Lathe Motor Tripping

[rogerguzzi]

Hello All

               The motor on my big lathe trips the earth leakage relay out I have tried removing the earth but no difference!

It is a capacitor start type and flameproof (ex petrol pump)

Do you think it could be the capacitor that is causing the fault as I have 2 spare motors I could take one from and I think change it in position(easier than changing the whole motor!

I looked at one in a spare but can not see any size reference!

Roger

[Nick Jones]

Is this an instant trip as soon as power is applied?

May be worth trying a capacitor swap, but as you say it does it with the earth disconnected I wonder about the leakage relay itself?

Do you have a meter with megaohms reading? Ideally you want a high voltage tester for the winding insulation but it might tell you something.

[JohnD]

In my ignorance, I thought that the capacitor on an electric motor was to deal with the large current they draw on starting.     My compressor has needed a new one twice, in about twenty years. It would chug-chug-durrrrrr and die on starting.  I know the sound now!

Is that what yours does, Roger?  Me, I'd take an earth current cut-out (RCD?) tripping  very seriously - they are there To save your life!

John

[DeTRacted]

The capacitor is probably a starting capacitor for the  motor John.    it is used to produce an extra pseudo-phase supply 90 degrees shifted from the real single phase supply and driving a separate auxiliary winding.  Having a second phase drive gives the motor much higher starting torque to get it up to synchronous speed quickly. 

The capacitor is disconnected by a centrifugal switch once the motor is running at speed. 

Unless it has an earthed  metal case, failure of the capacitor should not cause an RCD to trip as it only results in excess phase current rather than an earth leakage but it could trip an RCBO or an MCB as they also trip on excess supply current. 

An RCD should only trip if some current is being diverted to earth through a failure in insulation, instead of returning through the neutral wire. Essentially it looks for a difference between the supply and return current.   As Nick says, if the earth is disconnected a diversion of current cannot happen so the RCD should not trip.  (Of course disconnecting an earth is NOT to be recommended .........)

 

[zetecspit]

As others have said, the RCD compares current on L+N. More than 30mA and it trips (often less than 30mA when tested)

So some current is escaping somewhere, if not through the earth wire then though the machine to earth, or you. But that 30mA is the assumed safe limit for humans. 

You really need a proper tester,my gripe with testing it is not easy to do with an appliance running. I do some PAT testing, that uses 500V , joins L+N and checks resistance to earth. Done to not damage whatever is being tested. 

[JohnD]

There you go!   The Common Room of the Sideways University has experts (and I don't mean drips!) in any subject!

Thanks, to m'tutors!

John

Edited May 7 by JohnD

[Sigma hurricane]

3 hours ago, JohnD said:

In my ignorance, I thought that the capacitor on an electric motor was to deal with the large current they draw on starting.     My compressor has needed a new one twice, in about twenty years. It would chug-chug-durrrrrr and die on starting.  I know the sound now!

Is that what yours does, Roger?  Me, I'd take an earth current cut-out (RCD?) tripping  very seriously - they are there To save your life!

John

To be fair John lots of motors especially compressors have soft start for this exact reason however they manipulate the voltage via solid state semiconductors. The high voltage and current to get motors going is normally what kills the windings and eventually results in the motor “grounding” out. Either that or bearing failure tends to be what kills motors.

[rogerguzzi]

Hello All

             Thank's for the advice it looks like I am in for a motor change!!!!

It trips out the instant the starter is pressed 

I may try disconnecting the earth elsewhere just for test but with gloves on and not touching any metal that could be live(no desire to die )

I suppose 44 years is not bad and I have 2 more I can use so no cost in £ss just muscle power(or lack of it?)

Still we have been out on a good run today Breakfast a Church Stoke (wales) then Llanfair Caereinion railway for Ice cream then back to Craven Arms Discovery Centre for Tiffin!!

It's a Hard life but we do not mind doing our share!!

Plus only 172 miles covered and Spitty still doing 45mpg

Roger

 

 

 

[mpbarrett]

13 hours ago, zetecspit said:

As others have said, the RCD compares current on L+N. More than 30mA and it trips (often less than 30mA when tested)

 

If I remember its a combination of current and time, I think the standard is 30mA for 30mS, higher leakage will trip it faster. All based on what the human body can survive with out being killed! 
Is it a  RCD device or combined RCD and overload, RCBO? If it is it could be tripping due to high current or leakage...

BTW RCD;s do fail so its worth using the test button occasionally. Had a MK RCBO  in our DB fail to trip when the test button was pressed.

mike

[zetecspit]

Yes, thoroughly encourage everybody to test their RCDs AT LEAST twice a year. They can get sticky.

I have tested RCDs to be as low as 24mA, not that it really matters. Most around 27/28mA Trip times always well under 30mS, some in the teens, which is reassuring.

[rogerguzzi]

Hello All

               No luck 

I fitted a different RCD (Same)

I fitted a 3 pin plug to motor wire to rule out starter and wiring (Same)

So I have removed the bloody heavy Barsteward while lying on my side (I am sure it was easier 40 years ago?)

Now just have to remove the pulley and refit the bloody thing with head under the swarf tray lying on my side(DEEP JOY!!!)

Roger

ps I could cut the wall out of the garage and work easy? (Perhaps not!) 

[ahebron]

RCD protect people
MCB protect equipment.

I have had new RCDs fail, fail,fail, new one arrives and old rcd pass, pass, pass. 
Very infuriating but still replace as old one had a fault.

 

[zetecspit]

7 hours ago, ahebron said:

MCB protect equipment.

Being picky, the MCB is there to protect the wiring from being overloaded. 

[ahebron]

Yep, thats equipment

[spitfire6]

Disconnect the motor wires/connection and see if the RCD/RCBO trips.

[egret]

On 5/7/2025 at 7:34 PM, rogerguzzi said:

 

 

 

 

 

Forgive the tangent here, but nowadays it's rare to see such a lovely colourful carpark!

[rogerguzzi]

6 hours ago, egret said:

Forgive the tangent here, but nowadays it's rare to see such a lovely colourful carpark!

A few more for you

Roger

[RogerH]

I like roads with grass down the middle

 

Roger

[rogerguzzi]

11 hours ago, RogerH said:

I like roads with grass down the middle

 

Roger

Hello Roger

                      That's a road coming down off the Long Mynd and tests your brakes for sure 

We had just been to the Glider station for Breakfast recommended and some good views from up there!

Roger

ps about 1 in3 to 1 in 5 and narrow!