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Induction motor tripping RCD

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RichardG

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My Draper bench sander now trips the RCD within a second of turning on. Can anyone point to a fault finding guide?

I’ve checked the starter capacitor with a multimeter and there’s no DC path but of course I’m not checking it at 240v when it may be breaking down.

Thanks


Richard
 

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Does it do the same when plugged into another circuit? If it does the motor or cable is the fault. If it doesn't then there is something going on with the wiring. Overloaded, breaker getting worn et cetera.

Pete
 

RichardG

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Yes, it does the same on another circuit. I’ve got table saw, bandsaw, planer thicknesser (all induction) all on the same circuit without any issues so I’m sure the RCD is OK. I just wondered if there was a guide for testing out an induction motor to confirm it has failed before replacing it.

Richard
 

Woody2Shoes

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RichardG":21wrtpwd said:
My Draper bench sander now trips the RCD within a second of turning on. Can anyone point to a fault finding guide?

I’ve checked the starter capacitor with a multimeter and there’s no DC path but of course I’m not checking it at 240v when it may be breaking down.

Thanks


Richard
An RCD trips because the current in the phase line is different from that in the neutral line by more than a certain amount for longer than a certain period. If it's an rcd it's reacting to a phase/earth fault or a neutral/earth fault most likely. Are we assuming there's only a single start/run capacitor? Does the wheel and motor turn freely by hand?
 

RichardG

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I’ve finally found the parts diagram and only one is shown. The belt turns fine by hand. I’ll strip it down and see if there are any insulation worn away or cracked. It’s done amazing duty so perhaps it’s time to stand down and buy a new one...
View attachment draper sander parts.pdf
 

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Sheptonphil

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With my Jet 1100a chip extractor, I had to change the breaker in the consumer unit to a type C instead of the usual type B to stop it tripping on the initial startup if I had anything else running at the time. The type C cured the issue.

Phil
 

Sideways

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Be clear about what is happening. An RCD trips if the live and neutral currents aren't (almost exactly) equal. That usually means current leaking to earth somewhere.
An RCD does not protect against overloads unless it's a combined RCBO (RCB + Overload) device, so unless you have RCBOs your problem should not be anything to do with overloads.
I fault found an induction motor recently where overheating damaged the insulation inside the motor and allowed current to leak to the body of the machine. This an "earth fault". The RCD tripped immediately as it should.
I tried the motor on a non RCD protected circuit and it ran. The varnish on the motor windings had been damaged but there was not enough current leaking to earth to blow a fuse. RCDs are far more sensitive than a fuse and provide better protection which is why we now have them.
Lastly - DO NOT DISCONNECT THE EARTH WIRE while you are trying to figure this out. The whole machine could become live at anything upto 240V with the potential for a fatal shock.
 
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