Myford super 7 lathe tripping consumer rcd

[Nick Clarke (Suffolk)Participant @nickclarkesuffolk]

[Nick Clarke (Suffolk)Participant @nickclarkesuffolk]

[Nick Clarke (Suffolk)Participant @nickclarkesuffolk]

[Nick Clarke (Suffolk)Participant @nickclarkesuffolk]

How do I wire up the capacitor? What are the wires on what looks like a thermal cut out?

[Oily RagParticipant @oilyrag]

Quote from Andrew:

That seems very fast? I was under the impression that standard domestic RCDs normally broke in 100ms, or 40ms as needed. Note that's milli, not micro.

I was told this by the industrial electrician, he only changed my domestic consumer unit RCD which powers the workshop unit (on a 30 amp cable) as well as all the RCD's in the workshop unit for the longer time delays. All the remaining domestic RCD's were left at 10 micro sec. This was over 20 years ago so I stand to be corrected as milli's may have shrivelled to micro's over that time.

He told me, and I have no reason to doubt it, that the RCD monitors the in / out flow of electrons, after all the conundrum of electical energy is that we allow the electrons to flow into our houses and back out again back to the power grid! Remember the guy who put a meter on the negative side of his power supply and deducted it off the input side as he said it had been returned to the CEGB. Interestingly a 3 phase supply uses the earth connection as the return conduit to the grid. The time delay required can be calculated by the distance from RCD to unit and back again divided by SoL in units to suit.

This forms the answer as well to Nicolas's comment – the RCD does not detect an earth leak! It monitors inflow versus outflow of current and if there is an imbalance it assumes an earth leak, it does not monitor the earth connection directly.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Nick Clarke 5 on 02/01/2021 19:44:15:

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OMG! that is a bit of a mess. If you have not worked with AC motors before you really need professional or experienced help to make that safe, Ther are too many issues to even start explaining. To be honest a new motor would be the best approach. Depending how much use, and the type of work, you will give the lathe it would be worth considering a 3 phase motor and VFD. They can be bought as a package for the Myford 7 series.

Robert G8RPI

[not done it yetParticipant @notdoneityet]

Quote from Andrew:

That seems very fast? I was under the impression that standard domestic RCDs normally broke in 100ms, or 40ms as needed. Note that's milli, not micro.

Quite right Andrew, not quite as tight as that – it depends, I believe, on the excess over the trip current rating. None of these devices trip in less than a ms. A 10mA device (usual domestic) must disconnect within 40ms. Higher trip levels may be needed for circuits (with VFDs, for instance).

[Oily RagParticipant @oilyrag]

Andrew (and NDIY),

I stand corrected; maybe the domestics were 40ms and the workshop30 amp feed was upped to 100ms. In my defence a lot of electrons have flowed over the years in and out of my machines!

Martin

[old martParticipant @oldmart]

A 1/2hp three phase motor and VFD from the inverter drive supermarket with a remote switch with all the controls would cost about £200. A similar capacitor run motor single phase would be around £50 and a capacitor start motor would be about £75.

[not done it yetParticipant @notdoneityet]

Posted by old mart on 02/01/2021 22:07:41:

A 1/2hp three phase motor and VFD from the inverter drive supermarket with a remote switch with all the controls would cost about £200. A similar capacitor run motor single phase would be around £50 and a capacitor start motor would be about £75.

I’ve never yet paid £200 for a three phase motor and VFD. The advantages of the VFD are very considerable, IMO. VFDs can go wrong but there is very little to go wrong with a three phase motor, unless it is abused – they either work or not when bought second hand – particularly cheap for a star connected example that is suitable for converting to delta.🙂. A pair of new bearings is chicken feed, if required.

[duncan webster 1Participant @duncanwebster1]

There seems to be some confusion, as far as I know RCDs don't trip on overload, they trip if the live and neutral current are mismatched (usually due to earth leakage). It's MCBs that trip on overload, and they can and do trip when connected to VFD as that takes a large inrush current to charge the capacitors. When I had a VFD connected to a spur fed from a 15A MCB it was a problem, replaced the MCB with a fuse, no more problem. It didn't blow the main 30 A ring main MCB. You can get slow blow MCBs, but they wouldn't fit my distribution box

[EmgeeParticipant @emgee]

Some older BS numbered RCD's offered a bit better protection as they were designed to trip within 200ms (mil secs) at the rated tripping current and within 40ms at 5 times the rated current.

