Pump capacitor causes earth fault

[younes]

Hi all,
I have a Ravenheat combi boiler that developed a fault over a month ago. It stopped working and found out that the pump capacitor had developed a crack. The original capacitor is a 2.6 uF, I replaced it with 2.5 uF as that was the closest I could find locally. It worked fine for over a month then the boiler started tripping the RCD whenever it is operated. I located the fault to the capacitor again. However the capacitor looks fine externally. I replaced the capacitor once more but the problem reoccured after a day.
I wonder what the problem could be?
any help or suggestion is very much appreciated.

 

[matt1e]

I seriously doubt the cap has an earth fault, is it encased in plastic or metal
And what voltage rating is the capacito you put in?
it's more than likely an earth fault in the auxiliary winding
How have you narrowed it down to the cap btw

Matt

Edit sorry I don't know where I got earth fault from
Ignore the first sentence

 

[younes]

Thanks Matt for a speedy reply,
The voltage rating of the capacitor I put in is 440 V (got it from Maplin)
I suspected the capacitor because when I remove it and operate the boiler the fault stops happening and it works until the overheating protection kicks in as the pump is not spinning. I replaced it for the second time with a capacitor from an old pump rated (2.0 uF) but it did not last more than a day.

You may be right and the capacitor may only be a manifestation of another problem, I will remove the pump this afternoon and check if any winding has and earth faults.

Is the capacitor connected in series with the winding?

Younes

 

[matt1e]

Is the capacitor connected in series with the winding?

Younes[/quote

Yes mate

 

[Agile]

The cap is in series with a winding but the main winding is separately connected.

I also agree the pump has a fault and that is usually water leaking into the winding when its an earth leakage fault.

Tony

 

[matt1e]

The cap is in series with a winding but the main winding is separately connected.

Tony

mmm I feel another debate coming on.......... but I'll refrain as I'm going out gigging so haven't got time to be pedantic
plus I think I know what you mean Tony:

"The main winding is connected differently"

Matt

 

[younes]

Thanks Matt and Tony,

You're right, the problem was water leaking into the winding.
The leakage originated from the auto air vent which is positioned just above the pump. The water seeped through the screw opening of the motor connection cover (which was meant to be water tight, but I suppose the heat generated from the damaged capacitor compromised its waterproofness).
I dried the pump by warming it in an oven for about 20 minutes (as suggested in one post in this forum).
The boiler has been working for almost 2 days now without any issues.
Thanks again for saving me the cost of a new pump

Younes

 

[Porque223]

Apparently the wet windings (primary fault) caused the caps (secondary problem) to be abused in some way?

 

[Agile]

I expect that I suggested drying the pump in an electric oven to see if that will dry it out and save it.

I am glad that my advice was helpful!

There are two windings in the pump. The main winding is across the mains supply.

There is a second ( start/run ) winding connected to the mains in series with the capacitor to provide the phase shift to create a torque.

Most of these pumps have a cap about 2.6 uF and many lose some of the capacitance after some years typically reducing to about 0.6 uF to 1.6 uF at which lower value the torque measured by the "finger test" is significantly reduced!

Tony

 

[Porque223]

Somewhere I saw a link that talked about max temperatures and times for drying out motor windings.

A Megger should have detected this insulation leakage current but maybe a plain DVM on the high ohms scale would also do.
Depending on your meter, with a 12v battery and a resistor a better megohm test may be possible. The meter would be on the mA or uA scale and the series resistor would be there to protect the meter in case the insulation shorted while taking the measurement.