Grinder motor dilemma-need advice

B

Brian.Evans

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Well gentlemen, I need some advice on how to proceed. I kind of already have some ideas, but I want to see what you all think.

I posted a want ad over at Garage Journal's forum. I was hoping for a decent deal on a used motor. I got an email from a gent that has two 3hp, 3ph motors. He is asking $150 for the pair. He's driving through my area in a week or two, so no shipping.

Now the downside. He sent me pictures tonight. One is an Emerson and one is a Reliance Electric. Both are 1750 rpm, HOWEVER....they are open enclosures and 145t frames. The 145T doesn't matter a whole lot to me, as I can just have my machinist friend make the drive wheel shaft hole larger. The open enclosure scares me though. For $150 or a bit less, I could have some nice motors, but not if they are just going to smoke out in a month. What do you all think?? Help please!
 
Do you need 2 motors? I say pass. Chek local l ads, post a want to buy motor post there. Chek eBay also. I bought a Baldor on the bay for a very good deal. Make sure you buy a motor that is inverter ready. good luck.
 
I can send you to an eBay supplier link for a 1.5 or 2 hp 3ph motor for $159 including shipping. TEFC, footed , 56C frame NIB w/ a two year warranty....
You don't need 3hp.
 
Thanks guys. Some answers to questions:

I'm building an EERF/GIB clone. I know it's a face mount design, but I could modify it for a foot mount design fairly easily I think if a motor fell from the sky into my lap, which was what I was hoping would happen when this responded to my want ad.

I have found a 1.5 hp Elektrimax 3ph 3600rpm on eBay for $95 shipped brand new. TEFC, 56c, etc, etc. If I knew a 3600 would work just as well or almost as well as a 1750, I'd just buy it and be done with it, but I'm not sure.

Brian, I think you sent me a link to PJT Industrial, which Jeff emailed me back on a 2hp Leeson for $209. Do you have a different supplier?


It sounds like these motors really aren't what I want, and at a price that isn't worth settling for something I don't really want. Man, too bad they weren't 56c, TEFC, huh? I know 3hp is overkill, but if they were perfect otherwise, man whatta deal!

Let's throw this out there, just for fun since my mind is already made up against it.

What would be your personal number to work around all the "issues" and buy these motors?
 
medicevans said:
I'm building an EERF/GIB clone. I know it's a face mount design, but I could modify it for a foot mount design fairly easily I think if a motor fell from the sky into my lap, which was what I was hoping would happen when this responded to my want ad.

I have found a 1.5 hp Elektrimax 3ph 3600rpm on eBay for $95 shipped brand new. TEFC, 56c, etc, etc. If I knew a 3600 would work just as well or almost as well as a 1750, I'd just buy it and be done with it, but I'm not sure.
Click to expand...

I'd buy that.
I'm not sure why 1700rpms are supposed to be better, but nothing wrong with that motor.


if the 1700 vs 3400 rpm really bothers you, you can always put a 3" drive wheel on instead of 6"
 
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I have a 1800 RPM Elektrimax on my GIB. 1.5hp 3ph 56c and footed too. It works fine.

There's reasons for using a 1725 rpm motor vs a 3450 if using a VFD. Besides it being what was advised I also researched it and can simply say in a fuzzy alcohol induced state that it has to do with the windings??
The seller on eBay I'm speaking of is Boattechcorp or something like that.
 
given everything that I have read about using an open face motor for grinding metal I wouldn't buy them. Sometimes I get the urge to buy something because it is such a 'good deal', but ultimately it slows me down from completing the goal I set for myself. Google iron horse electric motors. Probably Chinese and I don't have a clue about quality but I think they have some for less$$. The bearings might wear out faster on a 3600 rpm motor, but I'd guess it would be a little smoother. If belt speed is an issue you could use a smaller drive wheel.
 
Guys, I'm looking for 3ph motors because I'm going to run a VFD, don't know if I said that already or not.

1800 vs 3600 rpm in a VFD setup:
Power=rpm x torque. An 1800 rpm motor is four pole and will have more torque for a given hp than a 2 pole 3600 rpm motor. I think. Someone double check me on that?
 
Brian Ayres said:
I have a 1800 RPM Elektrimax on my GIB. 1.5hp 3ph 56c and footed too. It works fine.

There's reasons for using a 1725 rpm motor vs a 3450 if using a VFD. Besides it being what was advised I also researched it and can simply say in a fuzzy alcohol induced state that it has to do with the windings??
The seller on eBay I'm speaking of is Boattechcorp or something like that.
Click to expand...

Brian, maybe you could give me a direct link to his page. I'm not finding anything.
 
Baldor quotes numbers that are 2x torque for a 1750 motor compared to a 3450 motor. Don't settle on the motor. Get what you want and need and then you won't have to look back. If you find a deal on a 3 hp, don't pass it up. Your vfd would need to have the capabilities for 3hp then so double check that. Not many can handle 3hp.
 
Power is torque x RPM, so to get the same power at half the RPM, torque has to double.

If you design for the same belt speed at the rated frequency, everything evens out: at half the speed, you have twice the torque, but the drive wheel is twice the diameter, so the tractive effort applied to the belt is the same.

I think most of the reason for going for a 4-pole motor is that motors are designed for streamlined manufacture, so everything that rotates in the motor will be the same for the 2-pole, 4-pole and (if they make variants with more than 4 poles) the 6-pole, 8-pole and so on. The frame will also be the same. Only the static windings will change between the different variants.

By using a 4-pole motor, you can therefore be pretty sure that it has been designed to run mechanically at the 2-pole speed of 3600 RPM, without compromising it's longevity. Electrically, it will generally run fine up to 100 Hz (3000 RPM) and may even do a bit more.

Using a 2-pole and going to 100 Hz (6000 RPM) with a drive wheel of half the diameter would also work, but the top speed isn't necessarily within the design speed range for the motor. It'll probably work OK, but may not last as long and it'll certainly be noisier.

If you use inverter duty motors, they'll tell you what speed they are designed to run to, but you'll pay a lot for them.

If like most of us, you buy on price, you can be reasonably sure that any fairly modern 3-phase, 4-pole motor will run to at least 100 Hz without problems, whereas for 2-pole motors, you might need to do some further research on the specific motor.
 
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