Dewhurst switch

[Malcolm BeggParticipant @malcolmbegg82759]

I would like some help wiring the Dewhurst forward / reverse switch on my myford lathe,The motor is a Crompton Greaves single phase,Capacitor start Cap run,(i will attach a diagram) and hope that someone out there can let me know which wires go where, and what wires I need to connect / disconnect I have tried different ways as wiring as per instructions but I am not getting anywhere,some suggest taking all the connections outside the connecting block on the motor ie RED Black Blue and Yellow and connect it to the Dewhurst switch using terminals 2,5,6.+7,1 + 3 being for supply, 4 & 8 not being used,What I would like to know is where do I connect the capacitor cables as they are then connected to terminals at the motor but to nothing else.

Thanks in anticipation

Wunatey

[Malcolm BeggParticipant @malcolmbegg82759]

Wiring switch to myford lathe

[MalcParticipant @malc]

Malcolm,

I have sent you a PM re: the ML7 wiring.

Malc.

[Les Jones 1Participant @lesjones1]

Hi Malcolm,
Your diagram of the motor only shows one wire from each capacitor. Where does the other wire from each capacitor go ? I assume the purple wire and the red wire connected to terminal AZ are the main winding and the auxilliary winding. You will need a multimeter (Or a battery and bulb) to identify which is which.

Les.

Edited By Les Jones 1 on 12/03/2015 08:47:50

[john fletcher 1Participant @johnfletcher1]

I'm not familiar with your particular motor (Crompton Greaves), but I will explain about ALL cap start and cap run motors and that might help a bit. There are two windings inside the motor stator, one with a low resistance which is the run winding. Then there is the second winding with a much higher resistance the start winding maybe 30 or 40 ohms, this winding is the start. In series with the start winding are the TWO capacitors ( a large value and a much smaller value capacitance wise) and the centrifugal switch which is usually inside the motor, this winding, at the point of switch on is connected in parallel with the run winding. Both windings connected to the 240 volt mains. As the motor gains speed the centrifugal switch opens, disconnecting the large value capacitor, so now the motor has its run winding energised across the 240 volt mains and the start winding with the smaller value capacitor also across the mains. In normal circumstance you just leave the capacitors where they are already connected in the motor terminal block. Are you also wanting to reverse the motor? If you are then you will need to locate the run winding as that will be the easiest to change over. To reverse a single phase motor you need to reverse the connection to one of the two windings NOT both.Ted

[Michael GinnParticipant @michaelginn27937]

Hi Malcolm

My Myford is wired as follows

Running winding A1 to Drum 5, A2 to drum 7

Starting windings Z1 to Drum 2, Z2 to Drum 6

Mains to Drum 1 & 3

Don't forget Earth!

This is also the information in the Myford installation manual

[Neil WyattModerator @neilwyatt]

Some capacitor start, capacitor run motors have two identical windings, one with a capacitor, one without. reversing is by switching the capacitor over between the windings. They tend top be lower power motors, I know it's true because I have one and a hand-written slip of paper inside the terminal box gave instructions for reversing it. The paper was wrong, so I had to work out what it was an how to wire it from scratch.

Neil

[Les Jones 1Participant @lesjones1]

Hi Malcolm,
Disconnect the red/green capacitor wire from terminal A. Disconnect the purple wire from terminal AZ Do these tests and post the results BEFORE applying power so I can confirm my understanding of how the motor is wired. Also can you type out the three lines of text with the reversing instructions as I cannot read the last part due to the poor picture. (For other readers this is picture 2 in Malcolm's pictures which he set to me in a PM) Measure the resistance between the purple wire and terminal K I believe this is the centrafugal switch so it should read almost zero ohms. Measure the resistance between terminal K and terminal Z I believe this is the start winding so I would expect a reading of between 5 an 20 ohms. Measure the resistance between terminal A and terminal AZ I believe this is the run winding so I would expect a reading between 3 and 10 ohms. If these facts confirm my understanding of the wiring then this is how to connect it.

Terminal 5 on switch to terminal AZ on motor
Terminal 7 on switch to terminal A on motor
Terminal 2 on switch to red/green capacitor wire (that was removed from terminal A)
Terminal 6 on switch to purple wire (that was removed from terminal AZ)

You will need a two position terminal block to connect to the red/green wire and the purple wire.

