How To Rebuild A Three-Phase Squirrel Cage Motor

I'm talking here about rebuilding motors of the size you will see most often
in a home shop, motors between 1 and 7½ hp. These motors are getting more
expensive around the Seattle area especially with the closing of Boeing
Surplus, so the ability to rebuild them is becoming more important.

It is a fair amount of work to rebuild a motor, so prudence would dictate that you should start with a motor that still works. I have a phase converter in my shop which has receptacles built in, and I made up a little cable with large insulated alligator clips on the free end, which plugs into my phase converter via a disconnect box. That way I can clip the alligator clips onto my motor's wiring quickly and throw the switch on the disconnect box to spin up the motor. Most of the time this setup just gets in the way, but it is very helpful when working on 3-phase motors. Start with a motor that works! If yours does not work (and I've run across some that don't) check continuity in the wiring and check the connectivity of the wiring. There is often a wiring chart on the nameplate or printed on a label inside the motor's wiring box.

MOTOR BEARINGS

Look at the nameplate. Many motors' nameplates call out the motor bearing numbers. If yours does not, don't worry, just don't throw away the bearings until you read their numbers. Once you have the bearing numbers, order new bearings. Try to get good bearings if you want a quiet motor. Bearings should be electric motor rated. Some bearings have an EM in the part number which stands for "electric motor". 

A note on bearings. Bearings come open, shielded, or sealed, on one side or on both sides. Shielded bearings don't have anything that bears on the inner race to restrict its rotary motion. Most sealed bearings do. If you dig down into your bearing manufacturer's datasheets you will learn their nomenclature and you can look up whether their sealed bearings are contact seals or non-contacting seals. The latter are rare but work very well indeed on electric motors because contact seals do bear on the inner race so some of the motor's energy must go to overcoming that resistance, and thus the bearings will heat up more and probably have somewhat shorter lives as a result. On the other hand, contact seals probably keep out dirt and contaminants the best, so like many things it's a tradeoff.

I don't recommend buying cheap no-name import bearings even though it is tempting because of the price. I have rebuilt motors using cheap bearings and have been dissatisfied with the noise levels that resulted. When I first switched to top quality bearings I was amazed at the difference. I have had good luck finding bearings on ebay even though that site isn't at all what it used to be. Order the bearings as early as you can in the process. Some quality bearing manufacturers you can look for include Fafnir, SKF, FAG, Toyo, Koyo, NSK, NTN, Nachi or NHBB.

PAINT

If your motor is pretty clean and has an intact coat of paint, you won't need to strip and repaint it. Mine never seem to be -- I nearly always wind up repainting the motors I rebuild! Now is the time to decide on the color and get the paint purchased or ordered BEFORE you need it!

DISASSEMBLY

Clean off a few square feet on the bench. I like to lay out a clean towel. You will need a soft-faced hammer and a pressurized air hose with a blowgun fitted, plus a few other hand tools for this job.

Put the motor on the bench. Remove anything attached to the shaft, like a pulley or a key. Before you proceed, look at the motor well or take a digital picture of it. This is to remember which end the shaft comes out, and which way the motor's junction box is oriented. While you're at it, take a small hammer and a small sharp chisel and make some witness marks across the end bell cracks in an area that won't show, so you can't possibly have any question of the orientation of the end caps during reassembly. I like to make two cuts forming a V. One V for each end bell. Make one V narrow and the other wider and you'll never get the end bells mixed up. 

If the motor is totally enclosed fan cooled (TEFC) remove the air shrouding screws and the shrouding, retaining the screws and remembering which ones they are. Now you have exposed the fan blade, which is often plastic, and which can be difficult to get off intact. It must be done, however, so loosen the clamp bolt carefully and do what you can to pry it open. Slip steel flat bars behind it as close to the central shaft as possible, and using as little force as possible, try to lever the fan blade off a little at a time. At any rate, remove the fan blade, hopefully without hurting it. Replacement TEFC fan blades may be available from the manufacturer if they are still in business and your motor is still supported for parts. If you intend to use the TEFC motor in conjunction with a VFD (a common arrangement these days) you may well be permanently removing this fan blade and replacing it with a muffin fan fitted to the shrouding, one which will deliver a constant air flow no matter how the variable frequency drive changes the power frequency. In that case, keeping the TEFC cooling fan intact isn't as important, although I try to save them anyway.

All motors seem to have 4 long skinny bolts with regular hex nuts holding the motor end caps on. If you see any corrosion or paint on the threads around the nut end, apply quality penetrating oil. (I like Kroil but I use others, just don't use WD-40.) Remove the 4 bolts entirely. Now take a soft-faced hammer and give the end of the shaft a good whack. The rear end cap (away from the shaft) should pop right out. Once the cap is loose from the motor shell, pull the cap off the motor's rotor (also known by the more technical term "armature"), holding the rotor shaft so the motor doesn't suddenly drop onto the windings when you remove that end cap. The next step is very important! Look in the end cap's bearing boss on the inside. You should see a wavy washer in a pile of old grease. Remove and save the wavy washer.

