Clarence and Tim enabled me to pick up a 3 phase 3hp motor from our church because it was going to be scrapped after some repairs to the AC system. Being unable to say no to a free motor that I cannot currently run or currently have no use for, I was happy to take it home. Why can’t I run it? It is designed to run off of three phase AC power but residential houses only receive single phase AC. So, if I tried to hook it up to 240V from the wall it wouldn’t spin at all without spinning the shaft by hand or rope which is not very practical. Even when running it’d be electrically unbalanced and could damage the motor. Don’t worry though there are ways to run three phase motors off of single phase AC without damaging them. More on this in a future post though. As for not having a use for it, that is probably temporary.
So what does this thing look like? It is bigger than my other motors so I have put a picture of a regular soft drink can in there for size comparison.
When I picked it up it had dust and grime on the front and back which isn’t surprising. The curious thing though was that there was a rubbing noise when the shaft was spun by hand. Three phase motors don’t have brushes or centrifugal switches so nothing should be rubbing at all. As a result I thought I’d crack it open and see what was the matter. Most motors come apart pretty easily. Just remove the four bolts holding it together, one which the green arrow is point to, and the end shields (or bells) can be removed. The end shields aren’t pressed on but they may need to be loosened with a rubber mallet.
Before you remove the end shields, mark them and the body like I’ve done in the picture below so you can realign them when you reassemble the motor.
Once the end shields are loose they can be removed and the rotor (part that spins) can be removed. Once I removed the rotor it was pretty obvious what was causing the rubbing noise….rust.
Not surprisingly it was on the stator as well.
My normal rust removal techniques can’t be used on an electric motor. Electrolysis is out for obvious reasons. Steel wire brushes of any kind and steel wool are out because you don’t want bits of ferrous metal left inside of the motor. I suppose a brass brush might have worked but I didn’t have one. So, I ended up using a 3M scotch brite pad. Sure, they don’t list this use under the applications for some reason on their website but I assure you it works. Usually you’d want to use some kind of oil to aid in rust removal but you’re supposed to keep these surfaces oil free.
After some scrubbing here’s what we have. I didn’t get every bit of it off because it’ll start coming back anyways but a large majority of it was removed.
While it is apart you want to remove the dust on the cooling fins with a soft brush and vacuum it all throughly. Blowing it out with compressed air is also an option but be sure not to damage the wiring. Also check the bearings in the motor. Depending on the type of bearings you may or may not be able to see the grease. If you can, look at it to see how it looks. If it looks dry or chunky replace it with new grease. Spin the bearings over by hand and see how they feel. If they feel gritty then the grease has dried out in them and they need to be replaced. If they spin very freely with little drag then they’ve probably gone dry and need to be replaced.
As every Haynes manual says, “Reassembly is the opposite of disassembly.” Put the front end shield (one without a center hole in it) back on and then gingerly put the rotor back in followed by the other end shield. On my motor I chose not to separate the end shield from the rotor so it all went on at once. Be sure to align your index marks. Reinstall the bolts, snug them up, and you’re done.
Now when the rotor is spun by hand there is no rubbing noise. At this point I’d normally turn it on to make sure it runs well but that’s not an option. In order to run it I’ll need to construct or buy a phase converter. I’m sure you can guess which will happen but I need to find some parts and another 3 phase motor first.