WM14 mill earth fault - again. Diagnostic advice?

A while back I posted about my WM14 mill intermittently tripping the workshop RCD. I thought I'd fixed it by cleaning out the tachometer, but not so - it's doing it again. I'm going to have to strip it down to find the fault.

A logical first approach is to separate the power supply from the motor and test each individually. Can anyone suggest a suitable dummy load for the PSU? The motor is brushed DC rated 500W.

Any advice would be welcome!

Robin.

Robin

Have you checked for a build up of carbon dust within the motor, sometimes it can cause arcing to the frame and cause problems in the control board.

Emgee

The old para flood lights use a 500W globe. Or 5 x 100W light globes.

That did occur to me but all I did was take the brushes out and apply a (powerful) vacuum cleaner to the holes. Perhaps I should take the motor off, dismantle and give it a good clean? Worth a go.

Thanks, Robin.

You don’t need ‘arcing’ to trip a RCD.

Leakage from Neutral or Positive to Earth will trip an RCD.

Are you sure it is not the RCD that is at fault - not unknown ! Where are you ? You only need a thin film of carbon dust to cause a leakage that would trip a over sensative RCD.. Noel

Before you take the motor apart try cleaning spray. I don't know what it's called or where to get it from, electrical wholesaler? Screwfix?

After the motor I'd carefully inspect the wiring looking for frayed edges, loose connections, and wire whiskers. Not highly likely but vibration can cause intermittent faults. All of it from mains-plug to motor. If possible run the machne from an extension lead to make sure the mains socket and house wires aren't guilty. (I've come across two damaged sockets!)

Whilst inside the machine, give the electronics a careful clean. The RCD will flip if anything allows current to flow to earth, such as graphite loaded cast-iron dust between tracks, or muck bridging the insulation on heat-sinks etc. Condensation can do it too.

Hoover and brush gently to remove obvious dust and swarf, then wipe round with Q-tips moistened with a switch cleaner like Servisol. (Don't overdo it in case the cleaner dissolves any of the plastics present.) Look for areas where airflow drops dust in the works. Otherwise, the state of the Q-tip after a wipe will indicate how dirty the board is. or not! No need to clean a clean board.

Dave

I replaced a heating element in a combi oven, the resistance between the element and the outside was 2400 ohms, measurable on a cheap meter and allowing enough current leakage to trip the 30mA RCD. The new element was above the max 20M ohms of the meter. You may be able to check the motor this way before and after cleaning.

Thanks for replies. First, I don't think Emgee was suggesting that arcing was the only, or even the most likely, cause of the earth fault - just a possibility worth investigating. Given the intermittent nature of the fault it seems a sensible suggestion to me.

noel - I don't think it's the RCD. When I started investigating the fault I bought a 30mA 'local' RCD (the type that goes between the wall socket and the machine plug) hoping to avoid taking out the CU every time the fault occurred. Sometimes the CU RCD wins the race, sometimes the local one. I'm in the Derbyshire Dales near Matlock.

lee - thanks for your suggestion. I think I have some Servisol somewhere, if that's what you're thinking of.

Dave - I'm pretty sure it's not the socket. I have a socket checker (one of those things you plug in and lights come on to indicate fault conditions) and it passes. Also I can run my drill press (1/2 HP induction motor) from the same socket without any problem. But thanks for the suggestion.

I'll dismantle and give it a thorough inspection and clean before attempting to test the PSU / motor components individually as the consensus seems to be that is best first step.

old mart - testing with a multimeter gives >40 Mohms - the fault only occurs when the motor is working.

Writing that, something else occurs to me. I always wind the speed pot down to minimum before stopping the machine. In the past, when I've restarted it has started smoothly at minimum speed, but now it makes a groaning noise and I have to give the chuck a manual nudge to get it started. It seems likely (correlation) that the two things are related, but I can't see how!

Robin.

To draw what I hope will be the final line under this topic I think I have at last sorted it out. Following the guidance of a forum member who has the same machine and PM'd me I was able to strip it down and eventually found four dodgy connections between armature windings and commutator segments. I re-soldered the connections and all is now well. For now at least! I shall never know if that had anything to do with the earth fault because I cleaned out the carbon dust from the brushes at the same time. Anyhow, the machine is back in service.

My thanks to all who have offered advice on this, and to the forum in general - the kindness of strangers heartens me.

Robin.

Glad you got it sorted.

A note for future readers of this thread. It is not a good idea to use switch cleaner e.g. Servisol on the commutator and brushes. It contains a small amount of lubricant that is left on the ccontacts. This can be absorbed into the brushes and left on the commutor. It interferes with their operation and causes the dust to stick.
A simple solvent is best for cleaning brushgear.