Newer EN numbered RCD's are designed to trip within 300ms at rated current but still 40ms at 5 times rated current.

The best test to confirm a faulty trip time is a Ramp test, here the meter injects fault current in stages and records the value of current before the device trips.

Emgee

[HopperParticipant @hopper]

Posted by Nick Clarke 5 on 02/01/2021 19:44:15:

 

He then went on to a successful career terminating unwanted pregnancies.

Best seek professional help on that lot or a new motor at the prices suggested above. Not worth risking. It's a mess. If you are not electrically experienced and competent it could be a time bomb.

Edited By Hopper on 03/01/2021 01:17:27

Edited By Hopper on 03/01/2021 01:17:59

[SteviegtrParticipant @steviegtr]

Posted by Andrew Johnston on 02/01/2021 17:28:37:

Posted by Oily Rag on 02/01/2021 11:48:27:

I believe the domestic time function is around 10 micro seconds, my RCD was changed to 30 micro seconds and I have not had a problem since. 30 micro seconds is still ample time to protect humans from a fatal electric shock.

That seems very fast? I was under the impression that standard domestic RCDs normally broke in 100ms, or 40ms as needed. Note that's milli, not micro.

Andrew

Steve. I don't want to get involved. Too many electricians on here. I am once such.

[john fletcher 1Participant @johnfletcher1]

Nick if you send me your email address I will explain to you regarding your motor connection details, but not on here, to many electricians. John

[SillyOldDufferModerator @sillyoldduffer]

OK, so Nick has a motor that someone bodged in the past. The good news is it ran before and the rats-nest rewire might well explain the RCD. There are several sticky whiskers of wire and other dodgy connections that may have come into contact when the lathe was moved.

On the downside:

• Why did the previous owner disconnect the terminal board? Could be he was trying to fix a fault and the motor itself is iffy.
• The motor has been 'got at', meaning Nick can't trust anything and now has to work the wiring out from first principles. As there are half a dozen different single-phase motor configurations, this is a challenge.

Single-phase motors aren't my thing, so I'd be very much obliged if a more experienced member would comment.

• The bodge uses two pairs of wires, which I believe are the start and run windings
• Two dirty blues wires are left dangling. Nick said he thought these are a thermal cut-out, and I think he's right. They can be ignored. (Except tape up or remove the exposed wires!)
• There's no sign of a centrifugal switch.
• There's one taped up blob marked 'CAP'
• The terminal plate is marked: A, AZ, S and T, which doesn't quite line up with any of the single capacitor motor configurations in my book. (But the wiring has been bodged and we don't know how the terminals were originally connected.)

However, I suggest the motor has been wired as a Capacitor Start and Run like this circuit:

With a multimeter on a low ohms range identify the Start and Run Windings. The run winding has a lower resistance than the start winding. I believe the light coloured wire is the top of the winding as laid out in the diagram.

Health & Safety Warnings! Proceed with caution. Mains electricity is a dangerous shock hazard, doubly so if the worker is stood on a damp concrete floor. Don't touch anything when the power is on. Instead switch on and off from a distance. Capacitors, especially if bodged, can explode. Not like a firework but enough to cause a startle accident, or spray noxious chemicals in your eyes. And finally, incorrect connections or a duff motor might cause an electrical fire. Any sign of smoke, smell or heat, turn off immediately. If it starts, don't immediately assume all is well: supervise until happy nothing nasty is developing. I'd replace the capacitor with the correct type – who knows what's inside that package!

Another hazard is I may have got the circuit wrong. For that reason wait to see what others say, and if you try it remember it's risky. No guarantee it will work or be safe. But I don't think Nick can do worse than what's already been done.

