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1950's motor and VFD-- any insulation concerns?

J_R_Thiele

Stainless
Joined
Jan 22, 2003
Location
Columbia Missouri
I just got a bridgeport with a J head "pancake motor- built in 1954.
I was thinking ov using a TECO FM 50 to power it. Does the old insulation present a problem- or a possible problem? I have read the early VFDs had voltage spikes leading to concerns. How are the newer VFD's?

(I do plan on using the belts to may major speed changes)
 
Most of the "voltage spike" issues you have heard about are in relation to motors using higher voltages, i.e. 380 - 600V. The insulation used on older motors was typically rated for 1000V peaks, plenty sufficient if your line was 480V because the old rule was, you needed to be able to handle at least 1.5X the line voltage. But VFDs can cause what is called "standing wave generation" because of the high speed switching of the transistors they use. Depending on a number of other factors, the peaks of these waves can reach to over 3X the line voltage. So on a 480V supply, that comes to over 1500V, far more than the insulation could withstand. Newer motors are now using insulation rated for 2000V.

But on a 240V system, the peaks are no more than about 800V, still below the capability of that original 1000V insulation. One caveat though is that insulation degrades over time, so that 50 year old insulation may no longer be even 1000V capable. You could test it with a 1000V megger to see for sure, or take it to a motor shop and have them do it for you. If it fails, have it rewound with new inverter duty magnet wire and fuggetaboudit. Or you could hook it up and run it until it fails, then get it rewound. Who knows, it might last 5 more years like this.
 
I'd worry about it IF it pukes.

People have been using VFDs on old motors ever since VFDs were invented. Failure can happen with a VFD, but it can ALSO happen just using the motor on utility power. Sometimes it can cause premature failure of the insulation. Usually they work fine without issues. My opinion is if it fails, it was probably not in good shape to begin with. IF it doesn't, that's a good thing. So, use it until it fails, which may not ever happen. If it fails, get it rewound with the best wire they offer, or replace the motor with a better one. Bridgeports didn't use crappy motors, so don't worry about it. :cheers:
 
I have done some significant work on insulation systems in motors recently.....

It turns out by actual test that most any motor will take 1.5kV to 2kv phase-to-phase IF the insulation is correctly in place.

Often in newer motors it is knocked out of place as the coils are hammered down (yes, they do that) and the wire insulation will fail where the main insulation is missing.

OLD motors were usually more carefully built, and unless the insulation has partly carbonized from long use, it will probably stand up to anything a low voltage inverter will give it. Or a higher voltage, if your wires are not long...

Old insulation relied more on spacing (thickness) and varnish. new insulation relies more on the properties of the material. So an old motor probably has thicker insulation, and a good varnish impregnation.

That will generally do fine. Better that than a new motor with the insulation out of kilter.

usually an "INVERTER DUTY" motor is one that has passed a slightly more stressful test......it isn't usually made ANY differently. The others either failed the 'enhanced test", or were not tested.

I DO know about the "enhanced" testing, but I am not at liberty to comment on that, other than to say that it is a good test for the purpose, and a motor that 'fails" is not damaged.

"Inverter duty" wire is largely "pixie dust". The motor manufacturers won't like to admit that outright to the motor-buying public, but privately they do.
 
The booklet that biddle supplies with their meggers has an excellent discussion about
insulation quality, and the how it behaves under test.

A megger test on an older motor reveals quite a lot, but the other thing to remember
is that leakage is pretty much a direct function of temperature. If the temperature
goes up, the leakage will as well. The reasons for this are actually pretty fundamental
but the result is that if insulation leakage is marginal at room temperature, it is
worth heating the motor to operating temperature and re-testing.

It sounds like the motors that fail the 'inverter-duty' test don't actually do so
in a puff of smoke or a shower of sparks.

:)

Kinda makes me look twice at a motor I would buy, that *isn't* inverter duty rated!

Jim
 
I have not been able to find any evidence that 230volt "legacy" motors fail any more frequently when powered from a VFD. As a matter of fact I found one study that showed that 230volt "legacy" motors fail less frequently when run from a VFD. Why the rate of failure would be lower is a matter of speculation.

One source has done some fairly extensive testing on legacy motors found that the insualtion is better than current standard motor offerings and many met inverter duty ratings.

JST is correct that inverter rated motors are not made any different than standard motors. All of the motor bodies are tested after winding and the ones that "pass" go get special paint and a sticker and a higher price tag. All the others get sold as "standard" motors. That bit of information came from a former supervisor who worked for a well known motor manufacturer.
 
Just like a 1N200X diode...

Hee hee... "Inverter-Duty"... "Enhanced Testing"...

Yeah, just like the 1N200X series diodes... All parts made the same, the ones given a 1N2007 simply passed the 1kv test... while the lower specs (1N2004, etc) leaked at lower PRVs... otherwise, same dude.

Well, totally avoiding the scientifics- I took my '58 BRJ mill, and cut the 3-phase plug off. Wired it direct to an Allen-Bradley AA08A (bulletin 1305?)'s A_B_C terminals, hooked in a 20K pot, a direction/stop switch, and two 100-ohm 500w wirewound resistors (to the DB terminals, and have been running it on single-phase 240v (L1, L2) for the last eight months. I put the motor belt on center sheave (so the sheaves are closest to equal sizes) and haven't done anything but cut metal since.

Works great, hasn't smoked.

Only think I'd really be concerned about (in terms of using a non-such-rated 'inverter' type... is running it slow and not having it provide itself with enough cooling airflow. In that case, rip out the original fan, and put a decent-flow 'muffin' type fan on the housing, make a suitable shroud to direct cooling air into the motor.

Also note, and I'm not certain how much it really matters... Bridgeport used to heavily advertise it's method of machine cooling... motor cooling air went through the head in many places to keep the various drive segments cool. Perhaps it mattered, and perhaps it only mattered when trying to split ten-thousanths... I don't know, but be aware of it anyway.
 
To answer the OP: I've done it, I do it, and will do it again if thats what I have to work with.

Something I haven't seen mentioned ( ok, Jim and Dave touched on it ) is insulation breakdown temps.
Growing up as a wee apprentice I was told "If you can't leave your hand on that motor, there is a problem." Now days we have insulation made to run all day at 100°C+

A VFD can "encourage" a motor to run hotter (I'll let some one smarter than me blabber about how.) I don't offer this as something of great concern, but something to keep in mind if you plan on working a motor real hard.

FWIW
Doug S.
 
But VFDs can cause what is called "standing wave generation" because of the high speed switching of the transistors they use.

Here's good link to what causes insulation failure in a motor on an IGBT PWM VFD. yaskawa.com/site/dmdrive.nsf/(DocID)/MNEN-5JFQPL/$File/AR.AFD.05.pdf

This is a newish phenomenon and you're just not going to see it in a home shop environment. Now in an industrial setting, yes. Essentially, the fast rise times of the IGBT devices, coupled with motor leads in excess of ~50' can lead to excessive voltage overshoot and corona discharge. Read the PDF for more, easy to understand info. In most applications, the output line reactors people put on their retrofits are totally unnecessary.

As far as the differences between inverter duty and standard duty, I only know that we were seeing failures prior to the introduction of the common inverter duty motors once the later generations of VFDs were introduced. Usually the insulation had holes in the first three (outer) turns of the windings where the motors were long wire distances from the drives. Are all of the motors basically the same now? Probably. My favorites are the TENV Inverter Duty motors simply because I don't have to worry about how slowly it's turning (or not!) at 100% torque. And yes, it's sweet.
 








 
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