replacement for Dewhurst switch

Hi all,

The Dewhurst switch on my S7 switch is arcing and getting hot. The motor is the usual single phase motor. Spares seem to be impossible to get but, I would like to replace with a NVR switch and a modern tumbler switch for reverse in anycase. Problem being I have no idea were to start looking. Local industrial switch suppliers did not have much idea. Can any one help? Been a lurker on here for a while but as a fairly recent starter the hobby am so far not in a position to offer advice, only ask for it!

Thanks in anticipation!

I got a modern tumbler reverse type switch off eBay from a Hong Kong seller. Very heavy duty unit. NVR switches are on the Bay too. Like you, I found local electrical supply shops fairly clueless on anything but standard domestic outlets and switches etc.

That said, if you have a Dewhurst, I think they are rebuildable to a certain point, but these days you might have to make your own replacement contacts for it.

Edited By Hopper on 31/01/2018 07:24:31

Hi Ian, I had a Dewhurst switch on my Super 7 some time ago and I had problems with arcing after many years . I bought some new contacts for it but when I took the switch apart to fit them there were some spare contacts inside on the drum which were not being used. I simply swapped the unused contacts for the burnt ones and the switch worked well again. Quick, cheap and easy. I now have a variable speed drive but I still have the spare contacts. I just saw some on ebay and, as I won't be using them, I thought I might as well put mine on there. If you need spare contacts let me know but I would try the cheap and easy option first! Good luck, Bob

Please never use your Dewhurst as an on/off switch. It is only meant for isolating or setting the connections for reverse before a proper NVR is used for starting the motor. You won't get arcing if you use it as intended.
It is the equivalent of throwing the point lever in the signal box while the express is still going over the junction.

Ian,

I have sent you a personal message please look for the INBOX at the top left of the home page which should be flashing.

Eric

A MOELLER T0-4-8213 cam switch costs about £27 from RS or Farnell and will fit in the Dewhurst switch housing.

There are sixteen terminals on the switch body, four pairs of which were already linked by Moeller. You need to add additional links as follows: 1-7, 3-5, 9-11, 13-15, 4-10 and 8-14. The switch wiring is then Live in to 2, Neutral in to 6, motor start winding Live to 9, motor start winding Neutral to 13, motor run winding Live to 1 and motor run winding Neutral to 5. You may need to swap 1 and 5 to get the motor to run forwards with the switch set to forwards.

Ian

For the cost of postage I can give you two Dewhurst switches, from which you should be able to make one good one. I went down the 1 - 3phase VFD route some time ago without any regrets.

Just send me a private message if you would like them.

Dave
The Emerald Isle

Hi Otley,

If you wanted to write that up as a short article with a diagram, I think a lot of MEW readers would find it useful.

Neil

Otley.

# 2 for Neils suggestion. Also perhaps details as to 3 phase reversing connections. Maybe shaft size too so a properly manly-man workshop style lever can be made to replace the simple knob.

I did a similar cam switch replacement on my Bridgeport some years back starting with a pure switch having no pre-made connections. Despite having a fair bit of electrical and electronics experience over the years it was definitely a sit down, read the data sheets carefully, think it through, draw out and then wire job. Defiantly not dive in and do. Mildly tedious for me but if its person's first time at connecting that sort of switch they could be in real trouble.

Although the standard pointer type knob is OK on the Bridgeport installation that's primarily due to it being up high where you can easily see it with near straight on viewpoint. Given the choice I think machine controls should have a nice hefty lever positioned for easy grabbing when things go wrong whose setting is obvious by both feel and vision.

I know VFD boxes and pendants normally have the full complement of forward / reverse / stop buttons but simple stop-go with a preset direction switch having a hard off position is objectively safer.

My Smart & Brown 1024 has separate forward reverse & stop buttons similar to the usual VFD set-up. No clutch. I have hit the wrong button a time or two. Bit of a shock when things take off going wrong way. The usual short term 50-points-off-the-IQ panic associated with brown-sticky-hits rotating-blades moments being less than helpful at such times. Standard gremlin procedure being to ensure that such button pushing finger misdirection should be reserved for when the machine is set for direct drive top speed.

Clive.