Les.

 

<amended with the correction Les gives below – Neil>

Edited By Neil Wyatt on 12/03/2015 20:37:47

[Les Jones 1Participant @lesjones1]

Correction to my last message.

The last line of the connections should read
Terminal 6 on switch to purple wire (that was removed from terminal AZ)

Sorry about the error.

Les.

[Adam HarrisParticipant @adamharris13683]

An indespensible reference is Tubal Cain's "Model Engineer's Handbook" – chapter 12 covers wiring for all the motors and fwd/rev switches you are ever likely to come across! Other chapters give in depth data on any other matters you are likely to ever need info on such as all the different tapers etc – amazon.co.uk.

Adam

Edited By Adam Harris on 12/03/2015 20:10:20

[Les Jones 1Participant @lesjones1]

Here is some information to try to maintain the continuity of this thread

This is the picture sent by Malcolm in a PM (Sent 12/03/2015 12:44)

This is Malcolm's PM in reply to my post in this thread 12/03/2015 at 15:22

Date Sent: 12/03/2015 18:52:42
To: Les Jones 1

Hi Les

The details on the motor reads:To reverse rotation

interchange yellow

& blue motor leads

The readings you asked for are as follows:Between Purple wire and K = .8ohms

Between K and terminal Z = 10.8 ohms

Between A and terminal AZ = 4.2ohms

This seems to be pretty near the readings you expected.

Thanks again

Malcolm

PS: I will await your instructions before I connect the motor up to lathe.

This is my reply to the above PM and the correction after spotting my typing error

From: Les Jones 1
Subject: Re: Wiring Dewhurst Switch
Date Sent: 12/03/2015 19:22:57
To: Malcolm Begg

Hi Malcolm,
I am now fairly confident that the connections I suggested will work. You may find that forward and reverse are transposed on the Dewhurst switch. If so swap over the wires on terminals 2 and 7 of the switch

Les.

From: Les Jones 1
Subject: Correction to connections
Date Sent: 12/03/2015 19:54:54
To: Malcolm Begg

Hi Malcolm,
I have just noticed that I have made a typing error in the connections. The last line of the connections should read Terminal 6 on switch to purple wire (that was removed from terminal AZ) Also if forward and reverse are transposed swap over wires to 2 and 6 on the switch. (Not 2 and 7 as I said in my last message.

Les.

Also thanks to Neil for editing out my mistake. (It was too long after the post when I spotted it for me to edit it.)

Les.

Edited By Les Jones 1 on 13/03/2015 08:59:53

[Les Jones 1Participant @lesjones1]

Just to complete this thread. This is the PM reply from Malcolm.

Message Viewer

Back To List | Reply
From: Malcolm Begg
Subject: Dewhurst Swich
Date Sent: 13/03/2015 18:04:50
To: Les Jones 1

Les

Tried the lathe today and it worked perfect,thanks ever so much,I had just spent hours on the internet and with many and various suggestions none of which worked! Once again thanks a million.

Malcolm

Les.

[Raymond Sanderson 2Participant @raymondsanderson2]

Posted by Adam Harris on 12/03/2015 20:08:41:

An indespensible reference is Tubal Cain's "Model Engineer's Handbook" – chapter 12 covers wiring for all the motors and fwd/rev switches you are ever likely to come across! Other chapters give in depth data on any other matters you are likely to ever need info on such as all the different tapers etc – amazon.co.uk.

Adam

Edited By Adam Harris on 12/03/2015 20:10:20

Adam having read your post I grabbed my copy off the shelf over the weekend as i am trying to fit a similar switch to my ML7. The switch is an Aussie made one almost identical possibly re-branded Kelly.
My motor is NOT CP but a Hoover.

Here's what happened with my copy of the book

I purchased Model Engineers Handbook by Tubal Cain 3rd edition some 3 yrs ago only ever flipped through it. Last night I dragged it off the shelf to do some ground work on electrics which is section 12 of the book. No section 12 not even section 11 or 13 I thought I was going bonkers as all I kept seeing was section 8, 9 & 10 yep they had bound the book with double those sections left out 11, 12 & 13.

Waiting to hear from the supplier here in Victoria.

[Author]

Posts