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

By www.facebook.com/quentin.thurtle

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
250W
RPM: 1425
220/240V
2.9A
50Hz

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!