Earth fault on lathe

[Chris BarryParticipant @chrisbarry]

Myford 254 has developed an earth fault

[Chris BarryParticipant @chrisbarry]

Hi all,

My myford 254 has just started tripping the house RCD as soon as I plug it in, I've traced the fault to the 110v control system but that's the limit of my electronics knowledge.

I've tried removing all sections of the control system from the transformer and reconnecting them individually but it keeps happening.

So my questions are, anyone got a hint or tip on how to fix it of further fault find? Or anyone in the Manchester area, specifically urmston who could lend me a hand in exchange for tea and biscuits.

Thanks in advance

Chris

[Engine BuilderParticipant @enginebuilder]

Have you tried disconnecting the supply to the 110v transformer. Could be the transformer that has the earth fault.

You need to remove both  the live and neutral as a N to E fault will also trip the RCD.

Edited By Engine Builder on 12/03/2014 16:09:34

[Chris BarryParticipant @chrisbarry]

I've had the 110v connections to the circuit removed but with the transformer earth still connected and it doesn't trip out, when I connect the positive of the 110v system it trips it. It was working fine last week and has just started tripping it.

Thanks in advance

[Neil GreenawayParticipant @neilgreenaway71611]

Hi All,

I have a Harrison M390 which has just started causing problems – tripping out the 3ph RCD on the generator we run it on – At first I thought it was dampness allowing earth leakage from some other electronic circuits also connected in the workshop (DRO's etc) so we ran a small heater to dry out the panels just in case – this has helped a little and allows it to run a little longer before tripping. I think the 110v supply might be part of the problem – it supplied the safety circuit and the brake caliper solenoid. The problem also started last week after a heavy downpour of rain – bit of a mystery but just need to get time to properly look into it.

Neil

[Diane CarneyModerator @dianecarney30678]

Someone I know had an earth leak on a lathe. He didn't know until he got a metal splinter in his thumb – then he knew! He didn't know he'd got a metal splinter in his thumb until he used the lathe either.

[CabengParticipant @cabeng]

Chris:

My myford 254 has just started tripping the house RCD as soon as I plug it in, I've traced the fault to the 110v control system but that's the limit of my electronics knowledge.

It is possible that the problem lies elsewhere, rather than on the 254. A 30mA RCD will trip when the TOTAL earth leakage current hits 30mA – and there are lots of sources of leakage current. Switched mode power supplies in computers, TV, wall warts, flourescent and CFL lights, wiring, workshop equipment (VFD's can be problematic) …. everything contributes a bit, there's always some of it around. Eventually something adds another piece of straw to the camel's back, and it trips. So something else could have caused an increase, brought the RCD near the limit, then when you plug the 254 in with its 'natural' leakage, the RCD decides to go home to mummy.

Of course, it might be a real problem with the 254, but it might be on some other piece of equipment around the house/workshop. Easy enough to check – disconnect everything else by removing ALL fuses except the one to the workshop, then unplug anything and everything left on that circuit, house & workshop. Including lighting!

Then plug the 254 in – if the RCD trips, you have a problem on the 254. If it doesn't, then you have the 'last straw' situation. Which could be more difficult to resolve!

I'll send you a PM later.

[john fletcher 1Participant @johnfletcher1]

To help Chris along, has a reader got a copy of the lathe circuit diagram and who can publish it here, which will help. Chris have you a tradesman friend with a YELLOW 110 volt power tool transformer? If you have, then you can disconnect your 110 volt one and feed the circuit via your friends, which will eliminate the transformer. Alternatively if you have a friend with a MEGGER he will be able to test your transformer in situ. The same friend could check out the motor for earth faults. I’m not familiar with your lathe Chris, what electronic controls does it have? TedTo help Chris along, has a reader got a copy of the lathe circuit diagram and who can publish it here, which will help. Chris have you a tradesman friend with a YELLOW 110 volt power tool transformer? If you have, then you can disconnect your 110 volt one and feed the circuit via your friends, which will eliminate the transformer. Alternatively if you have a friend with a MEGGER he will be able to test your transformer in situ. The same friend could check out the motor for earth faults. I’m not familiar with your lathe Chris, what electronic controls does it have? Ted

 

 

Edited By John Stevenson on 12/03/2014 21:53:21

[Chris BarryParticipant @chrisbarry]

Cheers everyone, john, I've got the wiring diagrams in the manual, will take a pic when at home and stick it up. Here

cheers cabeng for the tips, will have some fun tomorrow morning turning every thing off

thanks for the help so far everyone

[FMESParticipant @fmes]

I would suggest checking the RCD itself, they do go soft especially if they have tripped a lot.

[Les Jones 1Participant @lesjones1]

Hi Chris,
In your post today at 16:36 I am not clear on what you mean. I am assuming you mean that the secondary ( 110 volt) side of the transformer has been re connected and you have disconnected the 240 volt feed to the primary. In this condition it does not trip the RCD. When you then re connect the 240 volt live (Which you have called positive.) the RCD then trips. If this is the case then I am almost certain that the fault is with the transformer. A fault of any kind on the secondary side of the transformer should not cause the RCD to trip. (It could cause an over current fault which could possibly cause an MCB to trip.) The fact that it trips the RCD with just the live connected means there must be leakage from the primary winding to earth. As well as posting the wiring diagram a picture of the transformer with a ruler or other object to give an idea of its size would help in suggesting an source for a replacement transformer.

Les.

[jason udallParticipant @jasonudall57142]

I
Don’t know this machine.

That said I have had to fault find plenty without the wiring diagramms.

