On Thursday, 22 March 2018 21:20:53 UTC, Harry Bloomfield wrote:
Brian Gaff explained :
Its probably something like a brushed fan where the comutator is wearing and
causing momentary shorts over stressing whatever drives it. This sort of
thing used to happen to some equipment rack fans where I used to work.
it was just nobody had thought of what might happen if a crappy old motor
shorted out.

Twirling the fan blades inside its cast alloy casing, it doesn't have
the feel of having a commutator. That plus the extra wires into it,
suggest it might be a brushless type, using the switching sensors to
prove air flow - motor turning, there almost certainly is air flow.
Four terminals, with 100 Ohm across one pair seems to suggest the motor
is probably OK. It is a variable speed, to cope with variable demand
for heat from the boiler. I read or found somewhere that it runs on
around 320v DC, so maybe it uses the rectified 240v mains voltage.

Where ever the fault was which was tripping the MCB/blowing fuses, it
has cleared and the fault being displayed by both PCB's is a
PCB/controller failure. I have therefore flipped a coin and decided to
risk it by ordering a new PCB. I found one on ebay, new in its box,
which is a later version V10 (versus the V9) I had, for £60. Usual cost
for a new item is around £300.

200ohm on one pair certainly doesn't prove a motor ok. It might have a shorted turn, insulation that arcs over at full voltage, or other faults that could taken out an mcb.


NT