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DEWHURST reversing switches

DEWHURST reversing switches

i see a good few electrical topics here,usefully answered,here,s my q-
is there a difference between a 1 phase dewhurst switch and the 3 phase one?
i have removed a dewhurst from a redundant 3 phase machine and had hoped to use it on my myford ML10 which is single phase,i believe that reversing the connections to the start capacitor gives reverse direction on certain motors,but cannot see how to wire the switch to achieve this.
any guidance?

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PJ O'furey
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Re: DEWHURST reversing switches

Ah the Dewhurst Switch

PJ O'furey wrote:

i see a good few electrical topics here,usefully answered,here,s my q-
is there a difference between a 1 phase dewhurst switch and the 3 phase one? Yes I think so
i have removed a dewhurst from a redundant 3 phase machine and had hoped to use it on my myford ML10 which is single phase,i believe that reversing the connections to the start capacitor gives reverse direction on certain motors,but cannot see how to wire the switch to achieve this. You need to reverse the connections to the start winding not the capacitor but in effect it may be the same, reversing supply to capacitor I think refers to small capacitor run motors.
any guidance?

Your switch should have 4 layers if it is to be used on single phase. Note that you should not switch the load with the switch, there should be an additional stop start switch. The Dewhurst should be used to alter direction with motor stopped then power applied with stop start switch, this is not what people normally do and if you use a Dewhurst to switch the load over time the contacts spark excessively and burn out. Some while ago there was an advert on this site for spare contacts well worth searching fort if he still has any. I bought some in the Summer and the Ad was before that as I remember fitting them outside in the sun.
I've played with a Dewhurst and they are very simple far simpler than the Boxford type I still have a bag of bits for one of those.  I also know they are very common but they never seem that electrically safe to me lots of exposed contacts and a flimsy cover.  They must be all fairly old by now.
See here http://www.uphill.org.uk/pages/engineer … tips_2.htm
Best of luck,
Merry Christmas to all .
Ashley

Ashley (Remap Volunteer look us up  www.remap.org.uk  The best volunteering opportunity for practical people)

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Re: DEWHURST reversing switches

Hi PJ, Ashley,
The chap with the spare contact sets for Dewhurst switches is Bob Davis, from London (Wimbledon?). Email is  mechanalia(at)googlemail.com, change the "at" obviously,(at) is this site standard. Last phone no I had is 0208 788 6621.  I have had some from him, £20 per set incl p&p, and good kit.  Also a good idea to replace the old steel posts with new stainless steel ones, the old ones get pitted when the switch is used as on/off, as Ashley says tthis is not good for it, it should be preceded in the wiring by an on/off NVR switch (these days), or by a DOL starter (for us old fogeys).  When you replace the posts you find that some are in one layer and others in two, thus explaining how the current gets around the switch levels.  The trick is to remember which is which!   If I get time I'll post some pics of a dismembered Dewhurst which may help, however it is Christmas, and whilst I've got the sprouts in I still have gifts to wrap.
On a capacitor run motor you reverse the connection on the capacitor, on a capacitor start motor (more usual on a Myford), reverse the connections to the starter windings.

Bye for now, SWMBO is calling
Phil

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Re: DEWHURST reversing switches

PJ,
The 3~ and 1~ switches are the same and the reference Ashley has given shows a suitable connection. If fact you can reverse Either the Start winding OR the Run winding (but not both) to make the motor run 'the other' way and because of the connections at the motor end, you may well find it easier to reverse the Run rather than the Start winding. In the diagram, A1 & A2 are the Run windings.
The connections to the "Motor Winding", which is actually the Start winding, are rather dubiously shown in green, but are of course not the same as the Earth, also shown in green. The other end (from that connected to Pin 2) is connected via the internal centrigfugal switch to the capacitor and then to K1 in the diagram. The order of connection may be different eg winding-capacitor-CentrifSwitch-K1, but the order doesn't matter. It is also very possible that your motor terminals may have different nomenclature as it is not 'Standard', just 'fairly common' and Z/Z1 is reasonably widespread for the Start winding

The Dewhust switch was originally used as both 'The Switch' and the Reversing control, but I agree with Ashley that it is better to have an additional relay/contactor for the On/Off duty as it is then easy to arrange a No Volt Release (NVR) so that the machine will not re-start after a power failure, say. If you do so, it is preferable to use a double pole version so that it breaks both Neutral and Live. Also, run the contactor coil supply through the spare (4-8) contacts so that if you inadvertantly operate the reversing switch without first stoping the motor, the 'Centre Off' position of the Dewhurst switch will trip the contactor out. Even without this there is actually no risk of slamming from full ahead to full astern as it is only when the motor is at (or near) a standstill that the centrifugal switch is closed, and that is what controls direction.

The above is exactly what  have done on one of my machines and if you get stuck I can easily enough butcher a wiring diagram to match what I did.

- Richard -

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Re: DEWHURST reversing switches

Agree with everthing so far, but, if adding a relay for on/off, why not just use two relays, 1 – on/off 2 – Fw/Rev. Then discard the Dewhurst sw. I dont like them, cover too easly damaged with live contacts in close proximity, not a good solution.

