New guy motor question

M

MHM

Joined
Dec 5, 2018
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2
Very new. First time posting. I know enough to be dangerous.

Making a very much DIY 2x72. I have 2 less than ideal motors. Both are dp. I understand why these won't last long term. If you had to choose between the 2 motors, which one would you build a grinder with?





Ty in advance and hope the pictures work
 
There's really not much info to go off of from just those nameplates. These look like spa/hot tub motors? The 2 speed is likely 1725/3450 or something close to that; do you know what speed the single speed motor is?

If the single speed is 3450 rpm, that's too fast, IMO. If the single speed is 1725(ish), then either motor would be as good as the other one. Assumig it has a standard 5/8" shaft, like you would see on any other 56 frame motor, I'd fit it up with a 3 to 4 step pulley. That, or sell them and get a more ideal set up.
 
I bought a Motor VFD package online for under $300 to the door. Best money spent to finish my build. They are still going strong over a year of good use. Dealerselectric.com. Mine runs on 115 single phase input to the vfd and 220 3 phase to the motor.
 
NHM, what part of the world are you located? The type of advice given will depend to some extent where you're located. If USA - sell both motors and spend $250 to $300 for a 3ph motor and VFD drive.

If you're wanting to make knives and have only one grinder, variable speed is the ONLY way to go. I've been down that road with 3 (and 4) step pulleys and thinking that's all that's needed. WRONG! Once I made my first VFD drive grinder - no looking back.
 
Assuming they are both the same frame size, my inclination would be to use the 2-speed and build it direct drive: don't waste any money on pulleys, etc. Choose your drive wheel diameter for the high speed and take whatever lower speed the motor gives you: probably half the high speed. You will definitely not want to be doing everything at high speed and using the 2-speed gives you the option of using a lower speed, albeit not an optimized lower speed.

If/when the motor fails (or earlier if/when it becomes clear it is worth it), you can swap it out for a 3-phase motor of the same frame size, together with a VFD. You'll know everything else works because you'll have sorted any issues out while running on the single-phase motor.

I have a feeling that, if those are 56-frame motors, their 5 BHP ratings would seem likely to be a trifle optimistic.
 
Good advice Tim. 1 has the funny shaft. The other might as well. I'll know when I take the pump off. How difficult is it to replace shaft?
 
MHM said:
Good advice Tim. 1 has the funny shaft. The other might as well. I'll know when I take the pump off. How difficult is it to replace shaft?
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Not worth it. You can get a brand new 3 phase motor shipped to your door for less than $200. A chinese VFD is $100. If you have a surplus store nearby, or keep an eye on craigslist or eBay, I've seen used 3 phase motors for as little as $50 to $100 as well. Sell the motors for what you can, and put the proceeds into something you can use.
 
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