If the output of the 110 transformer has a fuse ( it should) isolate the output by removing it.
( ideally if you feel brave disconnect secondary 110 V side totally)..if unit?still “blows” rcd then?fault does not lie in 110V circuit.
Transformers can fail internally. .sorry but thats life.
If its the transformer then replace/repair.. ( repair depends on replacement cost obviously).but repair is not for the inexperienced. .think rewind….
But bearin mind simply removing the transformer might locate the stray connection… ( foreign matter say.).

[Chris BarryParticipant @chrisbarry]

Just got home from work will get pictures tomorrow AM.

It's a new transformer from last year due to the primary coil failing, the lathe has only been used a handful of times up over the last couple of years due to me moving between rented accommodation, really hope it's not the transformer again.

Cheers

[Richie MParticipant @richiem]

[Richie MParticipant @richiem]

Sorry I hope I haven't upset Chris!

Here are the 254 single phase diagrams.

if you need any other info please ask!

Cheers

Richie

[Richie MParticipant @richiem]

Another go at drawing 12898, hopefully a little bit clearer.

[Ian S CParticipant @iansc]

I assume you have eliminated any dampness, condensation from all parts of the supply, and circuit boards, switch gear etc. At one time I had trouble with condensation in the centrifugal switch in the (single phase) motor, required modification of the insulation around the switch gear AOK now. Ian S C

[Les Jones 1Participant @lesjones1]

Hi Richie,
Many thanks for posting the schematics. I have added them to my folder of Myford information.
Chris, If my understanding of the test you have done is correct the I am almost certain there is leakage between the transformer primary winding and earth. I suspect the transformer will be about 100 VA rating. This is a conservative estimate allowing for a 50 watt light on the 12 volt winding and 50 watts for the contactor coils. If there is space it may be cheaper to use a 12 volt lighting transformer for the light and a separate transformer for the 110 volts.

Les.

[Chris BarryParticipant @chrisbarry]

Hi Guys, thanks very much,

Richie thanks for the pics, didn't upset me at all

Below is the picture of the transformer in a state that doesn't trip the power, as soon i connect the red wire to the 110v positive connection.

**LINK**

it is in a damp garage, nothing i can do about that at the moment so damp might be a problem.

is their a check over across the pins of the transformer?

really appreciate all the advise from everyone, sorry i'm not he best at explaining what the problem/i've done with it.

cheers

[CabengParticipant @cabeng]

Hi Chris.

Dampness could have something to do with it. If you have an electric fan heater you could use it to warm everything up and drive out any dampness, see if that makes a difference.

[Les Jones 1Participant @lesjones1]

Hi Chris,
My understanding of what you had had disconnected was wrong. I though you had disconnected the mains live from the primary of the transformer rather than the wire connected to the 110 volt secondary. This means that the fault is not the transformer. Can you confirm that it is an RCD that it tripping out rather than an MCB. An RCD trips because of leakage to earth. An MCB trips due to too much current. I do not see how anything connected to the secondary of the transformer can cause earth leakage. Can you try reconnecting the red wire to the 110 volt secondary and holding the stop button pressed in and getting someone else to switch the power on at the socket. If it still causes the power to trip then the fault is probably on the wiring to the interlock switches. Do you have a multimeter to do some further tests if required ?

Les.

[Martin WParticipant @martinw]

Les

How about if the transformer had failed in an unusual way. Secondary failed i.e. open circuit with the earthed side of the failed windings totally isolated from anything else by the failure. However the live, red wire, side of the failed winding has some resistance via the failure to the main windings so when the red wire is connected there is a current path to earth sufficient trip the RCD. Its a weird failure scenario but plausible given the facts. Could be checked easily by totally disconnecting the 110V secondary and measuring this winding resistance. If there is a sensible resistance for the winding then this is a non starter.

If the windings are on separate formers then this is highly unlikely. However if wound on the same former then it could be that one or more of the primary windings could have slipped past the separator and being lying alongside the secondary winding.

Just a thought

Martin

 

Edited By Martin W on 13/03/2014 16:19:50

[Les Jones 1Participant @lesjones1]

Hi Martin,
You theory would fit the facts. The problem is it would require two faults. 1 the transformer failure mode that you described but it would also require leakage to ground on the wiring between the transformer end of the red wire and the stop button. Another possible theory is that there is a fault where one of the contactors closes as soon as power is applied thus applying power to the motor. If the motor had leakage to ground it would cause the RCD to trip. I rejected this theory as it would again require 2 faults. I also do not believe my suggestion that it could be an MCB tripping due to over current caused by a short on the secondary side of the transformer. I do not think with such a small transformer (I would guess about 50 VA) even a total short across the secondary would cause a large enough fault current to trip an MCB which is probably a 30 amp rating device.

Les.

[Chris BarryParticipant @chrisbarry]

Hi everyone,

Checked the micro switches and powering it up with the spindle stop pressed and it's come back to life. No idea what I've done to make it so, not tried drying it out and it's significantly damper today than it has been all week.

Yet again electrics prove to my simple spanner monkey mind that it's all black magic and whitch craft.

Thanks for all your help, have saved all this thread in case I get problems again.

Thanks again

cheis

[Neil GreenawayParticipant @neilgreenaway71611]

Hi all,

Got time to test things yesterday and found a few faults or so I thought! Ranged from continuity between 2 phases making me think that the main supply cable was faulty to a voltage reading between neutral and earth before tripping of about 150v! I narrowed it down after eliminating all other faults to the 415v to 110v transformer. Upon removal it was open circuit on the 415v tapping on primary side. Testing on 240v test block resilted in a small puff of smoke and the open circuit. New transformer needed. Lathe tested by manually pulling in the contactors and mo tripping observed with thr transformer removed. Still need to check the contactor coils are ok as cant get at them but fingers crossed after I replace the transformer.

Neil

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