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Re: DEWHURST reversing switches

PJ,
Decided not to post pics of dismantled Dewhurst, try these diagrams
Philhttp://www.homeworkshop.org.uk/components/com_agora/img/members/216/27122010-1554_Dewhurst-wiring.jpghttp://www.homeworkshop.org.uk/components/com_agora/img/members/216/Motor-terminal-markings.jpg

I didn't say it was a perfect plan.

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Re: DEWHURST reversing switches

I'm with Bill; relays are a much better solution, but of course there's a cost consideration: you won't get a relay and a switch to operate it, plus the box to put it in for £20. If you've got a spare relay it's a different matter, but it needs to be a 2 pole changeover, usually abbreviated to DPDT, or better; it matters not if there are unused poles. One of the advantages of relays is that they can be readily used in conjunction with control switches, wether they be limit switches or interlocks, and the control circuits can be at 24v rather than 240v though this means the added complexity of a transformer somewhere.

As regards the wiring of reversing switches I find it best to think of the motor in modules: the start winding, start capacitor and centrifugal switch is one module, whilst the run winding is another module. In each module the components are in series. The two modules are then connnected in parallel to the supply. Ideally the start module will be connected such that the centrifugal switch is the first thing connected to the line, not the neutral, otherwise the sequence of components in each module is unimportant. Given my comment of the centrifugal switch connection it follows that the best reverse is by changing the connections to the run module. In normal conditions there may be little difference whether the centrifugal switch cuts the line or the neutral supply to the start module, but under fault conditions it could make a big difference. If there is a run capacitor it should be part of what I have called 'the start module' in parallel with the start capacitor and centrifugal switch combined, but still in series with the start winding.

I hope that is reasonably clear!

Nick

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Re: DEWHURST reversing switches

thank you one and all for the most helpful and relevant replies,it is somewhat hazy here at the moment due to a port and brandy overload,but i will digest the info given here,see what i have in the electrical stores and get it done from there,the ability to reverse my little myford will greatly aid it,s usefulness in screwcutting and a few other other areas.
i have a bigger 'grown ups' lathe,but just love using the little ml10,it,s kitted with original swiss multi-fix toolholders,might be the give away-

thanks to all and best wishes for a happy healthy 2011

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Re: DEWHURST reversing switches

Hi,

I have an ML10 that I want to fit a Dewhurst reversing switch to, however, the Brook Crompton Parkinson motor (370 watts, 1425 rpm, 3.8 amps & 50 hz) currently has the following coloured wiring:-

Contact A - blue & black
Contact AZ - yellow, red & maroon
Contact Z - not used
Contact T - maroon

Do I need to alter the configuration of the wiring before fitting a Dewhurst reversing switch?

Many thanks

Derek

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Re: DEWHURST reversing switches

Hi Derek,
As you posted as an "offline guest" your post didn't appear on the front page, but went straight to the thread file.    Hopefully this will bring it out into public view.

Sadly the motor manufacturers didn't follow any set colour scheme for the internal wiring, so this varies, a constant source of annoyance now that the motors are all out of date.

Your motor terminals would normally be connected thus:    A and AZ to the running winding,  T and Z to the start wiring.   For one-way running start and run terminals would be bridged by an external copper strip to give just 2 connections for thelive input.     It would appear that someone has made a permanent connection by moving the wires around.   You will need to identify which wires feed which windings and reconnect them to the terminals.

Use the info in the posts above to help and see how you get on.   Note the link Ashley gave  to uphill.org doesn't work anymore.   

Phil

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Re: DEWHURST reversing switches

Hello Derek,
as Phil says, the earlier posts contain info on how to use the switch. You need to separate the four wires and take them out as individual wires to the Dewhurst switch, you can't have any interconnections within the motor other than the start to capacitor to centrifugal switch. The four being, two run and two start.  I'm pretty sure the Run winding is the Black & Red, the 'start winding' for this purpose includes the switch (yellow IIRC) & start capacitor which I think are the two maroon wires. It's easy enough to check. You need continuity between the ends of the windings obviously, one pair will be around a couple of ohms and is the Run winding, the other (not via the capacitor) will be around 6 or 8 ohms and is the start winding.

To reverse the motor, you reverse the polarity of either the start winding (normally) or the run winding, but not both.
This may help, taken from a machine manual of old, although I can't recall which:

http://www.homeworkshop.org.uk/components/com_agora/img/members/116/Dewhurst--capacitor-motor.jpeg

- Richard -

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Re: DEWHURST reversing switches

If you have a motor with a start-winding and centrifugal switch then this may help:

http://wktodd.webspace.virginmedia.com/cadet/wpimages/wp5164ceb1_06.png

Bill

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Re: DEWHURST reversing switches

Thanks Phil, I'll have a play and see how I get on.

Many thanks

Derek

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Re: DEWHURST reversing switches

Hi Richard, very informative and helpful, thank you.

Regards


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Re: DEWHURST reversing switches

Thank you for the diagram - much appreciated.

Regards


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