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Crabtree B15 3 phase Stop / Start switch

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Philip Slater15/02/2019 18:40:07
20 forum posts

Hi I'm in the process of modifying / upgrading my Tom Senior M1 milling machine. I've got the new motor and have a new VFD to power it. I was thinking of trying to utilise the original Crabtree B15 Stop / Start switch to switch the 240 volt supply to the VFD, I'm aware I would need to modify it.

Striped it down today and can see the start contactor and the main coil but can't see how the stop function works. I would have thought the stop relay simply broke the coil circuit causing the main contacts to open but can't see how it does this.

I wondered if anybody had come across a similar problem.

Split Pin15/02/2019 19:13:32
10 forum posts
1 photos

the coil is connected in series with the overload button circuit, terminals 95 and 96 this also acts as a stop button when used with its original enclosure. any additional stop button is also in series with the coil circuit. below a genric connection diagram


Edited By Split Pin on 15/02/2019 19:14:09

Philip Slater15/02/2019 19:41:51
20 forum posts

Thanks for the circuit diagram Split pin, just checked my switch again and it looks like the stop push button contacts are u/s, they are not normally closed as shown. I'll try stripping it down tomorrow but it looks pretty sealed up. I've got nothing to loose. Thanks again.

Emgee15/02/2019 20:50:16
1438 forum posts
212 photos

The lid stop button pushes the small 1/4" diameter red piece to the RH side of the contactor to stop, open the contactor.


Philip Slater16/02/2019 12:24:36
20 forum posts

Spent 3 hours this morning stripping and cleaning the switch, interesting project. All sorted and dirty stop switch contacts all cleaned and operates great.

Only problem is it's a 415v switch which I'm trying to operate on 240v unfortunately there isn't enough pull on the electromagnet to pull the contacts hard on and the whole thing vibrates at 50hz. Looks like I'll be putting the thing in my box of museum bits unless somebody knows of a supply of 240v coils.

not done it yet16/02/2019 12:59:02
4470 forum posts
16 photos

Sometimes it is simpler, easier and more cost effective to use a standard three pin plug and socket.

Emgee16/02/2019 13:48:49
1438 forum posts
212 photos

Philip, most likely cheaper to buy a new enclosed starter with stop/start push buttons than pay for a Crabtree B15 240v coil.
If you have a small 240v trans-rec unit try that for the coil supply, sometimes the DC voltage will hold the poles closed even on a lower voltage than coil rating.


john fletcher 116/02/2019 16:42:22
578 forum posts

Hello Phillip, if you feel happy working with electricity and have an Ohmmeter then it might be possible to modify the coil. Its not always possible as some coils were impregnated with a hard resin. As you haven't much to lose, you can take the starter apart and carefully remove turns of wire from the coil. First measure the coil resistance, remove the tape, unsolder the wire end which is attached to the outer end of the coil and very gently start to unwind the wire. Now all this takes time, don't be in any hurry. When I'm doing this for a friend, I mount the coil on a piece of wood and mount it between two slack centres in my lathe and providing the wire pulls off freely I unwind lots of layers. The NEW resistance needs to be very close to the ratio of 240 divided by 415 of the original resistance, that is why you need to test first of all to find the original resistance. So keep checking the resistance as you go. some times it helps to warm up the coil, not too hot otherwise the coil form distorts and then it won't go together again. I suggest you don't put it in the oven as the odour given off can be awful to some, and you might be looking for lodgings. I've found the actual resistance is not critical a few 100 either way.once you are down to 240 divide by 415 ratio then you have to resolder the wire,assemble the starter and connect it to the mains for testing. I test the starter when I'm in the workshop for a couple of hours so that I can monitor things. When things are correct the starter should snap in. Switch OFF disconnect and feel the coil with a finger it should be slightly warm after an hour or so. Mostly it works out OK and worth a try. John

Les Jones 116/02/2019 20:05:27
2120 forum posts
146 photos

I don't see the point of of trying to use this starter on the input of the VFD. If you want to put an emergency stop button on the input to the VFD such as this one will be easier than trying to modify your existing 415 volt starter. The start and stop buttons will be connected to the low voltage control terminals on the VFD. (Some VFDs may require a small low voltage 12 or 24 volts relay as well as the two buttons)


Edited By Les Jones 1 on 16/02/2019 20:08:43

Edited By Les Jones 1 on 16/02/2019 20:09:50

Philip Slater17/02/2019 08:53:01
20 forum posts

Thanks for the replies. John, I happy playing with electricity but I'm guessing by removing wire from the coil and so reducing its resistance makes it a stronger magnet? I assumed, maybe wrongly, the 240v coils would contain more turns of wire. I'm obviously lacking in my theory knowledge.

