Setting up surface grinder with VFD for slow speed honing

The insulation of the motor is at risk in an older motor but I do not see that the low speed presents any greater issue than would be seen at higher frequencies, the most significant factor is the carrier frequency. The pulse voltage is the same but the the pulse width would be smaller. Ideally you would want to stay around 2-3 Khz carrier frequency. The other consideration to tame the voltage spikes to an older motor is to use a drive reactor, these are placed between the VFD and the drive. They should to be sized for the maximum motor amperage and the operating voltage of the motor. Be careful and look up the specifications as many sold are for higher voltage motors. I would look at TCI KDR drive or output reactors. Probably recommend the KDRA28HC1 below even though it is rated for a 3Hp motor, a high Z will have a higher impedance to smooth spikes. I do not see a problem running the motor a low frequency but the Hp is greatly reduced, also at low speeds the motor over current may trip even with a low load. I had this issue when using VFDs for axis drives at below 10Hz.
https://transcoil.com/products/kdr-output-load-reactor/
High Z
KDRA25H, KDRA26H KDRA27H C1 denotes enclosure
https://www.ebay.com/itm/new-TCI-KD...amps-10-6-3ph-3-phase-protection/112477816224
Low Z
KDRA26L, KDRA27L or KDRA27L
https://www.ebay.com/itm/TCI-TRANS-COIL-KDRA26L-Surplus-New-not-in-factory-packaging/391875171464
https://www.ebay.com/itm/TCI-TRANS-COIL-KDRA28LC1-Surplus-New-In-factory-packaging/391750911526
Thanks, Mark. I will look into the drive reactor. I made a workholding jig and used the grinder today to hone 4 sides (8 cutting edges) of a Sandvik carbide scraper blank, and it worked just fine. Looks like I need finer diamond than 600 to get the finish I really want. I will possibly make a cast iron or aluminum lap and charge the lap with diamond grit. There is well more than adequate power for what I am doing at 5 Hz, and the machine and motor seem to behave themselves well. Mark, you exactly addressed the potential issues I am interested in. Thanks!
 
This is just in case what you are working on does not work as intended or maybe for the duration. I do not know how much you have invested in the project, but before you get too deep financially, check out the link below. I used one for five years on carbide, HSS, tunsten rig torches. work like a dream! If it’s too much, totally hear you, but if you find that it may be getting to the breaking point financially and to where it has gone past just being a curious interesting fun tool to build, this is very complimenting to any shop. We used one that was about $1200 new and it came with a couple different wheels. They were only available with 6 inch wheels at the time but may have bigger now.

http://accu-finish.com/
 
This is just in case what you are working on does not work as intended or maybe for the duration. I do not know how much you have invested in the project, but before you get too deep financially, check out the link below. I used one for five years on carbide, HSS, tunsten rig torches. work like a dream! If it’s too much, totally hear you, but if you find that it may be getting to the breaking point financially and to where it has gone past just being a curious interesting fun tool to build, this is very complimenting to any shop. We used one that was about $1200 new and it came with a couple different wheels. They were only available with 6 inch wheels at the time but may have bigger now.

http://accu-finish.com/
 
bob mksj stated the correct worry for motors not designed to run on a vfd. the windings can flex, rub away the insulation and short out the windings either to themselves or the case. but this would only happen under significant load. I don't know a way to measure that flex, but maybe someoneelse out there might have a way to measure it bil
 
This is just in case what you are working on does not work as intended or maybe for the duration. I do not know how much you have invested in the project, but before you get too deep financially, check out the link below. I used one for five years on carbide, HSS, tunsten rig torches. work like a dream! If it’s too much, totally hear you, but if you find that it may be getting to the breaking point financially and to where it has gone past just being a curious interesting fun tool to build, this is very complimenting to any shop. We used one that was about $1200 new and it came with a couple different wheels. They were only available with 6 inch wheels at the time but may have bigger now.

http://accu-finish.com/
I have $10 invested in the project for a Chinese 600 grit coated steel plate, plus a little electricity. The surface grinder is regularly used as a surface grinder. It's VFD is only being slightly reprogrammed to also work for lapping. I am not going to go with the drive reactor, and take my chances for the little bit I will be using the grinder for lapping. I have used the Glendo and Accu-finish grinders before. They are nice machines, and quite good for the job, but way too costly for occasional hobby use IMHO. I have also used a similar grinder that Ulma Doctor made from junk laying around his shop along with a Chinese wheel, and it also works well. All that is really needed is a flat lapping wheel, a slow and preferably adjustable speed motor, and an adjustable tool rest. Ulma Doctor's creation does all of that except variable speed, and it does the job just fine. If I was working full time doing this kind of work for compensation, then my choices might well be different. Thank you for the idea and link, webphut.
 
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