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Re-wiring a forward/reverse switch

Dewhurst & Partner Ltd Type A

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John Coates07/04/2010 20:24:10
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558 forum posts
28 photos
My Barker lathe has one of these fitted. No matter how I have a go a swapping the wires round, Reverse is "normal" i.e. rotating downwards towards the cutting tool, and Forward is "wrong" ie. rotating upwards
 
I know that looking from tailstock to headstock Reverse and Forward are correct in that Forwards is clockwise and Reverse anti-clockwise, but I would just like to have it seem more natural with regards to the mandrels rotation towards the cutting tool
 
Anyone have any experience of this switch ?
 
There are two cables from the single phase motor (don't ask me what it is) entering the switch on the Reverse side and a single cable for a normal plug entering on the Forward side. Neutrals for both wires are connecting to a bolt fastened to the casing
 
If the Forward side (left) has connections A through D going down vertically, and Reverse (right) has 1 to 4 similarly then the current wiring is:
 

A - red (plug cable)
B - red (motor cable)
C - black (plug cable)
D - unused
 
1 - red (motor cable)
2 - black (motor cable)
3 - black (motor cable)
4 - unused
 
Thanks for any help

John
mgj07/04/2010 21:20:53
1017 forum posts
14 photos
Normally to reverse a single phase motor you just swap the two wires over on the capacitor.
 
In the absence of a diagram, I'd track the wires back from the capacitor and swap them round on their terminals. That should make reverse forwards and forwards into reverse.
 
The wires (2) out of the capacitor are not normally differentially coloured so it doesn't matter which way round they go, but what does matter is which way the phase changes, which is how a motor is reversed, and what the capacitor does - in part.
 
Then before I actually started anything I'd ask Les Jones! (or send him a PM)
 
 
 

Engine Builder07/04/2010 21:21:18
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267 forum posts
Hi John.
There will be 4 wires comming from the motor, 2 go to the start winding and to 2 the run winding. If you put a continuity test on the 4 wires you can identify the 2 windings.
To reverse the direction you need to reverse the wires to one winding only.
I assume when you say the neutrals are connected to the case you mean the EARTH wires.
Neutral shold not be connected to the case.
David
Engine Builder07/04/2010 21:23:48
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267 forum posts
@ mgj
swaping the capacitor wires will not work, you need to swap the winding wires.
David

Edited By Engine Builder on 07/04/2010 21:28:48

Engine Builder07/04/2010 21:39:15
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267 forum posts
Me again!
John, if the switch we are talking about is a Dewhurst type A, I have the wiring diagram for this.
David
John Coates07/04/2010 22:02:44
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558 forum posts
28 photos
Hi David
 
Yes it is a Dewhurst Type A
 
Apologies for my typing mistake. It is both earth wires that are attached to the casing
 
regards
 
John
mgj07/04/2010 22:36:52
1017 forum posts
14 photos
Engine builder - thankyou . I do apologise.
 
Mine reverses on the drum switch by reversing the wires to Z and AZ, and I had assumed that was the capacitor, but you are dead right - they are the start windings. Sorry - should have checked.
 
Yellow and blue if I remember right.
 

Edited By mgj on 07/04/2010 22:44:08

Les Jones 108/04/2010 08:53:26
2292 forum posts
159 photos
Hi John,
                 I agree with David (Engine builder.) The two pairs of wires from the motor will be the start and run windings. The start winding will probably have a capacitor connected in series with it. (At motor end.)  Label the four wires from the motor with the switch terminal number / letter. (As you have two blacks and two reds) Disconnect these from the switch to avoid paths through the switch causing confusion. You will either find two pairs showing continuity of a few ohms or one pair showing continuity and the other pair open circuit  (Or a vary high value of a few hundred K ohms if there is a safety discharge resistor  across the start capacitor.) I you are using an analogue meter it will flick when first connected as the capacitor charges.  If you do get one of these results then connect one pair to their original terminals and connect the other pair to the original terminals but swapped over.
If you cannot identify the two pairs for any reason then DO NOT SWAP ANY WIRES OVER but reply with the results for further thoughts or suggestions.
Les.
 
Peter Gain08/04/2010 10:04:11
103 forum posts
An extra observation. The Dewhurst drum switch should not be used on load, it is an off-load device. The machine should be swiched off via the DOL starter, the Dewhurst can then be re-positioned & the machine re-started by the DOL starter.
Peter Gain.
 
Eric Cox08/04/2010 10:46:54
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557 forum posts
38 photos
Reverse is "normal" i.e. rotating downwards towards the cutting tool, and Forward is "wrong" ie. rotating upwards
 
I always thought it was the other way round, ie. Forward is "normal"
mgj08/04/2010 14:33:42
1017 forum posts
14 photos
Surely on this one the strong likelihood is that the terminals for the start windings will be marked, Z/AZ, or Z1/Z2. Dependent on the type of motor. Either way all one need do is to reverse these two.
 
If they are not marked then its time to scratch heads, agreed.
 
As for reverse and normal - presumably its whichever way you want it to be, since with a reversible AC motor either way is whichever?
Peter Gain08/04/2010 14:56:41
103 forum posts
Consult "Tubal Cain" Model Engineer's Handbook. Section 12 details a variety of possible connections including the different letter refs assigned by several different maunufacturers.
Peter Gain.
Engine Builder08/04/2010 17:30:33
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267 forum posts
John, from your description of the switch connections I can relate that to the wiring diagram I have. You need to interchange the connections to 1 & 3 on the swich. I can email a copy of the Dewhurst instructions to you or anyone else if you let me have your email address.
 
John Coates09/04/2010 20:36:06
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558 forum posts
28 photos
Hi David (Engine Builder)
 
Message sent with my email address. Thanks
 
As I am a Beginner in many things, not least electrical testing, when I follow Les' advice and connect the red and black motor wires to check for continuity, when I show a reading (i.e. continuity)  does this mean I have found a matched pair from either the start and run winding?
 
I will be using a Gunson automotive multi-meter (as that's all I have)
 
Thanks ever so much every one for the help thus far. I must say everyone is fantastic on here and tolerant of endless questions from newbies like me, and as helpful as my motorcycle owner's club website with the technical stuff
 
John
Engine Builder09/04/2010 21:21:20
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267 forum posts
Yes John
You will get continuity between the 2 wires from each winding so of the 4 leads comming from the motor you will find  you can get a reading between 2 pairs. From your description and reference to the diagram I have sent you, it seems each winding has one black and one red cable.
David
 
 

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