Before I fitted my S7 with a VDF I used one of these without any problem

H

Having bought some electrical equipment from Hong Kong which turned out to be potentially lethal, I'd be very careful over Harry's suggestion. The Moeller one is more expensive, but at least you can be confident it won't kill you

I've replaced two Dewhurst switches on my lathe and my mill with this item on ebay. 292412206423.

This is a replacement switch sold by Chester.

It has stop and start switches and a reversing switch and includes an NVR function.

I did these changes several years ago and they have worked with no trouble. I bought them from Chester as they were the only supplier with a switch that did all the functions I wanted.

The only thing was they didn't come with a wiring diagram so caused a bit of head scratching. I still have my wiring diagram, somewhere, if it is needed.

For your safety it is certainly worth putting an NVR switch in even if you keep the Dewhurst.

Thanks for all the responses, especially JonBerk. I have decided to go for the Chester switch as it is a pretty compact unit that I can put where the Dewhurst switch is now.

Thanks also for all the offers of Dewhurst spares but I feel that a modern alternative is the way to go.

Ian

Agree with JonBerk that those Chester switches do an effective job. Used 8 or so over the years, mostly for other folk. No complaints and no actual problems with the ones I used myself.

Except for the missing wiring diagram. Certainly a head scratcher as, if I recall things correctly, they aren't arranged quite how you'd expect from normal practice. If Jon can't find his diagram I may be able to find mine.

Effective and reliable though they are they were never my preferred way of doing things. About four things I didn't like about them and one thing I don't really approve of. So many years since I've last had anything to do with them that I can't remember what rubs me the wrong way. But working on weapons systems research (and testing) for most of my employed life has perhaps skewed my personal sense of rightness. In a practical world you hafta use what you can get / afford and settle for 'Well it works OK.".

Clive.

Seen a Dewhurst new on ebay last week.
Went through two on an ML7 used to get loads of use daily. Used to get from a local machine/electrics dealer.

Here is my wiring diagram. It is a long time since I looked at it, so first of all is a warning, check out your switch with a meter before using this diagram.

Without the switch in front of me I'm going by my, unreliable, memory.

First off you need a box to put the switch in - Maplins do a range of plastic boxes which can be modified to suit. Get one that is deep enough to take the switch and the Terminal Block. The Terminal Block is just a piece of chocolate block connector. You will need some four wire cable going to the motor, preferably armoured if the motor is exposed.

If I remember, the switch came with wires connected, but all the wires were coloured black! The numbers in circles are the numbers on the terminals on the back of the switch. There seems to be some duplicates but why I can't tell you.

I've written in Biro on the diagram but I can't remember why, it looks like they may be wires from the NVR.

Good luck!

Jon

Thanks Jon,

Just received my switch and will definitely need that diagram!

Ian

Ian,

I'm just wiring up one of those Chester switches. The wiring as supplied is as shown in Jon's pic, (minus the terminal block) but in the latest iteration the wires are not all black. The mains input wires are the short black pair attached to terminals 6 and 8.

Be aware that throughout this switch, contrary to domestic wiring convention a red wire does not indicate "live" nor does black indicate "neutral".

The pair of red wires coming from the centre terminals of the rocker switch are one set (i.e. one is live and one is neutral) and are used on one of the two windings on the motor. Similarly the black wires coming out of terminals 7 and 5 are another set for the second winding.

I've used coloured tape to identify the various polarities as in the pic below.

26231873978_da9915166a_c.jpg

This is the rear of the rocker switch showing the pair of red leads that feed one of the windings on the motor.

40106234341_a5354d209c_b.jpg

To check which of these is the live or neutral, I first wired terminals 5 and 7 (the black pair) to a lamp. With your live mains input attached to terminal 8, terminal 7 is your live to the lamp. The lamp lit when I switched on. Then I removed the neutral and substituted on of the red leads. It didn't light up, so it must have been another live feed. Changed it for the other red, the lamp lit so I'd positively identified the neutral.

It all sounds complicated but when you study the switch unit it's pretty simple.

Two other things I'm going to do is make much better provision for earthing than the old Dewhurst did: there was no earth to the motor casing at all, just to the switch body, plus add in an emergency stop button.

Tried to embed the pics but the album function wasn't having it.

Hope this helps, Eug

Edited By Eugene on 05/02/2018 23:24:53

Edited By Eugene on 05/02/2018 23:29:52