VFD with non inverter duty motor

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14 Oct 2018 19:56 #118793 by OT-CNC
This is not linuxcnc related so moderator feel free to relocate this to off topic or other section.

I'm planning on using a recently acquired Baldor VFD with my CNC'd Bridgeport knee mill. The existing 2hp motor is not an inverter duty rated motor I believe. I have concerns about life expectancy on the windings. I don't want to switch out the motor as I'm still using the var-disc setup. Main purpose would be to allow rigid tapping (I have encoder fitted to mill already). I would probably be running it mainly at 60Hz and in back gear for tapping.
I'm seeking some advice on settings that would pose the least damage to winding insulation in the long run. Any advice?

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17 Oct 2018 12:01 #118937 by andypugh
It might be OK, there is a good chance that the windings would have been inverter-rated had inverters existed at the time the motor was made to test against.

I believe that filters on the motor cables can help reduce the high voltage transients. (And are a good idea anyway to reduce electrical noise).

This can be as simple as taking a few turns though a ferrite ring with the feed wires..

The bottom two posts here seem to suggest that you probably have nothing to worry about:
www.chaski.org/homemachinist/viewtopic.php?t=79705

And my mill (not a Bridgeport, though) has been running fine on a VFD for years with the original motor.

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17 Oct 2018 13:22 #118940 by jmr
Hi,
I am not quite sure, but I believe it was stated somwhere that running at designed motor parameters should be good for all motors, this is why you input main motor parameters into vfd. I also believe it was stated that it is ok to go +/- 20% (or was that 10%?) with the motor speed, and vfd will keep other parameters under maximum. It is possible to go further but there is no warranty, as with low speed the cooling may not be adequate, and there will some looses or possibly dangerous voltage spikes if the supply is not sinusoidal (so the filtering may help). The windings should have been insulation-tested at higher than operating voltage at the factory, so I believe the temperature is most dangerous but it does not rise up instantly so short overload is no problem.
Otherwise the parameters are kept within boundary from 0Hz up, and some VFDs have current "boost" for higher torque at low rpm. So I too say it should be fine, but remember the torque at 2Hz will be small...
I have been using standard 50Hz 370W 3phase motor from the 50-60s running mostly on 60 or 70Hz with all kinds of abuse, for years now with no problem.
That is of course not a 2HP motor... Someone who put a lot of VFDs on old machines could help here, anyone?

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18 Oct 2018 16:22 #119002 by OT-CNC
Thanks for the responses and link guys.
I'll set the settings to stay within 15% of the rated speed for starters. I'll be tapping in back gear so no need for electronic high torque at low rpm.

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