Myford super 7 lathe tripping consumer rcd

It's allowed!

From the way i am reading your problem the RCD is in your house . What else have you got on the same circuit in your workshop that is using energy .The starting load that your lathe requires may just be enough to overload the RCD and cause it to trip, worth checking or just switch everything else of in your workshop and then try starting your lathe What size is the RCD that trips ? Or are you getting and voltage drop due to the extra load. some RCD have been increased a small amount in size IE 15amp to 16amp.

Regards Bee.M

Very true. I also assumed (sorry) that the OP does not have a three phase machine running from a VFD, for instance!

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

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.

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

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).

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

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.

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

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

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

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

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

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