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Connect a Dewhurst Switch to Single Phase Motor

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David Couling19/06/2021 22:27:07
10 forum posts
9 photos

Hi Guys,

I've added a few pictures of my Myford Lathe motor in my album.

I want to connect this motor which is a single phase motor to a Dewhurst switch but I'm not sure if it is possible the way this motor is wired., since there are only three terminals..

Mains power comes in on the AZ and A terminals.

Resistance between black and red is 3.9 ohms

Yellow and blue when switch is closed is 14.6 ohms

Could some kind person point me in the right direction how to do this if it is indeed possible.

Kind regards,

David

duncan webster19/06/2021 22:51:12
3597 forum posts
66 photos

If you type Dewhurst into the search box you will get endless answers on how to connect a Dewhirst switch, and why it is or isn't a good idea!

David George 119/06/2021 22:54:55
avatar
1677 forum posts
497 photos

Hi David the Dewhurst switch is not the best to switch the power on and off on a lathe as it dosn't switch the mains off and if there is a power cut and if the power is restored the lathe may start dangerously. A NVR switch is recomended and you can fit a Dewhurst switch, or similar, if you need to change over direction.

David

Emgee19/06/2021 23:02:16
2201 forum posts
272 photos

David

Yellow and blue wires from inside the motor are connected to the Start winding, this is the pair you need to change over to reverse the motor, beware the danger of the chuck being unscrewed off the spindle, best to add some kind of retention to the chuck.

Dewhurst switch schematics are available on the net FOC.
If you want the Dewhurst to last forever only use as a reversing switch with the spindle stopped, best have a DOL starter with Start and Stop buttons as your main control, this will also provide motor overload protection and no volt protection in the event of losing the power supply.

Emgee

David Couling20/06/2021 09:34:43
10 forum posts
9 photos

Hi Guys,

Thanks for your inputs. I think I will fit an NVR in addition to the Dewhurst switch but I need a way to easily reverse the lathe for metric threading.

Emgee from your response would I be correct to say you don't see any difficulties in wiring up the Dewhurst switch with these available motor connections?

I identified the start winding but I'm not sure of the run winding and the Dewhurst switch wiring requires a connection to the start and run windings.

Any thoughts?

Cheers

David

Emgee20/06/2021 10:11:34
2201 forum posts
272 photos

David

See this earlier topic for more info.

**LINK**

Best to add a TRS gland to the input cable to protect against damage on the threads and provide security.

Emgee

Paul Lousick20/06/2021 10:48:36
1868 forum posts
666 photos

David,

Not all single phase motors can easily be reversed.

Im not an expert (someone please advise if wrong) but I understand that there are 2 coils in a single phase motor and to change the rotational direction you change the position of the phase and neutral cables on only one of the coils. Reversible motors have separate terminals for each coil. Swapping the phase and neutral position of the power cable does not work.

The photo of the terminal box in the link above for a reversible motor has 4 terminal + earth. Whereas the photo in your album only has 3 + earth.  Permanent reversal probably done by swapping wires to one of the coils and possibly done with a Dewhurst switch by adding an extra terminal to the missing space in the terminal box. (be carfull if you are not an electric expert)

Take care when running the lathe in reverse if you have a screw-on chuck

Paul.

Edited By Paul Lousick on 20/06/2021 10:59:39

David Couling20/06/2021 11:48:24
10 forum posts
9 photos

Hi Guys,

Thanks for your inputs.

Yes I will fit a TRS Gland to the motor Emgee and thanks for the interesting link.

My opinion now is that it will not be possible to connect this motor to the Dewhurst switch, since it only has three terminals. In all other examples where this is done there are four terminals...I guess for the start and run windings.

I have a detailed diagram of an 8 contact Dewhurst switch which requires the input of two wires from each winding and the mains supply.

As I mentioned previously, I've identified the start winding so I could reverse the direction of drive but that alone doesn't help.

I don't think all of the wires from the run winding have been brought out of the motor (cost cutting perhaps?) so without them I can't connect to the Dewhurst.

I am aware of the danger of the chuck unscrewing when driving the lathe in reverse but it is only intended for slow speed metric threading purposes.

Perhaps its possible to dig into the motor windings and find the ends of the windings and bring them out to another terminal. I think start windings are made from a finer gauge of copper wire than the run windings...so it may be possible to identify them.

Had anyone tried that before?

Cheers

David

Dave Halford20/06/2021 11:59:47
1820 forum posts
19 photos

David,

One leg of the start winding goes to the centrifugal switch, hence no post is needed. The other start leg is commoned to the run winding, the cap is commoned to the other end of the run winding.

So you need to identify the end of the run winding the cap is not connected to.

Emgee20/06/2021 14:40:38
2201 forum posts
272 photos

David

It would be much simpler to just use a 240v 16A rated DPDT switch to achieve reversing, connections for this arrangement are given in the previous article.

You will still need to use a no volt release function as well as stop/start control.

Emgee

David George 120/06/2021 19:15:32
avatar
1677 forum posts
497 photos

This is my M type Myford switch set up.

20180711_165528.jpg

I start and stop with the N V R contact breaker and change the direction with a rotary switch. If I rotate the rotary switch it has a spare contact which breaks the power to the main switch and after the chuck has stopped I then start the lathe in reverse. You should only start a motor in reverse if it has stopped rotating as it may continue turning in wrong direction or it nay damage contacts etc.

David

john fletcher 120/06/2021 20:52:46
742 forum posts

Since you have quoted meter readings I'm assuming your motor hasn't got a capacitor attached.

You need four terminals to enable the motor to reverse. As you say you have ohm meter reading Black and Red wires of 3.9 Ohms this will be your run winding. the Yellow and Blue 14.6 Ohms will be the start winding. you will have discard the present arrangement and create an extra terminal. A 2BA nut and bolt will be Ok for the extra terminal. Dewhurst, connect Black to 5 and Red to 7 then Yellow to 2 and Blue to 6.As others have said you need a proper motor starter with RED and GREEN push buttons which will give you No Volt protection. The contacts on the Dewhurst switches are weak, they arc and soon burn away. To gain access to the terminal strip you might have to remove the motor end shield, some motor have the screws inside and you can't get to them with end shield on. Make marks using a scriber to give you guidance when re assembling the motor other wise it can be tricky. Careful you go and let us know how you get on. John

David Couling20/06/2021 21:30:01
10 forum posts
9 photos

Thanks John and all who have offered help with this issue. Yes, it does have a capacitor connected.

I'm now toying with the idea of replacing the motor with a 3 phase unit and a variable frequency inverter as a few others have done and also an NVR switch.

The existing motor is not in great shape, although 100 times better than it was after being removed and stripped and oil and swarf cleaned out.

Whatever, I think the idea of fitting the Dewhurst switch also is not great, and a 16A DPDT switch would be a better option.

Cheers

David

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