Yes Les I've seen the NVO open switches and will maybe go down this path. At the moment I'm putting an emergency stop on the low voltage inputs to the VFD. I suppose it's the tight Yorkshire man coming out of me, never throw anything away. It would seem a shame to loose a bit of 1953 history.

Clive Foster17/02/2019 10:04:13
2126 forum posts
73 photos

Fundamentally a VFD is a stop / start box. It just uses magic smoke hidden inside electronic package thingies instead of nice obvious contacts driven by a solenoid and has all sorts of other clever stuff inside too. (We'll not mention a manual that is not only large enough to be a serious drop hazard if picked up when not wearing hard toes boots but also a major threat to the sanity of any normal person trying to read it. Such are not normally considered advantages.)

Indeed the Eaton DE1 series VFD boxes are actually sold as a substitute for ordinary start stop boxes as being a simple way to get the usual VFD advantages of soft start and, if needed, speed setting. They require only the addition of simple switch controls to do basic motor stop / start, forward / reverse and, if needed, two speed duties. The internal parameters are preset to work fine on ordinary motor duties. Its claimed that any ordinary electrician can fit them with no special training. No display or buttons so they get hidden inside a cabinet. However if you look at the manual you find they are actually a full on VFD having all the bells and whistles inside which can be set-up in the usual way if you buy the programming widget.

Best thing to do with your Crabtree box now its fixed is to wrap it up and store safely ready to be re-fitted if the machine ever has to go back onto full fat three phase.


Les Jones 117/02/2019 11:57:30
2120 forum posts
146 photos

Hi Philip,
I just have the stop, emergency stop and interlock switches in the low voltage control inputs to my VFD I just use the switch on the 13 amp socket to switch the power to the inverter when the lathe is not in use. My view is that would be very unlikely to have a fault occur on the stop control input of the VFD at the same time as I needed to use the emergency stop. It would more likely fail with normal use of the stop button when it would be noticed and repaired.


Rod Renshaw17/02/2019 12:15:38
86 forum posts

Hi all

+1 for Les's approach. Many years ago when I first got a VFD I was told it was not considered good practice to turn them on and off at the mains too frequently as this would shorten their life. Similarly it was not considered good practice to have any switch gear between the VFD and the motor as this might generate voltage spikes that might damage the VFD. No idea if there is any evidence to back up these views but based on the opinion at the time I wired my VFD directly to the mains, (via a fused, switched outlet ) and wired the output of the VFD directly to the motor. I turn the mains switch on when I go in the workshop and need the lathe and turn the mains off when I leave the workshopfor the day. I have had no problems adopting this approach and my original VFD is still going strong.

I use the VFD's own low voltage switching terminals to control the motor to stop and start the lathe and control speed etc. I felt it safest to keep the mains and the low voltage switching far apart so I use only the stop and start buttons on the starter (which has no mains supply at all in this configuration, so the voltage of the coils is not important) and use a low voltage power supply and a relay wired to latch to simulate the action of the contactor.


Emgee17/02/2019 12:40:54
1438 forum posts
212 photos

Hi Rod, what happens if you are using the machine controlled by the VFD if there is a mains power failure ?


John Rudd17/02/2019 13:20:12
1367 forum posts
58 photos

It would be interesting to hear from the 'experts' like Transwave as to what they advise on their plug n play packages that are pre-wired.....

Do they advise leaving the vfd powered up or turning off when leaving the machine for some prolonged period......?

Personally, my big lathe, Chester 626 mill and Cheater 9 x 20 run vfd's, the minilathe and minimill run 'similar' electronics.....when not in use, all are powered down at the mains....

Rod Renshaw17/02/2019 13:41:24
86 forum posts


The latching relay drops out, because its power suppply is interupted, to give me a NVR action.


Les Jones 117/02/2019 14:31:54
2120 forum posts
146 photos



Edited By Les Jones 1 on 17/02/2019 14:34:14

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