Introduction: Wiring Up a Brooke Crompton Single-phase Lathe Motor (Myford Lathe)

Here are a few notes about wiring up my small lathe motor. They are specific to the particular motor referred to in the title and shown in the photos (250W, 2.9A, 50Hz, 220/240V AC).

The notes are simply to show my own set-up. This Instructable deals with potentially lethal mains-voltage wiring, and should only be tackled by someone who is competent. It relates to UK voltages and currents (230V AC). Consult a qualified electrician. Always use an appropriate fuse. Always ensure all exposed metal parts are earthed correctly.

I am not going to put a myriad of safety warnings on the steps, common sense is required. No doubt, people will comment if I have made mistakes on any of the pages, so read any comments which appear.

Step 1: Basic Wiring

This type of motor consists of two separate windings.

One winding is called the "Running winding" and is connected to terminals marked "A" and "AZ". Myford suggest that "AZ" terminal is connected to the AC live and the "A" terminal is connected to the AC neutral (if they are exchanged, then the motor would simply go in reverse).

The other winding is called the "Starting Winding" and is connected to terminals Marked "Z" and "T". Myford suggest that for forward rotation, "T" is connected to AC Live and "Z" is connected to AC Neutral. The starting winding has a centrifugal switch in series, and electricity only flows to the winding during the start-up; once it is up-to-speed, the switch disconnects the winding from the mains.

The four terminals can be seen in the first photograph. It looks like they are already wired-up, but they are not - the wires you can see are for the motor's internal wiring which goes to the two windings. You can clearly see the labels (except for the "T" label which is partially obscured). On my motor, there are two labels which have no terminals ("K" and "S").

To make the motor run forward, the connections mentioned above ("AZ" and "T" should be connected to AC live and "A" and "Z" should be connected to AC Neutral). In addition, the Earth connection must be connected through to the metal body of the motor using the screw terminal shown bottom right.

The simplest connection for a motor would use a 13A plug with proper 13A 3-core flex and a 13A fuse. Inside the motor, on the terminals shown in the first photo,  a wire would link the "AZ" and "T" terminals together which would then connect to the brown, live flex wire; another wire would link the "A" and "Z" terminals together and connect to the blue, neutral flex wire; and the Earth (yellow and green) flex wire would connect to the Earth screw.

If you want to have the possibility of reversing the motor, then you need to take all four terminal connections outside the motor so that the starting winding wires can be swapped over. The second photo shows this with my motor as it was received. NOTE, BE WARNED: confusingly, the red and black colours do NOT relate to the live and neutral mains connections.  Myford suggest using the colours shown for the motor connecting wires (Black for "AZ"  and "A"; red for "Z" and "T") I would prefer four different coloured wires to avoid confusion at the other end! My motor was not brilliantly connected - the red connecting wire going to the "Z" terminal is showing a bit too much copper wire - however, it was running fine.

Finally for this step, I wanted to connect my motor up in a way which achieved three things.
I wanted:
  • to be able to remove the motor easily (for easy transportation of the lathe);
  • to have a simple forward-only connection at first;
  • to be able to add a reversing switch easily in the future.

I did this by producing the die-cast connecting box shown in the photos. To remove the motor, the three mains wires are pulled out from the terminal block and left hanging to the left hand side, then the trunking, its gland and the terminal block is simply lifted out of the die-cast box and left permanently connected to the motor. I put a label in the base of the die-cast box, to remind me about the connections in the future. Two photos show the trunking being lifted out of the box, and the cut-out which allows this to happen.

Not only should the Earth wire be connected to the Screw terminal on the body of the motor as mentioned above, but the motor body should also be electrically connected to the metal mounting strap as shown in the next photo. This is because the motor is actually supported at both ends on a rubber mounting (electrically insulating). It is possible that there would be no electrical Earth connection from the motor through to the metal of the lathe without this connection being present.

