View Full Version : Rotary converter befudllement.
05-01-2009, 08:16 PM
I have a surface grinder. It has a 3/4 hp 3 phase motor for the table and a 1/2 hp 3 phase motor for the spindle.
Is there any reason why a rotary converter cannot also be used as a motor???? and... if so... what's wrong with using the table motor as the rotary convert to supply power to the spindle motor. The machine doesn't need "3/4" hp to run the tables.
A couple of run caps on the table motor ... and I can convert this machine to single phase.
Nothing I thank . all you need is a static converter to start the large motor first and run the second motor from it. I did that once for a while until I came up with a better way . What I do not know is their any long term affect.
05-01-2009, 08:44 PM
I don't think that you want to do that on a surface grinder. A rotary converter is actually not extremely smooth running (they feel "rougher" to me than a single phase motor, while a good 3ph motor is very smooth) and because the table motor is bolted into the system, I think you will see it in the surface finish.
05-01-2009, 09:28 PM
A 1 1/2 or so idler motor would probably be a better choice. That should give you the power you need and no surface problems.
05-01-2009, 09:39 PM
Why couldn't I use the 1.5hp idler motor to drive the table? is it really "not smooth"? I have room to mount a separate idler inside then i'd have three motors in the grinder. Not big deal, but...
My other option is to put in two cheap'ish Vfds, then at least I'd have variable speed tables instead of the fixed 19 or 31 feet per minute.
05-01-2009, 09:47 PM
Re-read what MickyD said. Rotary converter motors buzz and humm. Not what you want touching a precision grinder. Cheap VFDs seem like a good idea. Also, changing the wheel RPM is handy, and changing table speed also is good. If it is hydraulic feed, some pumps have a sweet spot with the least cavitation, and a VFD allows you to find it. Also is true for the grinder itself. My grinder has a resonance at 60hz. I run the wheel at 63hz and it is a much smoother machine.
05-01-2009, 10:13 PM
Mine purely mechanical drive -1957 B&S Model 2L... the last model before hydraulic.
Might be nice to get rid of the big box of starters, transformers and relays.
So.. I buy buzz and humm. But why do they?
05-01-2009, 10:37 PM
Buzz humm why? You feed a 3 phase motor 1 phase across 2 windings that want a 120deg phase relationship, but you are giving both of them only 1 phase, so with only 1, so there is no other phase to reference a phase shift. Of course until you spin it up, but then the phase are different magnitudes. Not exactly, but a good short explanation.
05-01-2009, 11:29 PM
My rotary converter sounded like it had bad bearings, so I replaced them. It still had the same noise, never figured out why. I have since gone to VFD's and I'll never go back. They are wonderful, once you get past the price. I buy them on eBay, with excellent results so far...
05-02-2009, 07:26 AM
They do vibrate and buzz a little - one reason it is important to keep them mounted on isolating pads vs. hard mounted. If you hard mount them bearing and winding life can be shortened due to "sharper" vibrations. I have a 15HP model from American Rotary and when my cnc mill is really working, you can put your hand on the rotary and feel a difference in the vibration. With no/low load it is very smooth, but when I load it up the buzz is there. I would also not try to run the surface grinder on a static converter, they are rougher feeling than even a rotary converter when you load them up. If the grinder is your only 3ph machine, you might look at getting a pair of small VFDs for it. Small Teco units are under $100 now and it would be simpler than rigging a rotary converter and cheaper than buying a new one.
05-02-2009, 12:03 PM
I run my whole shop off a single multi-stage RPC. But my surface/tool grinders (though still "in process") use a GS2 VFD.