I dislike this kind of problem because the raft of unknowns make everything difficult. It's much easier and safer to buy a new motor because they come with instructions. Most of the mystery and danger is removed; you just have to make a competent job of the wiring.

The circuit above only runs the motor in one direction, being a guess it has no provision for Reverse. If the circuit is correct, reverse is achieved by swapping over the two ends of the start winding between Z and AZ. That's what the Dewhurst switch does. My advice, leave that until the motor is proved to run safely. Or not!

I've an uncomfortable feeling the motor is a dud because of what's been done in the past. Even if my circuit is correct the motor may still pop the RCD. If it does, trust the RCD! It's a safety device issuing a warning that something is wrong. The motor demands a high-voltage insulation test, for which you need a nearby friend with a Megger, or a professional electrician. But rather than paying a man to tell me the motor was dangerous, I'd just replace it. (About £100, unless you upgrade to 3-phase and a VFD)

Dave

[not done it yetParticipant @notdoneityet]

One practical tip Dave omitted to mention. Keep one hand in your pocket while testing live circuits – it’s safer that way.

[noel shelleyParticipant @noelshelley55608]

Nobody has commented on the need for a centrifugal switch on a capacitor start motor which is what the 2 brown wires are. Not a thermal switch ! 4 wires on the windings are red / black, the run windings and blue/ yellow the start windings. Wire yellow through the capacitor then through the Centrifugal switch then out to terminal 2 on Dewhurst reversing switch. Blue out to Terminal 6 on dewhurst. Black to Term 5, red to term 7. Live to term 1 on dewhurst, blue to term 2 and neutral to terminal 3. Terminals 4 and 8 are not used. If the notation on the dewhurst reversing switch is wrong swap over the wires on terminals 2 and 6. RCDs trip due to an imbalance in incomings and outgoings, an MCB will trip due to current above its rated value. A spit phase starting motor can draw 1200% above rated run current. That's where a DOL may save a motor, as it will take the starting current for a few seconds before a sustained high current will cause it to drop out. Noel.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by not done it yet on 03/01/2021 11:49:20:

One practical tip Dave omitted to mention. Keep one hand in your pocket while testing live circuits – it’s safer that way.

What brown wires?
What about the two blue wires on the Stator?

I'm not sure how you can tell what wire is what unless you are psychic. Even if we knew the motor make and model the wire colours could change during production lifespan. I can't even tell for sure what the winding colours in the photo are.
As a minimum the windings need a proper isulation test, all the crimps need re-making (which will probably make them too short) and ALL existing wiring replaced and connected correctly. I'm pretty sure the OP is nor equipped do do all that. No work done previously by someone who used yellow/green wire for live and neutral connections can be trusted.

A new motor is cheap compared to the time required never mind the cost of the workshop burning down or someone being killed or injured by electric shock. Even a minor shock around running machinery can cause uncontrolable movement of body parts into moving parts or sharp tools.

Tha old, probably worn out motor / can of worms is not worth repairing. I have the knowledge, tools, parts and test equipment required and I would not even atempt to repair that motor if it was mine. I think it is irresponsible to give the OP advice other than to get a professional to look at it.

Robert G8RPI.

[Dave HalfordParticipant @davehalford22513]

Noel,

For my money the red/blue and red/black wires are the centrifugal switch, you can see the contacts in photo 2 of Nicks last post.

Nick,

You can't use those blue crimps in such a confined space nor leave all the exposed copper visible, especially those chocolate block blue and brown wires.

If 'rats nest' was how you bought it, then someone knew the motor was bad. Bin it and buy a new one, they have much more room in the connector box.

If you do go 3phase, be aware the VFD doesn't normally work straight out of the box.

[Martin 100Participant @martin100]

Lovely use of green and yellow wire.

Maybe fixable IF you have suitable test gear, or at a push it’s but a few minutes of work using terminal blocks, an incandescent lamp and a 13A plug. But as ever you really have to know what you are doing.

Look on the bright side, the motor originally wired in that manner would probably kill you…now you have something to sell on eBay as an antique, or if you have morals, something for the scrap man.

[Author]

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