Thread: Mill Motor Runs on Line Power 3P/240VAC But Not on VFD - Why???

...they did a minor rebuild w/o a rewind (bearing, baked to remove moisture, etc...)...

I agree with the others, the gradual loss of torque was because the windings were shorted inside. What you have experienced is a classic consequence of running an old motor on a VFD for 7 years; the motor insulation breaks down INSIDE of the coil windings and you get turn-to-turn shorts, but no line to ground or line to line short, so it does not show up in standard tests. Your motor shop was inexperienced in doing the proper tests for motors used on VFDs. Not that it would have mattered though, the cost to rewind a small motor like that would exceed the cost to replace it anyway.

An inductance meter would show a difference in the windings. If there was a gradual decrease in inductance and the current value was within a certain range then this problem would not be noticed at time 0 but later at time 1. The equivalent circuits for motor windings are complex and short would change more than the inductance.
What prevents me from doing this is lack of a inductance meter, probably something I should fix.

Originally Posted by everettengr

you know a lot more than me regarding motor design and application. i am relying on the info provided by the mfg and the distributor and this motor appears to be exactly what i want. here is the placard. let me know what you think? BTW, it works great so far...
Mark

Attachment 269764

Well now I agree with you. The motor nameplate does not match with the data sheet you provided - the data sheet was quite confusing. You did good on the motor.

Originally Posted by markz528

Well now I agree with you. The motor nameplate does not match with the data sheet you provided - the data sheet was quite confusing. You did good on the motor.

sorry about that. i posted the data sheet given to me by the distributer and did not realize it was wrong. i'm glad to hear the motor is the correct design for my application.

question: should i manually enter the motor data into the VFD or is the auto-detect function in the VFD menu adequate?

Originally Posted by Jraef

I agree with the others, the gradual loss of torque was because the windings were shorted inside. What you have experienced is a classic consequence of running an old motor on a VFD for 7 years; the motor insulation breaks down INSIDE of the coil windings and you get turn-to-turn shorts, but no line to ground or line to line short, so it does not show up in standard tests. Your motor shop was inexperienced in doing the proper tests for motors used on VFDs. Not that it would have mattered though, the cost to rewind a small motor like that would exceed the cost to replace it anyway.

i am very frustrated that the motor shop made this mistake. it sent me on a wild-goose-chase buying another VFD and wasting a lot of time and $$$ looking for problems in the wrong places.

Originally Posted by everettengr

question: should i manually enter the motor data into the VFD or is the auto-detect function in the VFD menu adequate?

I have never worked with that particular drive, but I would be entering the motor parameters manually. Does it have an auto-tune function or just auto-detect? More complex drives would have an auto-tune function but I doubt this drive has that. And its really not needed for this application.

Looks like a Teco N3 VFD. Set A000=0 General Vector. Manually enter the motor data and then run the auto-tune A006=1. The auto-tune does not detect all the motor parameters, only tunes specific parameters that optimizes the VFD/motor performance. So you still need to set things like the voltage, base Hz/speed, poles, rated current, (A001-A007), this varies by drive.

With the new motor you should have a much wider operating range, so it is typical to over speed the motor (probably 120Hz in this case). You still loose torque above the base speed, but usually not an issue since you use smaller cutters at speed. Low end I usually set to 20Hz as the Hp falls off below the base speed and you loose the mechanical advantage of the drive. You still need to stay within the operating speeds of your spindle/bearings. You can also drive an inverter rated motors a bit harder short term depending on the application if needed. They can be pushed to 150-180% for up to 1 minute depending on the VFD/motor. I usually do not go above 150% overload unless I am running a inverter/vector type motor and this is application specific.

Originally Posted by mksj

Looks like a Teco N3 VFD. Set A000=0 General Vector. Manually enter the motor data and then run the auto-tune A006=1. The auto-tune does not detect all the motor parameters, only tunes specific parameters that optimizes the VFD/motor performance. So you still need to set things like the voltage, base Hz/speed, poles, rated current, (A001-A007), this varies by drive.

With the new motor you should have a much wider operating range, so it is typical to over speed the motor (probably 120Hz in this case). You still loose torque above the base speed, but usually not an issue since you use smaller cutters at speed. Low end I usually set to 20Hz as the Hp falls off below the base speed and you loose the mechanical advantage of the drive. You still need to stay within the operating speeds of your spindle/bearings. You can also drive an inverter rated motors a bit harder short term depending on the application if needed. They can be pushed to 150-180% for up to 1 minute depending on the VFD/motor. I usually do not go above 150% overload unless I am running a inverter/vector type motor and this is application specific.

thanks, this is a huge help.
yes, vfd is a Teco N3 and i have been running General Vector control and did run the auto tuning (A006=1). glad i got that correct :-)
i did enter the motor placard data A001-A007 and i have been very happy so far with performance. braking is amazing.
the motor placards lists 6-Hz/180-RPM specs at full torque and i believe the motor can be run at this speed based on the 10:1 turn down (this i why i choose this motor). i was drilling some large holes at about 250-RPM a few days ago and torque was not a problem.
but, i like the idea of exceeding base speed. i get about 2500-RPM spindle speed at motor base speed, so a little extra would be nice. it looks like that is easily changed with b005 setting.
thanks for all the suggestions.

