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.
<p>In rebuilding a Myford ML7 I came across the problem of how easily to reverse the Brook Crompton Motor so I rebuilt the electrical system. I posted a series of images of the rebuild on my blog www.ritasears.blogspot.co.uk and look at the workshop tab.</p>
Wiring up a Brook Crompton Parkinson Motors single-phase lathe motor 3 terminals A,AZ,Z ..does the Z=blue to body is this just left with only the blue from body or do i connect the body earth to it and the g/y to the body and blue =plug to A=black body and y/r=AZ to plug brown is this the correct please . thanks
<p>I'm very sorry, but I have no experience of any other motor, so I can't help. Mine had four internal terminals not three, so I'm afraid I'm not going to be of any use to you!</p><p>Best wishes - I hope you sort it out.</p>
<p>hi i know this is an old post but i have a brook crompton motor single phase that came off my old table saw i want to reverse the way it turns so i can use it on my band saw but mine is twin capacitor type. i looked online found a very old listing for this motor it says it can be electrically reversed so it can turn cw and ccw. but i can't find any wiring diagrams for this. iv'e done things like this before but this one has me stumped the caps have the ratings of 15uf and 20uf there is also a centrifugal switch inside as i have stripped back the motor already. i'v tried the simple change of the live and neutral feed but to no avail. could you suggest anything at all thank you dave </p>
<p>I have just installed my Myford lathe and used an NVR switch complete with emergency stop feature. However, the motor just does not start. There is power to the on / of switch (terminals 13 &amp; 23) but nothing at the outlet (terminals 14 &amp; 24). If I bypass the new switch assembly and wire directly from the mains to the terminal block at the back of the lathe, the motor works fine using the Myford on / off switch. I have changed the NVR switch for a new one, but no result! What am I doing wrong??</p><p>Thanks - Nick.</p>
<p>I.m trying to find at which terminals the capacitors connect as my motor does not have &quot;T&quot; connected but does have &quot;K&quot; could any one help with this. Cheers </p>

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