Then hold the rotor shaft roughly in a central position and give the exposed shaft end (not the motor's drive shaft, this is the shaft end you just removed the end cap from) a whack with the soft hammer to pop loose the front end cap out of the motor shell. Now remove the end cap and the rotor and put them down carefully on a clean place. Remove the end cap, which should not require much force. The bearings should stay on the rotor shafts when you remove their respective end caps.

Now the end caps can be completely cleaned. They may have zerk fittings leading to grease passages which lead down to the bearing cavities. If so, remove and retain the zerk fittings and use compressed air to blow the grease out of the grease passages from the inside out, aiming the flying grease glob cleverly into a garbage can. The reason I say to do it from the inside out is that otherwise it splatters on the inside and sometimes goes all over your shirt and maybe into your face. Clean out the inside of the end bells with carburetor cleaner and compressed air and small wire brushes and 3M pads as necessary. The outside cleaning treatment will vary with what kind of metal they are made of. If they're made of steel or cast iron they can be boiled in water with a few tablespoons of Cascade dishwashing powder added. Bring to a simmer and boil long enough to remove or completely loosen the paint and all grease. Carry the whole pan to a utility sink and rinse the parts under hot water, scrubbing on them with a stiff brush or even some steel wool. Work quickly as you want the parts to stay hot enough to self-dry within 60 seconds. NOTE - you can not use a hot caustic bath like the above on aluminum or zinc-based end caps, they will get pinholes in just a few minutes since the bath eats those metals!

After the end caps have been degreased they can go into a derusting bath. You can simply dunk them into a vat of Evapo-Rust with a naked light bulb nearby to warm the bath a little, or you can set them up for electrolytic derusting, or you can blast them using glass beads or other media. If the end caps are made of aluminum or zinc then you may be able to simply rub the rust away with a 3M pad. But get any rust completely removed. At this point the end caps are ready for repainting. The final step I do to the end bells is to locate your witness marks, which hopefully are still visible, and take a sharpie and make a mark on the inside of the end bell so you can quickly look at the inside to see roughly where your witness marks are, as they can be tough to spot once you repaint. If your end bells are made of steel or cast iron and you are going to paint the inside, then of course wait to mark with your Sharpie until after painting.

Moving to the main motor body, you will see a shell, an electrical wiring box, and inside, windings. This part of the motor is called the stator. Start by blowing out the stator and looking inside carefully for any insect nests or rust damage. Be careful around the windings! Don't do anything rash like stick a great madly whirling cup wire brush down in there! One faint touch to the end of the wirings and you might dislodge a lamination which means your motor will buzz horribly, so go gently! 

Completely remove and disassemble the motor wiring box, carefully retaining the screws.

Now look all over at the stator. Is it very rusty? If so, dunk the whole thing into Evapo-Rust overnight. It won't touch the winding varnish or the nameplate coloring, and it certainly won't dissolve the nameplate metal, but it will remove or greatly loosen the rust, particularly if the bath is warmed to 70 or 80 degrees F. When it comes out, rinse it with hot water and bake it in the oven at 170F for half an hour or so to drive out any remaining moisture. This treatment works better if you take the part out after an hour or two in the Evapo-Rust and scrub or scratch on the rusty areas, which will greatly help in letting the Evapo-Rust solution penetrate into the rust and dissolve it. I have had amazing luck with this method.

Now use the exact same method on the motor's rotor. First remove the bearings. I generally use cheap wheel pullers, which deform the bearings' shielding, but I generally don't care because I'm going to replace the bearings anyway. If you need to, or if you're curious, clean the bearings once you get them off, and look at their numbers. Derust the armature if needed, and polish the shafts afterwards. I keep a 3M deburring wheel in a bench grinder for polishing of this type - it works very well. Once the armature is clean and its shafts are polished until shiny, with no nasty nicks or dents, if you like you can take it to a balancing shop and get it precision balanced. This will cost $100-200 but then the motor will run amazingly smoothly. It normally isn't necessary. I've balanced the armature from a cheap drill press motor in my shop before. To do that, I carefully leveled a surface plate and bolted some thin stiff flat bars of tool steel onto some 1x2x3 blocks to form "knife blades" which I set the rotor on to find the heavy spots. Then I drilled the heavy spots a little at a time. I was able to get the rotor a lot closer to balanced. That motor ran much smoother afterwards, but it was way off beforehand.

Clean and derust the external fan shrouding if necessary, as well as the motor's electrical wiring box and its lid. If there is a label inside the wiring box, try to remove it and glue it back in after cleaning, or clean around it, or make a replacement label with the same information. Also derust any of the fasteners you can't or don't want to replace. Any fasteners you derust chemically should be dried carefully and oiled or they will rust very quickly. If the wiring box is not steel or cast iron use carb cleaner or brake cleaner to degrease it. I have heard that a mild acid like vinegar will remove white powdery buildup on pot metal parts, but I haven't done this so if you try it then watch it carefully to make sure it isn't eating the zinc itself.

