Dismantling a Brook Crompton AC Motor (from a Myford Lathe)




Smoke was billowing out of my  1/3 HP Brook Crompton motor when I switched it on. I immediately switched it off and stripped it down to see what the problem was (I guessed it was the capacitor - but found out that it doesn't have a capacitor start - it has two out-of-phase windings and switches one winding off when it gets going).

This Instructable is short on words, because there is not a lot to say, but has photos to show the steps taken to split the motor apart to enable it to be cleaned.

I also give some resistance values for the windings, etc.

The process was not difficult, and required no complex tools. It took about 3 hours, start to finish.

In the end, I seemed to do nothing except clean it up. When I reassembled it and connected it up, it ran as sweetly as ever! I found no fault. As you can see, it was filthy inside, with 40-50 years of crud - perhaps some of it caught fire to produce the smoke? There was no evidence of overheating inside.

The motor had the following info on its plate:
E78 KP6345
Spares cat: 1-V
RPM: 1425

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Step 1: Split the Casing Apart and Remove the Front End Cap

There are 4 long nuts and bolts holding the ends together. These need removing. I marked the casing with felt-tipped pen to make sure I could get it back together.

I undid the nuts most of the way and eased a screwdriver in the join at the shaft end of the motor. Then removed all four bolts completely and took the front end casing off.

Step 2: Remove the Rear End, the Wiring and the Rotor.

Ease out the rear end a little and the four internal wires can be seen. These must be removed before the end can be taken off completely.

I had the external connecting wires in place - these needed to be disconnected first (I first marked them with felt-tip pen so I could return them to the correct terminals).

Then I disconnected the four coloured wires from the connection board.

I carefully lifted off the end cap and fiddled the three winding wires from alongside the connection board. A fourth wire needed disconnecting from a spade connector (just pull it off carefully with pliers - it slides back on later).

The rotor can then be slid out from the motor.

Step 3: Remove and Clean Up the Switch Contact Board

Remove the two holding screws.

A paint brush is good for getting rid of the dust!

Step 4: Check the Operation of the Centrifugal Weights

When the rotor is not moving (or moving slowly) the contact is pushed closed by the black plastic collar which can be seen in these photos. As the motor spins faster, the collar is pulled away from the contact board by two weights which swing outwards - the contact is then broken and the start winding is disconnected, leaving the motor running on its running winding.

Note: the rotor has no windings as such. (There are solid conductors inside its iron laminations).

Step 5: Electrical Resistance Readings

Winding resistances between the four wires as measured with a normal ohm meter - completely disconnected from the mains:

There are two completely separate windings - each insulated from each other and from earth.

Z to T resistance =10.2 ohms
A to AZ resistance = 7.4 ohms

When wired up for forward rotation the resistance between the Live and Neutral wires is 4.2 ohms (note - immediately the AC is connected to the motor, the resistance would increase instantaneously, which is why the fuse does not blow).

When operating under normal conditions, just after connecting up, you see a spark from the centrifugal switch as the switch opens (disconnecting the start winding) and you hear a clear click from it. When you switch off and the AC is removed, you also hear a click as the motor slows down and the centrifugal switch makes the contacts again.

When stripped down, the contact shows an open circuit when at rest, and a short circuit when the little nylon cam is gently pressed (these are when you measure the resistance between the yellow wire and the spade connector on the other end of the circuit board).

Step 6: Clean Up and Reverse the Steps

I cleaned everything thoroughly.

I used virtually no lubrication (dust gets attracted to it) except on the felt inside the main support bushes at each end - I made sure these were wet with oil.

I used the felt-tip markings to make sure I replaced the casings in the right places.

When I reconnected it to the mains. It started and ran beautifully - hopefully for another 40 years or so!

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    8 Discussions


    Reply 6 years ago on Step 6

    I'm still not really sure. It was filthy inside, but I can't see what might have generated any smoke.

    I have one thought: When I put the motor back in, I noticed that the pulley/axle which it drives did not rotate very smoothly - it was a bit stiffer to rotate than I expected. Perhaps, on this occasion, it grabbed a bit more than usual and presented the motor with a huge force to overcome, causing the motor to overheat??? The smoke happened very quickly, so I'm not convinced.

    On hearing my thought above, someone suggested that I had seen smoke coming from the motor pulley as it slipped on the belt producing the smoke from the friction. This sounds possible (although there was no rubbery smell and I really thought the smoke was coming out of the motor - however, it did all happened very quickly).

    Either way, the pulley/axle was also cleaned and lubricated, so I hope I have all bases covered!

    Best wishes


    Reply 6 years ago on Introduction

    I just purchased a very nice myford super 7 but the motor same as yours is heating up even without a load, are these motors like that? would you know the normal operating temperature?


    6 years ago on Introduction

    Thanks, I've a 1974 Compton motor off a Myford Super7 and I bet it's as bad as yours inside. Its certainly a lot more oily, Will follow your instructions, many thanks for posting.


    6 years ago on Introduction

    Being such an open frame motor it might be a good candidate for the old pantyhose stocking over the whole motor trick. Either that or just pop an end cap off every couple of years and blow it out with some air. I wouldn't be shy about oiling it either. A motor like that needs oil on a regular schedule. Make sure how you mount it the oil fill is facing up too. When the motor burnt up were you stopping, and starting it a lot? Doing that can cause some types of motors to overheat. An induction motor will pull 6 times its running current starting. That current means heat. In that case a supplemental external cooling fan isn't a bad idea. But let that fan run for the duration that you are using the machine to increase your duty cycle. P.S. That trick works for cheap welders that don't have fans too! You can effectively double, or even triple a duty cycle with a cooling fan. Although I'm sure taking the winter coat that motor was wearing off of it like you did will help it out a lot too. Like you said it will probably be good for another 40 years now. Just make sure it is grounded well, because the internal flexible insulation will fail before then. That can lead to a shocking experience! I've had to rewire motors because of that.

    Phil B

    6 years ago on Introduction

    You are fortunate that the motor ran again. I remember a large motor on the blower for a church pipe organ. It smoked. The centrifugal switch contacts had pitted over the years and stuck together. That overloaded the starting windings, and they burned. The motor had to be rewired completely.

    I have always seen people make marks to identify the location of the end caps with the frame of the motor for reassembly. You used a felt tip marker. I have often seen marks with a chisel.

    Thank you for your Instructable.

    1 reply
    pfred2Phil B

    Reply 6 years ago on Introduction

    With a motor it is hit or miss whether it is OK once it cools down, or not. I think a lot of it depends on how badly it cooked. If you can catch it quickly enough the insulation can "self heal" as it cools and solidifies. I have a grinder here that I bought used and the windings look like they cooked in it at one point, but it is fine now. Of course when I bought it the guy I got it off of told me oh, it is fine. He was lying of course, but it turned out he was right, once I reset the thermal switch on the motor.



    His loss, and my gain.