# I want to use salvaged A/C fan motor for a disc grinder - Need capacitor hooked up?

#### davew252

Joined Aug 20, 2017
4
Hey, folks! I just had A/C replaced, but salvaged the condenser fan motor. Seems to be in good shape. Only about 4 years old. See photos. I'm a retired software dev., now a semi-pro knife-maker. I want to try using this motor for a bench-mounted disc grinder.
Question: Does it absolutely need to have a 10 uF cap. hooked up to it? If so, I'm pretty sure I can do that, but rather not mess with it if not really necessary.
Dave

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Joined Jul 18, 2013
20,875
You need the cap to operate the motor, it appears to be a 4 pole motor, which is rather low rpm for a grinder.
Usually 2 pole , double rpm.
Max.

#### davew252

Joined Aug 20, 2017
4
You need the cap to operate the motor, it appears to be a 4 pole motor, which is rather low rpm for a grinder.
Usually 2 pole , double rpm.
Max.
I think it should be fine for a knife-maker's disc grinder. It's really only used for polishing the 'flats' on the blade and the tapered tang. Just a light touch is all that's needed. Anything more than that (like flattening a slight warp) you would use your flat platen on the 2x72 belt grinder, or a 4x36, and then just polish it up on the disc.
Well, I guess I'll scrounge up a capacitor and give it a try.
Thanks, Max!

Joined Jul 18, 2013
20,875
It has to be AC motor run rated.
Max.

#### Externet

Joined Nov 29, 2005
1,518
I suspect a blower motor will not have the torque nor the speed nor the power to work as a decent grinder. It will stop/stall too easy at grinding. For very light usage, perhaps.

#### Tonyr1084

Joined Sep 24, 2015
4,703
I used to operate a low speed flat wheel grinder. The motor was far smaller than the one salvaged from the AC compressor unit and it was belt driven with a healthy speed reduction. The motor had plenty of torque, so if the original post wants to turn a disk for polishing a knife - I believe it will work. Especially if he uses some kind of reduction drive. The nice thing about my grinders was that there was a water arm held over the grinding surface to keep things cool and to remove the ground materials.

My recommendation is to find a cap for that motor. I'd go with one from an appliance repair shop. I'm SURE they have the right cap for that motor. Shouldn't cost more than $15.00 and probably a lot less. Then go with the reduction drive and a water source to keep the stone (or paper) clear of debris. #### BR-549 Joined Sep 22, 2013 4,938 You won't lose anything by trying. Let us know how it goes. #### MaxHeadRoom Joined Jul 18, 2013 20,875 Too bad you are not local, I have a quantity of new surplus 10μf motor start caps, N.A. origin. Max. #### Tonyr1084 Joined Sep 24, 2015 4,703 I remember one of my very first real jobs (not Taco Bell or Mc Donald's) - it was an assembly line in a heater factory. We assembled motors into their fan cages along with the squirrel cage impellers. The motors had starter caps. If you plugged one in without a starter cap the motor would not start. But you could spin it manually and it would go in that direction. I wouldn't recommend doing that on a regular basis, but I believe if you give the wheel a spin in the direction you want it should start and run. My band saw motor had a bad starter coil. Had to spin it before you turned the power on. Eventually it got so bad I had to replace the motor. Replaced it with a tread mill motor and the electronics that drove it. Now I have a variable speed band saw. #### MaxHeadRoom Joined Jul 18, 2013 20,875 The motors had starter caps. If you plugged one in without a starter cap the motor would not start. But you could spin it manually and it would go in that direction. . An old trick when either the cap failed or more often the centrifugal switch. Max. Thread Starter #### davew252 Joined Aug 20, 2017 4 Yes, this may just turn out to be one more "good learning experience". But if I don't try it... ya know. I was hoping the fan would be about the same weight as the disc. But the disc is a good heavy "billet" aluminum disc from Beaumont Metal Works. (See link.) But it weighs about twice what the fan does. With a totally different "moment of inertia", I suspect. So... heh-heh. But I also salvaged the furnace blower and motor as well. It's a 1/3 H.P., running a much bigger heavier blower, and has the 10 uF cap. with it. So if one don't work, the other one might. Just hate to buy a whole new motor if one of these will work. Even if I did order a new one, I'd still have to supply a junction box, switch, cordage and plug, etc. So here we go. Ha! Thanks to all for the tips and anecdotes. We'll keep ya posted. https://beaumontmetalworks.com/product/9-aluminum-flat-face-disk/ #### tcmtech Joined Nov 4, 2013 2,867 Looking at the tag the numbers show it's a 6 pole 1075 RPM motor and given its input voltage and current I would be surprised if it's over 1/4 HP. Unfortunately that's pretty slow and weak for any grinding operations. I have a 1/2 HP 3450 RPM bench grinder that run's a 6" wheel set and it's not good for much more than sharpening drill bits and lawnmower blades. Thread Starter #### davew252 Joined Aug 20, 2017 4 Looking at the tag the numbers show it's a 6 pole 1075 RPM motor and given its input voltage and current I would be surprised if it's over 1/4 HP. Unfortunately that's pretty slow and weak for any grinding operations. I have a 1/2 HP 3450 RPM bench grinder that run's a 6" wheel set and it's not good for much more than sharpening drill bits and lawnmower blades. Yup. It's a 1/4 HP. You bring up a good point. This is what I really need/want: , but it's$1,080. And a full 1 HP motor. With the VFD, of course, which I could probably live without. But... the HP.
I can get one of these: ,
which might just be the smart thing to do here. I got the 1.5 HP model to run my home-built 2x72 belt grinder, and it'll grind a leaf spring down.
Well, thanks again for the input. Guess I got some re-think to do here.

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#### Tonyr1084

Joined Sep 24, 2015
4,703
An old trick when either the cap failed or more often the centrifugal switch.
Max.
Actually, nah! The cap - I replaced that first thing. Made no difference. As for the centrifugal switch - didn't have one either. IF you turned the power on you couldn't spin the motor; it would lock down and not move. But if you spun it and before it came to a stop you turned it on - most times you could get it to run. However, eventually even that trick ceased to work. That's why I replaced it with the treadmill motor. Original motor was 3/4 horse. Treadmill motor is 2 1/2 horse. And at that - it's variable in speed. I don't know how many people own variable speed bandsaw's.

@davew252 I'm not so sure the fan motor will prove to be a sufficient grinder. I doubt they're very powerful. It's possible it's only 3/8 horse, which is pretty small. Spinning a grinding wheel will take some work. Then when you grind something - even polishing it - the wheel is going to slow down.

Another thing about fan motors is that they're designed to push so much air. If you put bigger blades on it then it will run slower, but push the same amount of air. Running slower it will eventually overheat. And unloaded the motor will spin at its max designed RPM and no faster.