "...the VFD induce voltage that builds up on the motor shaft ..." it sounds as though it takes a while for significant voltage to accumulate. Given the way I generally operate my milling machine -- long periods of setup intermittently broken by short periods of actual machining -- I wonder if there would be time for a significant problem to develop.

If a motor is running 8 hours non-stop, that might be another story.

Or maybe I'm misinterpreting what was said....

In any case, in the several years I've had my VFD, I've not noticed any problem.

04-27-2007, 03:33 PM
I like VFDs a lot. Very smooth acceleration on my old belt drive lathe. Most of the time I run only at about 20hz, giving me speeds not possible with just the motor alone.

This whole paranoia of inadvertently EDM'ing bearings makes me wonder if my a belt drive could turn into a Van de Graaf generator and make some sparks. :D

Alistair Hosie
04-27-2007, 03:52 PM
I have rotary and static convertors in both my shops , but I also have a vfd fitted to my milll and another one of my wood lathes for speed control. So I would use this on all my machines but it won't work on my metal lathe but they are very good don't hesitate to buy one .If it will run your machine then go for it they are becoming much cheaper by the month

04-27-2007, 08:31 PM
Please explain to me why a VFD would induce current in the motor shaft and direct it through the bearings? And why this wouldnt happen under normal 3phase line voltage?

Its the same power, especially with the new ultra high frequency carrier digital sine wave generation devices. Just differing frequencies, nothing strange happening with the magnetic fluxes other than normal?

Maybe I dont totally understand the situation but it sounds like an old wives tale to me.

04-27-2007, 09:02 PM
Completely guessing here, but from what I read, it's only a problem on 480+V systems. When you consider that it seems to be tied to higher voltages, and VFDs are often used to run higher frequencies, then I'm guessing it is either induction current or arcing. Any time you run higher voltage, or higher frequencies, particularly combined, both of those (induced voltage and arcing) become more significant. So a 240/480V motor might run just fine on 480 3 phase, but maybe there is not a lot of head room margin, particularly if tolerances (or wear?) stack up the wrong way. Then add in higher frequency and suddenly you start having some issues over time. And "inverter rated" motors probably have a bit more head room to eliminate the issue completely.

So, my guess is that as long as you don't go over 60hz, I'll bet it's no different than line 3ph. Lets see what others have to say...

04-27-2007, 09:20 PM
Here are my 2 VFD horror stories, and I have installed hundreds of drives. At a former employer we installed a 125 HP Mitsubishi VFD and a Lincoln Inverter duty motor, within 3 months the bearings were shot. Upon inspection at Lincoln, they found micro pitting on the bearings due to improper grounding. I am told this is not nearly the problem with newer VFDs and motor as was when Pulse Width Modulated drives were first coming on the market and IGBT were not as good as today. Recently, I installed a 15 Hp drive on a non inverter duty motor for a client, within 2 days the windings were destroyed and the motor suffered turn to turn shorts. (I wish I had photographed the damage). Inspection of the motor by the motor mfg, confirmed it was insulation failure from the VFD's carrier frequency stressing the insulation. What I am told by a major drive maker, is that running a VFD on a non inverter duty motor will eventually ruin the motor. It is a mater of when, not if. It may be many many years or several weeks. What really kills the motor is that the insulation in a non inverter duty motor cannot withstand the high frequency switching the the drive produces to make variable frequency. So, over time small holes appear in the insulation and eventually if holes from different phase are close enough you have a turn to turn short. With that said, I run VFDs on 3 phase motors all the time knowing the risk (and I tell my clients). But, I would never put a VFD on a motor that was special and could not be easily replaced.