Wiring up a Brooke Crompton single-phase lathe motor (Myford Lathe)

Step 4: Dewhurst drum switch

Picture of Dewhurst drum switch
dewhurst basics.jpg
Dewhurst motor connections.jpg
Dewhurst full circuit.jpg
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.
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