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Repair an old Zanussi washing machine with RCD tripping

Dave Empson -

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問題

My FJ1040 (part 914787007) Zanussi washing machine (1992) started tripping the RCD (residual current device switch) every few months. The machine carried on washing, once the RCD in the consumer unit (fuse box) was reset after some delay. After several years, the tripping became more frequent until it happened every wash or several times during a wash cycle. A repair was therefore required. It's reasonably easy to take the top and back off the washing machine, just needed a 9mm socket (and a socket extension arm to remove the motor).

修理

Internet sites suggest the emi filter (to stop interference with other electric items in the house) was likely to be faulty, but even after bypassing this, it still tripped. The next fault candidate was the heater, but I disconnected this (and loaded the machine with water at the desired temperature), but it still tripped. Note: to trip an RCD, the faulty component typically has an earth lead, so the next candidate for a fault was unfortunately the motor. (Unfortunately, because this component is hard to source and/or expensive eg £150)

I could see that the motor (part 1240369098) was the problem because immediately after after the RCD tripped, the resistance between the live and earth prongs of the unplugged mains plug measured about 50 kohms, easily measured with cheap multimeter (analogue or digital, no need for an expensive megger or PAT tester) and when the earth lead was pulled off the motor, the above resistance essentially measured infinity.

Also, I could see that the resistance, with the earth lead reattached, gradually went back up to an acceptable 150 kohms in 10 mins and back to a satisfactory 1Mohm after an hour.

This suggested that it was a thermal effect. A thermal camera showed that the commutator ( the stripey rotating cylinder bit) was getting hot, probably decreasing its resistance to earth. This was also proved by heating the commutator manually with a hair drier for a couple of minutes and watching the resistance fall. (A thermal camera also showed the commutator was heating up to about 50C.)

With the aim of reducing the temperature of the commutator, it was cleaned using 'electrical contact cleaner' and new carbon brushes (as one of the old ones had split and only cost £8). However this appeared to make thing worse, tripping the RCD up to eight times per wash!. Blowing the carbon dust out with a bicycle pump (in the garden) also had negligible effect.

I then reduced the spring load on the brushes (from 34mm/600g to 10mm/300g) by cutting cutting off 24mm of graphite brush and re-profiling the ends using fine (400grit) abrasive paper around the commutator, turning it backwards and forwards by hand. I also used this fine abrasive paper on the copper commutator to make it smooth and shiny. All the abrasive and debris must be cleaned out of the motor as well as possible (not perfect as the motor is riveted so not split open for cleaning).

All these actions were to reduce friction and improve current flow to minimise the commutator temperature when running the washing machine. The shortened brushes should still last a few years.There was also a slight hair-line crack in the insulation below the copper bars in the commutator, which I filled with superglue. (Not sure this was necessary).

On the internet I have seen another 'repair' option, ie removing the earth lead from the motor. Although this might work, it could be dangerous for anyone subsequently working on the machine with the motor exposed, as its surface could become live, so not recommended. Also modifying a circuit may have unintended consequences unless one really understands the design intent.

Another (safer) option would be to set up some cooling air for the commutator as the motor has no cooling fan, which might work.

Although the washing machine did still trip occasionally in the beginning, it was not really a hassle as I used a separate RCD adaptor (borrow or <£10 eBay) in the socket next to the washing machine so it didn't trip the whole house. This may or may not work for you as it's a matter of luck which RCD trips first. (People used to use RCD adaptors for electric lawn mowers before it was normal to have RCD's in the consumer units. So an old neighbour might have one to lend/donate.)

Update: The washing machine did trip 4 times in subsequent 7 full machine washes but since then the last 10 full wash cycles have run without tripping. I guess the brushes have worn in to a better profile.

アドバイス

A quick way to find out which component in the washing machine is leaking to earth, and so tripping the RCD, is to measure the resistance (highest resistance setting on multimeter) between the live and earth prongs of the plug after the plug is fully pulled from the mains socket immediately following an RCD trip. To be clear, don't put anything into the socket. (Don't alter the washing machine settings). Then, having taken the back off the machine, detach the earth leads, one at a time, from suspect components until the above resistance goes back to near infinity.

I guess the earth leak is due to insulation (mica?) breakdown under the commutator, although this can't be repaired, keeping the temperature of the commutator down, seems it's less likely to trip the RCD. ( I guess with insulators, resistance goes down with temperature, unlike metals)

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