Be aware that when you run the motor ABOVE its base speed, you will begin to LOSE torque. The torque that a motor can develop is based on the motor design, delineated by the ratio of voltage and frequency it was made for, aka "V/Hz ratio". So a 230V 60Hz motor is designed around a ratio of 3.83 V/Hz and as long as the VFD maintains that ratio (+-10%) it delivers rated torque. But once you go over 60Hz, you can't deliver more than the 230V, because that's the VFD input; it can't manufacture voltage from thin air. Do at 90Hz but still 230V, you have lowered that V/Hz ratio from 3.83 to 2.55 or 67% of normal. Then peak torque capability of the motor (what is used to accelerate and more importantly REaccelerate after load is added) drops at the SQUARE of that change, so you will only have 44% of rated peak torque. Run it up to 120Hz and you are at 1.91 V/Hz, 25% of rated peak torque. So yes, you CAN over speed the motor (so long as the motor bearings and balancing allow it) but you have to understand this aspect of doing so.

Originally Posted by everettengr

i am very frustrated that the motor shop made this mistake. it sent me on a wild-goose-chase buying another VFD and wasting a lot of time and $$$ looking for problems in the wrong places.

"Too late now.." but.. Your old VFD as variable oscillator + your Rigol 'scope, to "see" what was afoot, and you actually had in-hand all you needed to have tested the old motor yourself and "definitively", not just guesswork.

No need of a high-voltage-capable Megger.

Which "we" usually rented, anyway, anyone but full-time telco, industrial electrical gear, or holes and poles AKA "outside plant" utility folk who had need of them at least once a week, every week and could easily justify the cost of their always trustworthy - but never CHEAP - goods.

Rigol ain't yet done earning its far less costly crust.

Even a far less capable 'scope can be a good deal of fun as well as an eye-opener and problem solver/anticipater/preventer.

Originally Posted by Jraef

Be aware that when you run the motor ABOVE its base speed, you will begin to LOSE torque. The torque that a motor can develop is based on the motor design, delineated by the ratio of voltage and frequency it was made for, aka "V/Hz ratio". So a 230V 60Hz motor is designed around a ratio of 3.83 V/Hz and as long as the VFD maintains that ratio (+-10%) it delivers rated torque. But once you go over 60Hz, you can't deliver more than the 230V, because that's the VFD input; it can't manufacture voltage from thin air. Do at 90Hz but still 230V, you have lowered that V/Hz ratio from 3.83 to 2.55 or 67% of normal. Then peak torque capability of the motor (what is used to accelerate and more importantly REaccelerate after load is added) drops at the SQUARE of that change, so you will only have 44% of rated peak torque. Run it up to 120Hz and you are at 1.91 V/Hz, 25% of rated peak torque. So yes, you CAN over speed the motor (so long as the motor bearings and balancing allow it) but you have to understand this aspect of doing so.

got it, thanks. i will go easy and try a little extra Hz when needed, which is not too often. i would need about 80-90Hz to get 3300-3700 RPM at the spindle which is adequate.
i just rebuilt the gearbox, so bearings/belts are new and the whole motor/drive assembly seems to be working quite well. the new motor is silky smooth and a lot quieter than the old Fairbanks Morse.
the end result in this whole process is awesome. very happy with the results...

Originally Posted by thermite

"Too late now.." but.. Your old VFD as variable oscillator + your Rigol 'scope, to "see" what was afoot, and you actually had in-hand all you needed to have tested the old motor yourself and "definitively", not just guesswork.

No need of a high-voltage-capable Megger.

Which "we" usually rented, anyway, anyone but full-time telco, industrial electrical gear, or holes and poles AKA "outside plant" utility folk who had need of them at least once a week, every week and could easily justify the cost of their always trustworthy - but never CHEAP - goods.

Rigol ain't yet done earning its far less costly crust.

Even a far less capable 'scope can be a good deal of fun as well as an eye-opener and problem solver/anticipater/preventer.

Bill, by the time i determined the motor was bad, i was set on getting a new style inverter duty motor. it just didn't make sense based on cost as well as the benefits of a motor designed for VFD use only. getting full torque at 180 RPM is a big plus. i will save my next machine motor failure for the Rigol scope investigation process.

Originally Posted by everettengr

Bill, by the time i determined the motor was bad, i was set on getting a new style inverter duty motor. it just didn't make sense based on cost as well as the benefits of a motor designed for VFD use only. getting full torque at 180 RPM is a big plus. i will save my next machine motor failure for the Rigol scope investigation process.

Getting full torque at one revolution every fifteen seconds - as you can do on your 10EE in back-gear by diddling the "zero set" pot on the SSD, is nought but a parlour trick for all the USE it might have, but yazz... it ain't Dee Cee, so 180 RPM beats all Hell outta 400 RPM or above!

And TIME has a monetary value.

Yah done the right thing.



PS: Fishy bizness, but I just now waddled out into the shop and weighed a motor, just for the halibut.