Once the stator is completely clean, look very carefully at the wiring which pulls through into the wiring box. See if the insulation is cracking. If it is, get some shrink-wrap tubing of the appropriate size and shrink-wrap one wire at a time, making new labels with a labelmaker. Don't screw up the labels! Be kind to your wires - handle them with care, but do whatever you need to to make sure their insulation will last a long time as well as their label.

Now you have completed the motor's disassembly, and it is time to repaint.

PAINTING AND RELATED PREP WORK

I'm not a great painter, probably not even a good one. I don't know how to
paint using compressed air. But I have painted a few dozen shop projects 
and they all seemed to turn out OK, and I've learned a few things along the way. Of course it is very important to have the parts clean and degreased. It is also important to disassemble things and to set up your painting area so you don't get paint all over your bench, for example. I cut up little squares of paper (an office-type paper cutter works way quicker than scissors for this) and roll the paper up into little tubes and put them into holes and let them unroll until they mask the bores nearly completely. I started out brush painting shop projects, and I have learned a couple tricks. Use top quality brushes, thin your paint a little if it has been sitting for a few years, get a paint mixing propeller on a stick thingy at the hardware store so you can use a drill to mix the paint. If it isn't warm out, put a little Japan drier in your paint, this will help it dry quicker. Having come up in shipyard work, I have learned the value of good quality primer. I often will prime areas of projects that don't show and which I've derusted, but I don't want to take the time to make perfect looking. As for spray cans, I have long had a dread of trying to use cans that aren't brand new from the store because often they get the dreaded total plugup about a second after you start and then nothing I've tried will work to free them up. I think that sediment slowly sinks to the bottom of the cans, and the regular 1-2 minutes of haphazard rattle shaking doesn't mix them up enough, so that pile of goo is sitting there just waiting to clog up the tube. Someday I'm going to build a spraycan shaker - how hard can it be? Anyway, shake the can(s) until you can't stand it anymore, and then follow the directions. Might help to store spraypaint cans upside down too, don't know. I like to put big plastic bins over just-painted items and stick a trouble light with a low wattage bulb inside the bin. This warms the parts and the paint dries much more quickly. Motor parts often will fit into your oven, and you can warm them to 175F or so to dry the paint quicker.

Spend some time to mask the motor's nameplate carefully. I generally cover 100% with masking tape and then carefully trim the edges using a box knife. Splashing paint all over the nameplate is amateurish and a sign of sloppy workmanship, and it is to be avoided at all costs.

After the motor parts have been painted, remove all masking. Some masking tape adhesives can harden, so don't leave the masking tape on any longer than you need to, or buy quality masking tape in the first place.

REASSEMBLY

Motor bearings come prelubricated. I generally buy either shielded or sealed bearings. If the bearings are open, then grease them. The reason electric motors have grease in the bearing housings on the end caps of electric motors is the same reason those bearings aren't a super tight fit in those end caps - the bearings are supposed to be able to slide around a little. The armature warms up during use, so the motor's shaft gets longer. That's why the bearings are supposed to be able to slide in their housings, and this makes it necessary to lubricate the outer surface of the bearings so they don't get stuck in there. By the way, that's also the reason for that wavy washer you took out of the rear end cap - it puts a little side loading on the rear bearing to remove end play. That's why it was important to save that wavy washer. 

Press the bearings onto the motor shaft, making sure to only press on the inner bearing race. Push as evenly as possible. If only one side is shielded, put the open side towards the shaft ends, not towards the center of the motor. The bearings should be a light press fit onto the shaft and a slip fit in the end caps. If you find you need a shop press to push the bearings on you are doing something wrong.  

Once the bearings are solidly in place and the end caps' bearing housings are lubricated with a little grease, now you can tap the end bells into place, taking note of your witness marks so everything goes back the way it came off. Now's when you will be glad you marked the inside of the end caps with a Sharpie so you don't have to peer at the end caps with a magnifying glass under a strong light to find your witness marks! Tap your end caps in place, and reinstall the 4 long bolts with their nuts, now all derusted and oiled. Tighten those nuts to the proper torque which you can look up in Machinery's Handbook. 

Reinstall the wiring box, using the original gasketing if available. Make sure your wiring is all pulled through into the box, and put the lid on. 

Take a final look at the zerk fittings. Are they rusty anywhere? If so, it probably makes sense to replace them. They're very inexpensive and are available at any auto parts store or good hardware store. Reinstall the zerk fittings if the motor was fitted with them. Now load up your grease gun with quality grease that is compatible with the grease you used on the end cap bearing housings, and force grease into the zerk fittings until you see it come out around the shafts. Don't overdo this step, you don't want to jam too much grease in there, but the idea is to make a plug of grease that will really help keep contaminants and moisture out of the bearing area. Wipe away any grease that may have oozed out around the shafts.

Now reinstall the TEFC fan and shrouding, if needed.

Look around -- there shouldn't be any parts left on your bench! At this point I generally spin up the motor to make sure everything's OK and running smoothly, and to admire the sound of brand new motor bearings. If the motor spins up correctly, you can congratulate yourself on a job well done!