Strange fault ML7

[HopperParticipant @hopper]

Was this a lathe reconditioned and supplied by Myford Ltd?

[Haydn CallowParticipant @haydncallow19480]

No, not supplied by Myford. But the chap who refurbished it has done dozens and has been VERY helpful.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Haydn Callow on 26/08/2022 20:43:16:

With ref to this post….when I am up and running again, I have a dol switch ready to fit. Newlec 240 volt….do I just put in the mains wire to the Forward Off Reverse selector switch ? Then in future select direction of rotation and use the dol to provide power ?
thanks

Don't fit a DOL starter until the existing fault is fixed! It adds complexity and might introduce another fault. The lathe should work without it. (My impression is Myfords are rarely fitted with them?)

Broadly speaking, there are only three suspects:

1. The Forward/Reverse switch, now proved innocent.
2. The motor, for which a replacement is on the way.
3. The wiring.

RCDs work by comparing the current flowing in Live with the current flowing in Neutral. When all is well the currents balance exactly. If the currents don't balance, then Live is leaking to Earth. The RCD flips out if the current exceeds a few tens of milliamps, so sensitive enough to save lives if the earth leak is due to a human being electrocuted

If the RCD is tripping then look for a wiring or component failure of some sort. Chafed insulation, damp etc. Note the fault could be in the neutral line; if neutral is connected to the lathes earth, then the RCD will trip. A conventional fuse won't detect this fault, and the lathe will run "normally".

If nothing found, there's a legitimate reason why live and neutral currents don't balance perfectly. It is that suppression capacitors and filters earth unwanted noise to ground plus a tiny AC current. As the RCD sees the total imbalance caused by all the devices plugged in, its possible domestic devices are bringing the RCD close to the limit, and running the lathe takes the RCD over the top. Unplug everything in the house that shares the popping RCD and try the lathe again; if it runs OK, the problem is in the house. This is one reason why sheds and workshops usually have their own supply. I found this description of the problem readable, and it explains how to use a clamp ammeter to detect it.

Approach with caution!!! This site explains how to detect earth leakage with a multimeter. Poking at live mains with a multimeter isn't for inexperienced operators or anybody with wobbly hands. There's a reason electricians test disconnect circuits with a Megger: much safer!

Dave

[Haydn CallowParticipant @haydncallow19480]

Thank you for that comprehensive reply…I’m beginning to get my head round it……does the fact the lathe runs perfectly in reverse give you any clues ?

[noel shelleyParticipant @noelshelley55608]

I wonder if the centrifugal switch depending on direction of rotation is causing the problem as IN THEORY the same circuits are in operation for both directions ? The moving component is pushing the fixed component to one side on start up and causing a short circuit, that may be not present at rest ! If it's not the switching then it must be the motor. Without some test gear this may be hard to solve. Noel.

Edited By noel shelley on 27/08/2022 11:47:51

[JssParticipant @jss]

Changing the motor direction is done by swapping the polarity of one of the windings (usually the start winding?). If there is leakage at one end of this winding or associated wiring there may not be enough leakage to operate the RCD when that end is connected to neutral, but there will when it’s connected to live. Neutral to earth faults will also trip the RCD if there is enough voltage rise of the neutral due to load or length of circuit.

John.

Edited By Jss on 27/08/2022 13:29:16

[Haydn CallowParticipant @haydncallow19480]

Sorted, sent motor back to the supplier, there is a fault in the motor and a replacement is on the way.

thanks for all your input.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Haydn Callow on 01/09/2022 17:43:22:

Sorted, sent motor back to the supplier, there is a fault in the motor and a replacement is on the way.

thanks for all your input.

Hurrah!

Bad luck though and the inconvenience must have been annoying.

Did they say what the fault was exactly – I can't think why a motor would be OK in reverse but not forward. Mysterious!

Dave

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