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Old three phase motors and Variable Frequency Drives (VFD)

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Forum topic by TopamaxSurvivor posted 04-02-2010 09:31 PM 9353 views 1 time favorited 57 replies Add to Favorites Watch
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TopamaxSurvivor

14875 posts in 2367 days


04-02-2010 09:31 PM

Topic tags/keywords: three phase 3 phase motor vfd

I was working on a job this week where they were adding VFDs to some equipment with 25 year old 3 phase motors. These motors have been in daily use and working just fine since installation. Here comes the risk of using a VFD on an old motor; one of them worked OK, but the other tripped out showing ground fault as soon as the drive was started up. It was temporarily reconnected to line voltage and a new drive rated motor ordered.

Sometimes you win and sometimes you loose using drives on old motors. The motor insulation is seeing voltages as high as 1,000 volts. It was designed for 480. Of course the switching cycle of the transistors can be in the megahertz range. That doesn’t help the older motor either.

I just thought I would mention this for those who are considering old three phase equipment when the live in a single phase world.

That motor would probably run on a mechanical rotophase. It is anybodys guess as to how much longer it would have run. An electronic megaohm meter shows a dead short, but it works on the same principle as the drive. A regular ohm meter shows it being just fine.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence


57 replies so far

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UnionLabel

660 posts in 1891 days


#1 posted 04-03-2010 05:19 PM

Topamax, just a question. Was the power in connection Delta or Wye? Only asking cause I’ve seen some high phase Delta here that does some weird stuff to old machinery. Don’t know that much about it, but I know an industrial electrician who has run into three phase motors that acted just like you explained. Like a dead short.

Same thing, close or not so much?

-- Methods are many,Principles are few.Methods change often,Principles never do.

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TopamaxSurvivor

14875 posts in 2367 days


#2 posted 04-03-2010 07:46 PM

All most all pwer connections are Wye these days, at least here in Water World.

The motor is does have a dead short at the voltages teh drive is butting out.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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dfarr

19 posts in 2123 days


#3 posted 04-04-2010 01:21 AM

VFDs introduce transients with extremely fast voltage rise times into the circuits that can quickly destroy the insulation in older non-inverter duty motors. There have been many motor design improvements since VFDs came on the scene once it was apparent that drives are here to stay. Inverter duty motors have design changes like increased winding slot insulation, increased first turn insulation and increased phase-to-phase insulation to withstand peaks of up to 1600 volts. Another frequent mistake made is slowing older motors down below 1/2 rated speed (<30HZ) which creates overheating due to the motor fan not moving enough air.

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studie

618 posts in 1838 days


#4 posted 04-04-2010 08:32 AM

Your vocabulary is too copious for my immediate comprehension, please illusidade!

-- $tudie

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TopamaxSurvivor

14875 posts in 2367 days


#5 posted 04-04-2010 08:39 AM

I tried to keep it as simple as possible. Bottomline, you are always taking a risk putting a drive on an old motor. Sometimes you wins and sometimes you looses ;-))

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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jerryz

164 posts in 1969 days


#6 posted 04-04-2010 02:03 PM

Ahhhhh, my good friend, unfortunately it is pretty much like tossing a coin in the air and picking heads or tails.

That’s the nature of the beast.

Happy Easter to everyone….

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UnionLabel

660 posts in 1891 days


#7 posted 04-04-2010 04:21 PM

Tanks Tomamax, I wuz just axin’ fur my own persuites.

-- Methods are many,Principles are few.Methods change often,Principles never do.

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ToddTurner

144 posts in 2014 days


#8 posted 04-04-2010 04:26 PM

25 years ago VFD’s were very new. The motors built for that time usually were made for the standard 50-60hz operating range. A good VFD can be infinitely adjusted from 0-120 hz. THose old motors will not take it. Trust me, been there, done that. Im just wondering where you are using a VFD in a woodshop? Im always in for great ideas and i have access to all kinds of drives.

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ondablade

105 posts in 1889 days


#9 posted 04-04-2010 07:53 PM

It may be a little different to the US, but over here in Ireland i’m using a 4kW Omron/Yaskawa V1000VFD to step up the rpm of the 3 phase 50Hz motor on the Bill Pentz dust sysytem i’m building from the standard 2 pole 2850 rpm to the 3,450 rpm you get on the US 60Hz motor the blower was designed for.