The motor only takes 2.9A when it is running, but at start-up it takes considerably more, hence the 13A fuse. The actual type of wire used for connecting my motor is shown in the photo. Single strand or multi strand is good. 1.5 sq mm (15A rating) would be fine or 2.5 sq mm.

Step 2: Reversing the Motor

To reverse the motor, before applying any current, the start winding terminals should be swapped over. The connections to the running winding are not changed.

Reverse switching should NEVER happen while the motor is connected to the mains supply. The reversing and the starting/stopping of the motor are very different operations and must happen separately, with time given for the motor to come to a stop, before starting up in the opposite direction. In fact, by putting the switch in the starting winding circuit, the internal centrifugal switch will disconnect the winding from the mains, once it is up-to-speed, so operating the switch should not make any difference; however, the motor must be permitted to stop before power is applied again.

The switch needed for reversing the motor can be a 'normal' 13A or more, rating - it is not operated while the current is flowing, so only needs to be able to carry the maximum (surge) current. It must be a mains-rated two-pole two-way (DPDT) switch and have six terminals on the back (not just a 2 pole on/off switch). On ebay there are many mains DPDT rocker switches (230V AC 16A) which would be suitable.

Step 3: On Off Switch

Starting and stopping the motor generates some high voltages and will cause the terminals of an under-rated switch to arc and become damaged. Ebay lists many suitable on/off switches intended for bench drill-press operation - these seem to be an ideal solution.

In theory, an NVR (no volt release) on/off switch would be best. These are held "on" electrically and if a power cut happens whilst the lathe is operating, the switch would 'drop out' meaning that when the power comes back on, the lathe would not immediately start running (as it would if a mechanical on/off switch were fitted). This is a safety feature. Suitable NVR on/off switches are on ebay for £13 or so (the photograph shows one), virtually identical non-NVR switches are about half this price.

Step 4: Dewhurst Drum Switch

Reversing is often achieved with a drum switch like a Dewhurst Switch. These can be prone to arcing and sparking. Some sources suggest that they should not be used for switching the power on and off, only for reversing the direction when the power is already off. Many installations have an NVR on/off switch before the Dewhurst switch.

The terminals inside are numbered 1 to 8 but it can be hard to see the numbering which is punched into the base plate and then painted over - the third photo shows the numbering 1 to 4.

Terminals 4 and 8 are not used for a 'normal' single phase motor.

The live supply wire from the mains is connected to terminal 1
The neutral supply wire from the mains is connected to terminal 3

When the switch is in the 'off' positions, all the terminals of the Dewhurst switch are isolated from each other. When it is forward or reverse positions, the running winding is connected to live and neutral in an unchanging way (terminal 5 'AZ' is connected to terminal 1 - the live supply wire; terminal 7 'A' is connected to terminal 3 - the neutral supply wire).

To arrange the reversing, the remaining two terminals (2 and 7) are connected to the starting winding ('Z' and 'T' respectively).

Confusing when written down, but hopefully clear when seen in diagram and photograph form! The photographs of the Dewhurst switch have the live and neutral supply wires removed for clarity (you can see the felt-tip-pen 'L' and 'N' to remind me where they go). AGAIN NOTE Myford's rather weird black and red colouring of the wires to the motor, which do not relate to live and neutral in any sensible way.

Obviously, the Earth connection is maintained throughout - the metal body of the switch itself should be connected to Earth. Typically, the Earth connection is brought in from the 13A plug (or on/off switch) to a terminal which is fixed with a nut and bolt to the metalwork of the Dewhurst switch and then continues onwards to the Earth terminal on the metalwork of the motor itself.

Step 5: Contactor

Some installations use a DOL motor starter (Direct On Line) instead of a simple on/off switch.

There are various configurations - the one pictured has a starter switch (circuit breaker/contactor) with an NVR (No Volt Release) and an overload relay which will 'trip' out if too much continuous current is drawn (but not 'trip' during the starting phase which demands more current).