Reliance DC Flexpak 3000 drives causing motor bearing failures

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We have been using Reliance Flexpak 3000 DC drives to supply the
conveyance drives on our machines for about 7 years now and have
experienced frequent motor bearing failures due to voltages being
induced into the rotor and discharging through the tail bearing. I
realize this is a common problem in the drive industry but we also use
many other types of drives AC PWM, AC Sensorless Vector Drives, And
Minpak DC drives from many other manufacturers including Allen Bradley,
Lancer, Toshiba and we have not seen the bearing failures with these
other drive systems.
Does anyone have any suggestions to prevent these effects and
limit the voltages induced. We have run tests by isolating motors and
found 70 VAC on the motor casing to ground. In talking with Reliance
support they suggested building a filter on the output of the drive to
smooth out the waveform but they couldn't spec out capacitor and
inductor values because each filter application would be speed, load
dependant.
Thanks in advance for any help

 

[Cameron Dorrough]

We have been using Reliance Flexpak 3000 DC drives to supply the
conveyance drives on our machines for about 7 years now and have
experienced frequent motor bearing failures due to voltages being
induced into the rotor and discharging through the tail bearing. I
realize this is a common problem in the drive industry but we also use
many other types of drives AC PWM, AC Sensorless Vector Drives, And
Minpak DC drives from many other manufacturers including Allen Bradley,
Lancer, Toshiba and we have not seen the bearing failures with these
other drive systems.
Does anyone have any suggestions to prevent these effects and
limit the voltages induced. We have run tests by isolating motors and
found 70 VAC on the motor casing to ground. In talking with Reliance
support they suggested building a filter on the output of the drive to
smooth out the waveform but they couldn't spec out capacitor and
inductor values because each filter application would be speed, load
dependant.
Thanks in advance for any help

Hiya. Yes, you have a problem. When motors started failing around 1997,
VVVF drive manufacturers finally did something about it. In a Technical
Paper I have from PDL Electronics, dated 13 August 1999, it states "the
coupling mechanisms are usually attributed to dissymmetry effects and
homopolar flux effects within the motor design or electrostatically induced
rotor charge accumulation coupled from the load.". Cute.

There are a couple of ways to get around this:
1. Fit a filter, as you say.
2. Change the motor drive-end bearing to an insulated one.
3. Replace the drive.

For the drive to be showing these sort of problems it must be getting on 10
years old and you probably can't get parts for it any more.

My advice - just replace the entire drive with a "modern" one and be done
with it. Building a filter is a waste of time and money.

Good luck,
Cameron

 

[C What I Mean]

Yaskawa is just introducing a G7 drive that is supposed to get rid of this
problem. I sell Yaskawa and I just received a flyer for a seminar about
them. I haven't seen one yet, but from the info I received, they are the
next hottest thing to hit the market. They have a lot of other features you
may find helpful. I don't think the G7 is on the Yaskawa web site yet....
but it should be soon.

Craig

 

[Cameron Dorrough]

Yaskawa is just introducing a G7 drive that is supposed to get rid of this
problem. I sell Yaskawa and I just received a flyer for a seminar about
them. I haven't seen one yet, but from the info I received, they are the
next hottest thing to hit the market. They have a lot of other features you
may find helpful. I don't think the G7 is on the Yaskawa web site yet....
but it should be soon.

I don't know of any drives currently on the market that DO have this
problem. Is Yasakawa one of them, perchance? Personally, I've not heard of
them..

Cameron

 

[C What I Mean]

This phenomena of bearing pitting is indicative of VFDs with PWM output
sections... which is just about all of them. Filters, grounding brush kits,
or things like conductive grease or isolated bearings will help. It shows
up with some motors and not with others. It is hard to predict in all
cases. But, when the problem comes up, it is a combination of the PWM and
the motor as well as other factors. We use lots of VFDs and have never had
that problem with our systems that I know of..... But a few people we do
service for have the pitting regardless of what drive they use.

I don't believe it is a 100% predictable behavior given field wiring and
conduits and so on... But I may be wrong.. I haven't paid a lot of attention
to it because it has not been an issue for us to date.

Yaskawa is the manufacturer of many private labeled drives. They made EMS,
Magnetek, IDM, and Saftronics until recently. They are a very large
manufacturer of VFDs. Good drives as well. Anyway.. the reason I bring it
up at all is because of the seminar about their new drive. It has a
different type of output configuration supposedly. It might be worth
checking out. I will know more in a few weeks.

 

[C What I Mean]

I must apologize. I didn't read your post very well. I was talking about
pitting problems with respect to VFDs. You were asking about a DC drive.

I was half asleep... sorry about that.

 

[Cameron Dorrough]

To sum up: DC Drives started life as PWM and then became VVVF in line with
their AC counterparts. Initially, drive manufacturers didn't fit output
filtering (smoothing) for simple cost reasons and, in truth, on some of the
older motor designs you probably didn't need it. Going from PWM to VVVF
just made the problem more complex.

But, as time passed and more and more customers reported problems, they
fitted finally filters to the output as a matter of course. I repeat my
statement that I don't know of any drives currently on the market that have
this problem.

Cameron