Setting up surface grinder with VFD for slow speed honing

Bob Korves

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I need to rough and finish grind some carbide hand scraper inserts. For rough grinding I will be using my HF Baldor clone carbide grinder at high speed with a 300 (Edit: 150 grit) grit resin bond diamond face wheel. That should work fine, with a simple jig to hold the scraper handle for grinding the correct radii.

For finish honing/lapping, I bought a 6" (150mm) diameter, 1mm thick import steel plate, with a 600 grit plated diamond coating, and plan to use my surface grinder for the slow speed work, because it has a VFD. I had to open up the mounting hole in the lapping plate from 1/2" to 1 1/4" and then mounted it on a wheel adapter and installed it on my 1946 B & S model 2L surface grinder. The runout is minimal, and I want the capability of running the grinder at very low rpm's to do the lapping. Previously, I had the VFD set for 15-60 hertz, which allowed a measured 3266 rpm at the high end, and jogging at low speed to get the ISO 2 oil circulating in the plain bearing spindle. It also allows slow acceleration and deceleration to keep the wheels from slipping on the adapters and going out of balance. I want now to also get the maximum wheel rpm up to 3600 rpm, which is the rated limit of 7" surface grinding wheels (makes the wheels act harder when desired.)

So, tonight I reset the VFD for a range of 5-65 hz, which gets me up to just under 3600 rpm, and down to about 160 rpm, and infinitely variable between those limits. My question is about the low speed operation. The grinder has a 1946 1.5 hp GE 3 phase motor, which is running smoothly. I don't imagine that 65 hz is going to hurt anything, my concern is running as low as 5 hz for short periods of time, 5-10 minutes max, light loading. I really do not think the big old motor will overheat under those conditions for that period of time, but wonder if the VFD might add additional strain on the old motor that I should be concerned about.
 
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To allay any concerns you could check the temperature after grinding a few parts and compare that to the temperature reached with 5 minutes at low speed from an ambient temperature start.
You're working at almost zero load so it's likely that the motor current drawn will be quite low.
 
I don't think it is a cause for worry, but why not just find a worm gear speed reducer?
 
Bob, you could take an IR non-contact temp gun and check the temperature of the motor after a 10 min run. Or maybe just use the touch of your hand?…Dave
 
Bob, I have read a lot of internet chatter on this subject. People using muffin fans with temp sensors and all that. Maybe there is merit. But in my shop I can run my BP motor for most of the day on 15cps with no overheating. I have a magnetic temp gauge that I stuck on the motor. It maybe reads 10° F above ambient after I use the BP for the morning…Dave
 
Bob, I have read a lot of internet chatter on this subject. People using muffin fans with temp sensors and all that. Maybe there is merit. But in my shop I can run my BP motor for most of the day on 15cps with no overheating. I have a magnetic temp gauge that I stuck on the motor. It maybe reads 10° F above ambient after I use the BP for the morning…Dave
I am not at all worried about overheating. This motor does not get warm when grinding hard. It is 1.5 HP but looks like 5 or more HP, big heavy motor. I was (and am) more concerned about electrical noise, spikes, or other issues from the VFD at that low cps that might hurt the motor. To put a polish on a scraper blade takes a couple minutes and very little power. Of course, the surface grinder has a 1.5 x 120 rubber/fabric flat belt and a shorter 1" leather flat belt, and there are half a dozen idler pulleys and a whole clock maker's nightmare of mechanical stuff that runs the grinder and the power feeds, all of which are idling when just using the spindle. Amazingly, it all works fine after 70+ years.
 
I am not at all worried about overheating. This motor does not get warm when grinding hard. It is 1.5 HP but looks like 5 or more HP, big heavy motor. I was (and am) more concerned about electrical noise, spikes, or other issues from the VFD at that low cps that might hurt the motor. To put a polish on a scraper blade takes a couple minutes and very little power. Of course, the surface grinder has a 1.5 x 120 rubber/fabric flat belt and a shorter 1" leather flat belt, and there are half a dozen idler pulleys and a whole clock maker's nightmare of mechanical stuff that runs the grinder and the power feeds, all of which are idling when just using the spindle. Amazingly, it all works fine after 70+ years.
That’s a good question. I don’t see a problem. Can’t imagine the insulation falling of the magnet wire? Or the bearings getting brinell damage in a dynamic not static condition? Like I said, have been running my BP at the low cps since day one. And have no problems at all.
 
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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
 

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