Westinghouse Mobilaire Motor Rewind

This is a Westinghouse Mobilaire ma 4020 fan made in the 1950s. It belongs to my son-in-law.* He bought it used. After a few months it sparked quite a lot and quit. Wires in the motor had burned off, as can happen with very old enameled wire that loses its insulation and flakes off. Motor shops quoted ridiculously exorbitant prices to rewind it. I offered to do it for the cost of materials.

This is my first attempt at rewinding an electric motor, and it succeeded. I have read articles about the process and there are a couple of published Instructables on rewinding electric motors. This Instructable will share my experience and caveats I observed. It may be helpful to someone who has a Mobilaire in need of a rewind. Another option is to buy a new fan motor with the correct shaft size and make it to fit the existing mounts.

*My son-in-law repainted this fan black. It was a light gray from the factory.

22 and 24 gauge enameled copper wire suitable for 200 degrees C.

18 gauge stranded wire for connections between the windings and the switch

Insulating paper for use in an electric motor.

Varnish for use in an electric motor

DPDT switch

I made a drawing of the motor wiring scheme. I wanted to record the diameters of the two wire sizes used in the coils on this motor. I noted the direction in which each coil was wound whether clockwise (CW) or counter-clockwise (CCW). I noted which wires connected to which leads from the switch.

Make an abundance of notes. It is easy to second guess yourself later. I used a digital caliper to measure the thickness of the wires. Wires are best measured where the wire has not been bent and after the enamel has been burned off. Consult a wire gauge chart in a book or on the Internet to determine the gauges of the wires. According to my measurements, this motor used 23 and 25 gauge wires. I could find only gauges with round numbers. I ordered 22 and 24 gauge wire. It is safe to choose a slightly larger size, but not a smaller size. I calculated the number of inches needed for one turn of wire and multiplied by the number of turns needed to decide what size spool to order. You will likely buy a little more than you need.

I used a marking pen to make a curved arrow on each post to make for less confusion when actually winding coils in place. I also numbered the poles for reference later.

I did my best to unwind each coil and count the turns of wire in it. Soon it became apparent each of the six poles had 100 turns of 22 gauge wire and 25 turns of 24 gauge wire. (Oddly, some of the poles were not consistent and randomly had a few extra turns, but there did not seem to be a pattern.)

I used Nomex paper for electric motors. It would have been easy to buy a roll of it suitable for an electric motor shop. Finally, I found a seller on eBay who offered packets of five sheets. I used less than one sheet. I believe Nomex paper is mineral based rather than wood pulp based. It can handle temperatures of 200 degrees Centigrade. Motors do have a heat rise when in use.

I mimicked the use of the insulating paper in the motor as it came from the factory. I added an extra narrow piece as shown in the photo to make certain the coil wires stay inside the opening for them and slipped it in after the coils were wound and placed. Paper to insulate is cut wider than the slots for the wires so wire is not resting on metal and its crisp edges that could compromise the enamel insulation on the wire. Paper from the factory was bent over at the edges. Nomex paper is like thin plastic. It resists bending and quickly refuses to stay in place.

In the photo you can see my curved arrows to indicate the direction in which each coil is to be wound. They are a little blurred because the spray varnish caused the felt tip marker ink to smear and run.

Most motors allow winding the coils and slipping them into the slots. The poles on this motor are shaped in a way that makes that impossible. I decided to make a fixture that would hold the frame of the motor, but allow me to change its position while in use. The jig would also hold a spool of wire from which I could pull wire easily.

There are holes in the motor frame for the bolts that hold the parts of the motor together. I positioned rods so the motor frame easily slides onto them. When welds cool things usually move. I had to watch for and correct for that.

This fixture is made from scraps and I clamped it into my vise. A similar fixture could be made from wood.

As mentioned earlier, it is impossible to wind coils on a form and then drop them into the motor frame. I wound each turn into the motor in place one at a time. In the photo the smaller coils from 25 turns of 24 gauge wire have been completed. One larger coil of 22 gauge wire is in place and a second coil has been begun. Notice the spool of wire from which I pulled wire as I wound it.

Every other pole is wound clockwise. Poles between them are counter-clockwise. That means each coil is wound in the opposite direction of the coils on the next pole.

It should be easy to count turns of wire, but it is very easy for your mind to wander and you lose count. See the second photo. After ten turns I made a stroke. Ten strokes meant 100 turns for a complete coil.

McMaster-Carr offers a spray varnish they say is rated for high electric motor temperatures and will insulate the motor coils while holding them firmly in place. I sprayed a number of applications. I wanted to get enough liquid into the coils that it would flow deep inside them, but I did not want all of it to run out the bottom before forming a hard barrier at the bottom side. So, I sprayed and waited, then I sprayed and waited repeatedly. This seems like a good method for someone who wants to rewind one small motor at home. The alternative used by motor shops is to preheat the motor in a special oven, varnish the motor, and bake it for four hours. It is advised you do not use your kitchen oven.

