Motor cutout problem – do bimetallic strip cutouts ever “weaken”?

My parents have this 20 year old very solid classic East German vacuum cleaner that no-one's ever heard of, called a Vorwerk. The motor has a very elegantly simple cutout board and warning light system - resistor, capacitor, diode, neon lamp, bimetallic strip.

If the brush head motor is stalled, it goes over-current, the bimetallic strip pings down, the neon flasher circuit kicks in, and a little plastic button is required to be pushed to reset the trip.

Image of cutout board: http://minus.com/l1sWPSMZaJwvC

Thing is, is suddenly started tripping instantly. So, we changed the brushes but it still does it. The motor is running freely, and we very gently cleaned the commutator with a soft cloth ("before" pic http://minus.com/lbyVi4liz2VxJl ). Still doing it. I've seen people talking about using very fine emery cloth to clean commutators, and others warning against that. Could it be that that's causing the overload? Or could it be that the bimetallic strip has weakened with age and "pings" too easily?

If anyone thinks it's the bimetallic strip, and if I can't find one of the same dimensions (no luck yet), is there something else that would suit instead?

You need to troubleshoot this problem down to either the motor or the bimetalic strip.

I'm assuming you don't have access to a 1 kW+ Variac (variable autotransformer) to bring the motor up from zero volts. So, you can bypass the protection circuit and use a suitable fuse. Does the Vorwerk list the current draw? You could use a 3AG type fuse rated for 240 VAC and perhaps 5 amps more than the listed current draw. If the motor is ok it should start and run without blowing the fuse. If so, then the bimetalic strip circuit is the problem and it can be replaced with a suitable AC circuit breaker.

The fuse, fuseholder, and circuit breaker are available at Digikey (and others) and I'll help you select the parts when you're ready.

Thanks - I'm about to place an order from Farnell this afternoon, as I'm in the UK this would suit me best. Is there particular part I should be looking out for? Thanks again - BTW, no, sadly no 1kW Variacs here

Farnell stocks dozens of possibly suitable circuit breakers. It depends on the mechanical configuration and the current rating. Make sure it's rated for 240 VAC. I would use a thermal rather than a magnetic breaker.

The SCHURTER T11-211-10, 10 amp, 1 pole thermal breaker might work (if 10 amps is the correct rating):

http://uk.farnell.com/schurter/t11-211-10/thermal-circuit-breaker/dp/1651001

Thank you for the quick reply, David, and sorry for my slow thanks - "things" came up this week.

Anyway, looks like a sound and pointworthy suggestion. I'd be very surprised if a domestic hoover motor required 10amps, and unfortunately at just under 50mm that particular breaker is too tall, although in other respects, it looks ideal.

Thanks for setting me on the right track - Farnell can be a nightmare to find stuff on!
But I've got the idea now and will look further this weekend and let you know (with pics!) what we end up doing so you can throw your hands up in horror

John, I'm sure your work will look better than mine! When I do profession work it looks professional. But for my own things it works but isn't always beautiful.

You're right on the 10 amps. I keep thinking in terms of US 120 VAC where the current is double yours. You might order a variety of circuit breakers and use the one that allows normal operation but always trips when the motor stalls.