A two-hoss Reliance RPM III in 180 Volt windings goes right about 152 lbs Avoir. and is prolly triple to quadruple the cubic volume of your new AC motor.

That would tend to make the average vertical mill a mite TOP heavy if a body were not willing to apply whichever technology was best suited to the purpose, yah?

Most factory VFD mills in this category (manual, 3-5Hp) use a TEBC motor to provide consistent cooling at both RPM extremes, which is more of an issue with a TEFC motor. I have also used quite a few TENV inverter/vector motors in both mills and lathes (manual and CNC), that do not require active fan cooling. VFD specific 1750 RPM motors as others have noted, can be oversped 2-3X their base speed and maintain full Hp where other motors will start to drop Hp above 1.5-2X their base speed. Torque does fall off above the base speed, but you do gain some mechanical advantage at say 90 Hz of 1.5 vs. 60Hz. One reason why the motors are often oversized to compensate for the Hp/Torque loss below/above the base speed. Still a back gear is usually needed to effectively have sufficient power at the lower RPMs. I have a Sharp factory VFD mill and they run the inverter motor from 20-200 Hz, it is a direct belt drive with a back gear. The spindle RPM is something like 40-4500 RPM, back gear covers 40-450 RPM. If you wanted to overspeed the motor to a significantly higher RPM you would want to change the drive ratios so the maximum motor speed is at the maximum spindle RPM.

It looks like you purchased a great motor that is designed to be used with a VFD, so the end result through all your ups and downs is much better overall. I have heard of other people have problems/failures running the old pancake motors on VFDs, lots of different factors so kind of a crap shoot as to longevity of the motors.

Originally Posted by mksj

Most factory VFD mills in this category (manual, 3-5Hp) use a TEBC motor to provide consistent cooling at both RPM extremes, which is more of an issue with a TEFC motor. I have also used quite a few TENV inverter/vector motors in both mills and lathes (manual and CNC), that do not require active fan cooling. VFD specific 1750 RPM motors as others have noted, can be oversped 2-3X their base speed and maintain full Hp where other motors will start to drop Hp above 1.5-2X their base speed. Torque does fall off above the base speed, but you do gain some mechanical advantage at say 90 Hz of 1.5 vs. 60Hz. One reason why the motors are often oversized to compensate for the Hp/Torque loss below/above the base speed. Still a back gear is usually needed to effectively have sufficient power at the lower RPMs. I have a Sharp factory VFD mill and they run the inverter motor from 20-200 Hz, it is a direct belt drive with a back gear. The spindle RPM is something like 40-4500 RPM, back gear covers 40-450 RPM. If you wanted to overspeed the motor to a significantly higher RPM you would want to change the drive ratios so the maximum motor speed is at the maximum spindle RPM.

It looks like you purchased a great motor that is designed to be used with a VFD, so the end result through all your ups and downs is much better overall. I have heard of other people have problems/failures running the old pancake motors on VFDs, lots of different factors so kind of a crap shoot as to longevity of the motors.

i set the max speed at 90Hz and all seems fine. i did up size with the new motor a little (from 1-1/2 hp to hp 2) so i'm guessing that higher speeds with small end mills will work out ok even if some torque is lost. in hindsight i probably should have gone with the 3hp motor option which is the rating of my VFD. i do have a back gear in the gearbox, so that is always and option for low speed torque, but this motor is full torque at 180-RPM so i don't think it will get used much. when i need legitimate high speed, i use the cnc. overall it worked out ok and i'm sure for 99% of my projects this motor/VFD setup will be great and will outlast me.

no idea if the VFD caused the motor failure. that would probably involve some forensic analysis, but i believe it is highly likely...???

Originally Posted by everettengr

in hindsight i probably should have gone with the 3hp motor option which is the rating of my VFD.

No, you should have no regrets about a 3Hp.

Originally Posted by everettengr

no idea if the VFD caused the motor failure. that would probably involve some forensic analysis, but i believe it is highly likely...???

Has Teco ever reimbursed somebody for a motor failure. That will be something to read about.

Originally Posted by everettengr

Attachment 269588

It wouldn't be a bad idea if you could cover all the controls with a hood held by magnets. A box with a door is better.

[QUOTE=thermite;3442534]Getting full torque at one revolution every fifteen seconds - as you can do on your 10EE in back-gear by diddling the "zero set" pot on the SSD, is nought but a parlour trick for all the USE it might have, but yazz... it ain't Dee Cee, so 180 RPM beats all Hell outta 400 RPM or above!

QUOTE]

One revolution per 27 seconds on VFD at its slowest (5 Hz) on my Holbrook, but the only thing I've used it for that slow (so far) was milling a four-start, 3" lead "ballscrew" actuator with a spindle on the toolpost and some weird gearing to the leadscrew... Not a lot of torque though, definitely wouldn't have cut the "thread" with a static tool!

The cooling at such low revs (motor turning at about 90 RPM...) is GONE, I fitted a fairly hefty external fan which switches OFF at 30Hz and above (so it cools between uses and reminds me to power off at the end of a job), easy to program in most VFDs using their relay outputs.



Dave H. (the other one)