After winding tie the coils to keep them from spreading, which can mean windings may rub on the spinning armature. This motor generates so little heat that household cotton string worked fine. Also, the spray varnish saturates the string and it is rated for very high temperatures.

It is not difficult to avoid confusion on making connections. When finished winding coils, there are four wire ends. Two come from the left and two go to the right. Two are a smaller gauge and two are a larger gauge. (If you measure the resistance of the completed coils and apply Ohm’s Law, it seems there is a problem. The resistance is too low and too much current will flow for the size of the wires used.. But, this is an AC current motor, and each pole generates an inductance which adds a significant resistance not seen from Ohm’s Law.)

I decided to test one thing at a time. I connected a pigtail from lamp cord to the two larger wires that go to the main coil. But, before putting power to the motor, I took some precautions.

I used an Ohmmeter to check for shorts from coil wires to the steel frame of the motor. Use some high Ohm scales.

Turn the motor by hand to be certain the armature is not binding on anything, especially on the newly wound coils. It should turn freely without any drag or scraping sounds. (I had to make a collar to center the armature and align it with the frame of the motor to which the coils are wound.)

I made up a pigtail with an in-line 5 Amp. slow blow fuse. See the second photo. The 22 gauge wire will handle a bit more than 5 Amps. over a short distance. The fan draws less than that. If there are any shorts the protective fuse will blow and save my new motor windings. I also connected an Ammeter to see what the exact current draw is when the motor pulls its maximum current, which is the high speed setting. A higher than expected current draw would mean an internal short between turns in the windings.

See the first photo. I mounted the motor frame in a vise and made my connections. I made certain I was able to pull the plug from the outlet instantaneously if there were any indications of a problem. Notice the blur of the fan blades. The fan motor roared into life.

Had the fan run backwards, I would have needed to turn the motor frame over 180 degrees to change the direction because this is a shaded pole motor, not one with a starting coil and a run coil.

In the previous step I tested the main or high speed coils. The motor ran well and in the correct direction.

This motor is easily identified as a shaded pole motor using six poles. See the third photo. Notice a slot across each pole. In it is a black band that is actually a band of copper. A current is generated in the copper band that is 90 degrees out of phase with the current in the armature. This out of phase current creates something for the current in the armature to chase after, resulting in rotation of the armature. These copper bands can become quite hot. The moving air from the fan blades helps to cool them.

The first photo shows the wiring diagram for a two-speed shaded pole motor The colors for the original switch wires were yellow, red, and blue. I used blue wire to make new leads from the motor to the switch. I indicated the color of each piece by wrapping it with colored tape. I did not have any yellow tape and used orange in its place. See the second photo. It gives a glimpse of the new switch I added. You can make out the red and orange bands I added to identify the wires. The switch had bare contacts. I soldered the wires to the contact terminals. Then I covered all bare contacts with a liberal amount of hot glue. The switch is well insulated.

Connections between the windings and the 18 gauge stranded wire I used to connect to the switch were crimp connectors insulated with heat shrink tubing. I made only one such connection between any two poles to keep any opportunity for shorting between connections as low as possible. I did also solder the crimp connections to ensure the connection is good. The heat rise shown by means of an infra-red thermometer was found to be so low that there is no danger of failure of insulation or solder due to excessive heat.

In the circuit drawing H and N represent hot and neutral. A shaded pole motor adds an extra coil in series to create additional resistance. That lowers the voltage and slows the motor. This motor also connected the extra coil as if wound in the opposite direction from the main coils. (See my drawing from step 1.) That does not just add resistance, but creates a counter-emf that further reduces the magnetic field in each pole.

The original switch had a dead spot and one of the settings did not work, so I decided on a new switch. On eBay I found original switches as new old stock. Their configuration is off-on-on. A local supply house had a new rotary switch very similar, and it probably would have been fine. But, its current carrying capacity was only a little above the 3.75 Amp. operating current draw on this motor. I decided for a double pole, double throw toggle switch (DPDT) capable of carrying more current. Its pattern is on-off-on. That means the Low speed setting would be used directly from “off.” I wondered if the motor would struggle to start from a stop when moved to the Low speed setting, but the motor handles that very nicely with no problem.

The motor worked and the fan runs just like it should. Both speed settings work well. While this motor is a shaded pole design, Westinghouse used different motors on these fans. Some are capacitor run motors. I read that the original motors from 1948 were capacitor run motors. Those motors have a round shell. Westinghouse introduced the six sided or hex motors in 1954 as a cost cutting measure.

My expenses for this project came to around $75. Cash outlay would have been a little less if I could have purchased smaller quantities of things like wire and insulating paper, but I had to buy in the lots available, and that meant buying more than I actually needed.

UPDATE—I have since seen another of these fans. I believe it was made in the mid-1950s, and its windings are quite different. I did find some information at a forum for antique fans, but it does not give all that is needed. The burned out fan motor is so badly burned that it is not possible to remove windings and diagram the circuit. In cases like that, it is always possible simply to use the winding scheme I described above.