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Electrical but not as we know it....

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iansoady View Drop Down
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    Posted: 21 Mar 2020 at 10:39
I know this is slightly off topic but also know what a wealth of knowledge and expertise resides in these pages. So:

I've recently moved house and am setting up my workshop / garage. I have a Tormach mini lathe which is a 110V US version similar to the Clarke and other versions on sale in the UK and I power it via a 230v/110v site type transformer. This worked fine in my previous house which had antique wiring with fuses and no RCD.

The new house however is fitted with RCDs and I find that sometimes (but not always) when I power the transformer up it trips both the local one in the garage and the one in the house that supplies the garage. I believe this is because of an initial current surge to the transformer.

Any wise people here who can suggest a solution? The only one I have thought of is to replace the motor and control board with UK versions but they ain't cheap.
Ian
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Brian UK View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2020 at 11:36
Unlikely to be current surge.

I suspect somehow the transformer has a leak to earth. The RCD will trip if it sees a different amount of current in the live and neutral.

I had this with a TV aerial amp once, then discovered I had managed to wire the neutral and earth the wrong way round.
Check the plug.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iansoady Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2020 at 11:50
Many thanks for the prompt reply Brian.

Incorrectly wired plug was my first thought but no it checks out. The reason I thought of current surge is that this has been mentioned on DIY forums (but without offering solutions....) It's also odd that it trips sometimes but not every time which I would have expected if it was a wiring fault. Anyway I'll pop the top off and have a look inside.

Something of a "first world problem" given the current state of things......
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Post Options Post Options   Thanks (0) Thanks(0)   Quote c13pep Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2020 at 12:03
Easy solution is to beef up the RCDs, I know it`s not ideal but a combination of site transformer and a machine may be just to critical for the current RCDs

CHRIS
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iansoady Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2020 at 12:39
Yes, I was wondering that but am a bit reluctant to reduce protection.

Some further investigation tells me that in fact the lathe motor is a 12v (I think) 350 watt DC job and the mains power goes into a control board which then drives the motor. UK spec boards are available but I was wondering whether substitution of some of the components would make the existing board work on 230V UK mains.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Doug Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2020 at 14:25
If you're considering a new motor and board. When the motor failed on my Chester lathe it also took out the control board. I replaced it with a dc motor from a treadmill and a cheap dc motor control board and a forward/reverse switch off everyones favourite online auction. Had to mount the motor at the back of the lathe and but that was easy enough to do. Probably less than £50 altogether.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 21 Mar 2020 at 15:38
My first idea would be to check the insulation of the transformer. Any PAT testing equipment will do it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike H Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2020 at 19:47
Transformers can have switch on surge, especially high power ones, and especially if toroidal construction.

Go on Google and type "inrush limiter"

You should get lots of hits for NTC (Negative Temperature Coefficient) thermistors. Commonly used in machines, motors, vacuum cleaners etc. for exactly the same reasons. Next prob is calculating which one to use, so you could search for "inrush limiter calculator"




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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2020 at 20:54
But a surge alone wouldn't trip an RCD would it?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iansoady Quote  Post ReplyReply Direct Link To This Post Posted: 23 Mar 2020 at 10:08
I've had a peep in the unit and checked with a digital multimeter. No apparent problems although the input resistance seems to vary oddly.

It's an industrial unit with a 3KVA capacity so fairly beefy and way over specified for the application as the motor is only 350 watts. All I really need is a simple 2:1 step-down transformer with no complex circuitry.

Mike - I've had a look for the inrush limiters and they look like simple capacitors and seem very cheap. I assume they're just fitted across the input terminals?

<edit> I see that they're fitted in series on the input. May be worth a try at a quid or so a throw.</edit>

Something to do while I'm self-isolating anyway.......


Edited by iansoady - 23 Mar 2020 at 12:34
Ian
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iansoady Quote  Post ReplyReply Direct Link To This Post Posted: 23 Mar 2020 at 16:42
I've bought this and will install it when it arrives.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike H Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2020 at 01:42
Originally posted by Brian UK Brian UK wrote:

But a surge alone wouldn't trip an RCD would it?


Right, sorry keep seeing RCD as circuit breaker. Going senile

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wits Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2020 at 06:24
RCD’s can go “Lazy”...... as my electrician mate informed me .....
The house one would throw out for no reason random appliance or light turned on.....
Nice new RCD problem solved .....simples.....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2020 at 08:14
But this is throwing two out.

I would look for an alternative step down transformer.

I have a feeling that if you did a proper PAT insulation test on the transformer it may not pass.

But look at the label. Does it have that one rectangle within another symbol, to show it's double insulated?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iansoady Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2020 at 09:59
It's actually tripping three. There is a 2 channel unit in the garage with a master RCD - both the "socket" and the master trip. This is fed from an RCD in the house which also trips.

The transformer unit isn't double insulated but has a label saying it has a centre-tapped to earth transformer. But while I understand what that means I don't know what if any its implications are. Also I'm not actually sure what the difference is between an RCD and an circuit breaker and how I know what I have?

<edit> having looked I now see that all are MCBs ie circuit breakers.... </edit>

As Brian says if the inrush device doesn't work I'll look for a simple 2:1 step down transformer as the site box (which actually came with the lathe) is somewhat overkill for a 350 watt motor.

There are a lot of conversations on DIY / building forums about similar issues....



I'm comfortable with bike electrics but have not touched mains stuff since my technical apprenticeship with the CEGB 50 years ago.......


Edited by iansoady - 24 Mar 2020 at 10:03
Ian
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