reversing AC motors?

Hi, I'm looking around motor for a grinder. someone pointed out that compressor-duty motors are non-reversible. I didn't think this an
issue previously, but it occurs to me that if I build a buffing wheel opposite the grinder, off the same motor, for each to run in the proper rotation I would have to reverse the rotation of the motors.
Assuming a conventional, reversible motor (I'm not on the question of compressor motor or not here), does this involve rewiring the motor each time? Even if just 10 minutes of work, it would be a nuisance; since it would not just be a one-time rewiring, but every time I want to switch between machines. Could I build a simple device that would rewire it by flipping a switch, if this is the case?
btw, how much more efficient is 3 phase over 2? It seems a converter is affordable off of ebay, and it looks like it would make every subsequent motor cheaper, and/or more efficient and smoother-running?
thanks! -Bernard Arnest
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Why would they run in opposite directions? You want a grinding wheel to run so that the grinding face is traveling down, that is the same as a buffing wheel, down and away from you.
--
Steve W.

"Bernard Arnest" < snipped-for-privacy@mit.edu> wrote in message
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Exactly. Because they are on opposite sides of the same stand/motor, to conserve space.
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wrote:

You are sleeping, Bernard. ***
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OK I think I see what you plan on doing now. Your thinking of putting the motor in the middle and having two separate units mounted to run off of it, kind of like a small lineshaft arrangement. with equipment located to the front/back of the motor.
In that case I would not bother with reversing the motor. I would just run either a reversing 1:1 gearbox on one item or twist the belt driving one shaft. That way they all run the same direction and you won't have problems with loosening of the retaining nuts.
--
Steve W.
Life is not like a box of chocolates
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I dunno the context of the comment that compressor-rated motors aren't reversible, but that comment isn't accurate. Compressor-rated motors, as well as any other specific type of motor may be reversible.
I can understand that an air compressor manufacturer might specify a single direction of rotation, because a reversible motor wouldn't be required for their particular application, and the motors would cost less than a reversible version of the same model of motor (fewer power leads and/or terminals, lower cost). This would result in the availability of non-reversible compressor-rated motors, but there are many of the same which are reversible. Generally, any type of motor that wasn't produced for a specific appliance/machine application, will be available as a reversible model.
I think the application you're contemplating is a motor located between two separate shafts/arbors, and each shaft intended for a different use. In the assumed application, the nuts securing the grinding and buffing wheels may loosen when the motor is started, if there is no consideration given to preventing that.
On a typical single shaft arbor, the mounting/securing nuts are separately left and right hand threaded, since the arbor is always run in one direction only. Placing your wheels on opposite ends of a single shaft would probably be easier to implement (most certainly safer). Then there would be no need to reverse the motor direction.
Compressor-duty motors are built/intended for high starting and running torque, but aren't absolutely required for grinding or buffing applications. Another type of motor would be adequately sufficient, although a c-r motor would also provide very good performance.
Switching reversible motors to run in either direction isn't extremely complicated if the motor is a reversible model. There was an in-depth discussion within the last week concerning this same topic, here in RCM.
Motor efficiency is related to running hours, to be of any major significance. For cost-effectiveness, one might consider 3-phase power as less costly, as 3-phase motors are generally cheaper to purchase.
WB ..............
Bernard Arnest wrote:

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Compressor duty motors are commonly built with no access to the leads necessary to reverse them. They are considered 'specific purpose' and built as cheaply as possible. They are also usually not rated for continuous duty.
Wild Bill wrote:

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Hi, Regarding nuts loosening, I was considering just taking the belt off one machine and transferring to the other, wouldn't take but a second, so that it would only be running one at a time. But good point, I forgot about that.
I didn't realize I could just twist the belt; wouldn't there be friction where it crosses itself, and/or other premature wear? Or maybe not? That would certainly be a straightforward solution.
thanks! -Bernard Arnest
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LOTS of equipment out there with twisted belts. Also many with a twisted idler and two belts when you only have a short distance. In that situation you use a double sheave pulley and tip it 45 degrees to keep crossover from occurring.
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Steve W.

"Bernard Arnest" < snipped-for-privacy@mit.edu> wrote in message
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There may be LOTS of machinery using twisted belts but it is still considered bad practice. Twisted V belts should be heavily derated and make sure there is LOTS of space between adjacent pulleys.
Steve W. wrote:

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snipped-for-privacy@microsoft.net says...

That may be true of the motor that comes on a cheap compressor, but if you ask for a compressor duty motor from, for example, Leeson or Baldor it'll probably have a larger frame, higher insulation class, and higher breakdown torque than a general purpose motor. If single phase, it will also be reversible.
Ned Simmons
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Bernard Arnest wrote:

DON'T DO IT!!! The threads on the shafts are such that the acceleration of the motor at switch on tends to _tighten_ the nuts. Reversing the motor would then tend to loosen them. This causes the grinding wheel to become an unidentified flying object until after it hits someone.
Ted
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On Sat, 22 Apr 2006 21:24:53 GMT, Ted Edwards

A simple jam nut will avoid this - been doing it this way for 40+ years. Gerry :-)} London, Canada
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On my 16 speed import drill press (of which only about 3 sperds are useful) w/ a capacitor/centrifugal starting circuit (that clicks out at speed), I discovered that if you disconnect the starting circuit, the motor stalls, but will spin in either direction you set it in by hand, and then continue in that direction when the motor circuit connected. Yeah, a little rustic....
I figger this could be wired in properly, as single phase lathe motors w/ centrifugal switches are reversible. And in those, if you attempt to reverse the motor before the centrifugal switch drops out, it will continue in the same direction even tho the handle was thrown in the reverse direction! I screwed up more than a few tapped holes that way. :)
Often the techs at the motor mfr can be helpful w/ this. As can small motor winding shops. Bring it to them disassembled, I'll bet they can point to the "right wires" in about 2 seconds--no charge. Proly just need a dpdt switch from radio shack (two 3-way wall switches will work in a pinch). -- Mr. P.V.'d formerly Droll Troll

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