The variable speed is incidental so far as i’m concerned, it’ll be set and forgotten. The VFD does have some other advantages however from what i’ve read in recent weeks though:

1. The programmable soft start the VFD almost eliminates the additional current drawn by the motor on start up. (it would draw six times the full line current if started normally DOL or direct on line) This is important for me as i would otherwise risk popping the incoming 62A fuse on my 220V single phase supply.

2. A decent VFD gives very high levels of protection in the event of overloading and so on. It also avoids the need so far as i know for an NVR (no voltage relay – drops the contacts out for safety in the event of loss of supply) in the starting switch as it contains this function.

3. They give more or less full torque right down to the lower RPMs – but you need to be careful about the limited cooling that results from lowered fan RPMs.

The downside my supplier said is that it’s not reckoned to be a good idea to run a cheap eastern or old motor with one, presumably because of the insulation and other extra stresses it can result in if run over or under the stock RPM.

It also happens that for the majority of single phase to three phase VFRs that don’t step up the voltage (i for example am getting 3 phase power at our single phase 220V – instead of the usual 380V) you need a dual voltage motor connected in delta. This isn’t an issue for most modern motors, but older and cheaper motors may not have this facility.

The reason for this is that to get the same HP at 220V as 380V the windings have to handle a much higher current – which in the specific situation where old three phase motors are being run on single phase power via an inverter without a voltage step up transformer is presumably the reason they often quit.

Don’t ask any complicated questions as this is just what i’ve gleaned in the past week – i’m no expert!!! :-)

ian

-- Late awakener....

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jockmike2

10635 posts in 2937 days


#10 posted 04-04-2010 08:06 PM

Also older model Delta Lathes had VFDs on 2hp motors that ran off 110volts. I had a b*ch of a time trying to replace it when it went too. Had to buy a whole different system and run it off a 30 amp ac line. But at least it work and works good now.

-- (You just have to please the man in the Mirror) Mike from Michigan -

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TopamaxSurvivor

14875 posts in 2367 days


#11 posted 04-04-2010 09:09 PM

notottoman You can have the motor checked with a mega-ohm meter. They operate at high voltage. If it doesn’t read close to infinity, the motor will probably not run with a drive.

ToddTurner IMO, drives are getting so cheap, they make roto-phases and converters obsolete. They are so much more versatile.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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stefang

13304 posts in 2025 days


#12 posted 04-14-2010 06:20 PM

It’s all geek to me.

-- Mike, an American living in Norway.

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paleoelectronics

7 posts in 1069 days


#13 posted 11-21-2011 05:34 PM

New to this chat room and although not really a woodworker myself, I am an electrician who helps a lot of my cabinet building and blacksmith buddies with the ancient equipment that they pull out of barns around here. I am trying to find help about a VFD problem Im having with Chinese equipment in a brewery that I’m doing some work on. Its a 4.4 HP 220/480 volt Chinese motor transmission assembly that has been making a clanking sound when wired in the lower voltage ( Delta) configuration. This matches the system voltage which is a delta High leg. The drive operating this motor is a 5HP drive from Automation Direct. The unit operates poorly and will fault out on overload after a short while. However when this same unit is wired in the Wye configuration it operates properly with no noise whatsoever. I was impressed with the expertise I have read in this thread and was wondering if you could help me with this problem before I go bald from scratching my head

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TopamaxSurvivor

14875 posts in 2367 days


#14 posted 11-21-2011 06:17 PM

That is wild!! :-)) I’m not sure, but I would suppose it has something to do with the high swiching ferquency of the drive that is se the ground fault wired delta. Any reason you can’t just wire it wye and forget it?

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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paleoelectronics

7 posts in 1069 days


#15 posted 11-21-2011 06:41 PM

Yeah my intuition on it is that its causing some sort of ringback noise from the motor. A pal of mine has suggested an inductor on the output as a filter but before I start throwing extra parts on the system I would like to know why its doing this. We can and will run it in Wye but there is HP loss when you do that and the german in me tells me that its not proper to run a device differently than its designed for. Seeing as we have several of these out in the field I would rather be prepared for the next time this comes up

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