Any electronic/electrical experts out there?

[paul_h]

Got a generic wall split system thats blowing fuses on the indoor PCB.

It has an AC circuit board and a separate DC circuit board.

On the AC PCB there's a 3.15A fuse, a transformer and 4 diodes and a small capacitor to give power to the DC PCB
Across the active and neutral there's a 0.1uF capacitor and a ZNR.

Also there's 6 relays: 1 for comp contactor in the outdoor unit, 1 for reversing valve, 1 for outdoor fan, and 3 for indoor fan speeds, as well as the soldered on 2uF indoor fan capacitor.
It's a very simple PCB.

First time to the job with the a/c not working, I found the 3.15A fuse blown. I replaced it and powered the system up (not turned anything on, just had mains the the PCB), fuse OK with the transformer etc powered up.
As soon as I turned it on and the indoor fan started, the fuse blew.
I unplugged the indoor fan, replaced the fuse and turned it back on. Everything else worked, compressor was running, louvres operating, seemed that everything was good except the indoor fan motor.

I got a replacement fan motor and fitted it, fuse blows as soon as the fan is on.

What is on this PCB that is drawing too much current with the indoor fan running?
None of the relays seem faulty or welded. the ZNR is open circuit (could that just fail for a second when there's some current going through it- the indoor fan would be the biggest power drain on the indoor PCB I guess), but it looks new and tests OK.
I removed the 2uF fan capacitor and tested that, that looks OK too.

I want to replace the bloody a/c, but it's on the top of a steep pitched 2 story tin roof, would cost thousands to replace. GENIUS!

[Tradewinds]

Hi Paul,

Where did you disconnect the motor from? The motor end or does the new motor plug straight onto the pcb. Just wondering if you could have a short in the cable or relay. Does it trip on all three fan speeds. Is it possible that 2 of the relays are closed at the same time? ie one is stuck closed.

Just trying to think outside the box a bit.

[paul_h]

Fan motor just plugs straight into the board. So the new motor has all new wiring and plug.
I checked all relays to see if any are welded or stuck closed like I said.

I mean I have come across this problem before with horribly complicated circuit boards that are both AC and DC, and have thousands of circuits, resistors, diodes etc, like for example panasonic. Where fan motor dead = scorch marks and damage to the PCB.

But the board in question this time has practically Nothing On It! I can't see why it has failed and keeps blowing the fuse.

[install monkey]

is the transformer shorting/grounding? whats the make of this generic split? have u tried wiring the new fan motor to bench supply fused at 3 amp to check u haent got a duff new motor

[simon@parker]

just a thought yr fan motor isnt a DC one is it ? if yr bridge rectifier failed it wont be getting a smooth feed so will draw hi amps check the output from the board at the fan plug in point hope this helps

[install monkey]

good point

[paul_h]

Unfortunately I don't have any bench test equipment.
Transformer seems good because everything else will run if the fan isn't plugged in/ and it measures out OK
It's not a DC motor.


Ok I've got the ac PCB here and tested it.

When the fan motor isn't running, fuse doesn't blow
I worked out the DC signals, so with the fan motor unplugged I shorted the -12 to the relay coils, and all relays switch on and off fine.

When I have the fan plugged in, the fuse blew when set to medium speed straight away.
the fuse blew after 3 seconds running on low speed

I ran out of fuses so can't test high speed

But pretty much looks like the DC PCB must be OK, as the fuse blows when this is not connected anyway.

Looks like the transformer and everything else might be OK as I could switch the relays on and off fine?
Trans Primary 330 Ohm and secondary 2 Ohm

Also I replaced the ZNR with a MOV I had before my tests.

[mikeref]

Doubt new fan is the problem. Fan relays could be burnt at the points or have conductive carbon allowing energised relay to short out (through the fan windings) to neutral. To test certain 240 volt items AFTER doing insulation test, i use suicide leads, insulated alligator clips on fuse protected extension lead, to run the isolated and earthed component,such as your fan motor, to check current draw. Just a thought.. Mike. P.S. For anyone out there not electrically qualified...this type of testing is never to be considered as an option.

[paul_h]

New fan motor stuffed.

Found a fan from probably the same OEM, wired it up, and it all works fine.

Motor I found is from a 3kW unit, so no good to me because the a/c I'm repairing is 6kW. But it proves nothing wrong with the PCB (there really couldn't be as it's so simple)

The faulty motor wasn't from local sources though, so going to be fun getting it exchanged.

[mikeref]

Well, there you go!!. Buggered from the start. These problems are more than annoying.

[paul_h]

I like your sig, but it's missing a line. Isn't it supposed to go:
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needed to be.

[mikeref]

I like your sig, but it's missing a line. Isn't it supposed to go:
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needed to be.

Yes, thats right.. however, unfortunately, i cannot change that as there is a limit in this section as to what i can say..Mike.

[NoNickName]

A batch of motors low on insulation. Tipical chinese stuff untested.

[paul_h]

Don't say a batch of motors, please.
There's only one source for these and I don't want them all to be faulty
That's even if they accept my claim and are willing to send replacement, or if they have any more as this brand is non existent here anymore

[Gary]

When a motor burns out, excessive current flows through any and all contacts in series with that motor, burning the contacts. When the motor is replaced, the burnt contacts can deliver low voltage/high amperage to the new motor.

[install monkey]

oh! my bad- i'll make an ac eng yet,20yrs at it -refer to post 4 -nothing like being right for once haha

[paul_h]

When a motor burns out, excessive current flows through any and all contacts in series with that motor, burning the contacts. When the motor is replaced, the burnt contacts can deliver low voltage/high amperage to the new motor.

thanks for that, and mikeref saying the same thing, it explains some cases of why the PCB needs to be replaced or new relays fitted.

In this case though I checked the output loaded and unloaded. Mains is 248V.
When low selected output 248V on low, Med = 2V, High =2V
medium, output 248V on Med, 2v on low, 2v on high
High = 248V on high, 2v med, 2v low

When loaded with the motor running:
Selecting high speed: H=250v, m=260v, l=250v
Med speed: H=180v, m=250v, l=220v
Low speed: H=212v, M=260v, L=248v

This is all with the small 13W motor from the 3kW cooling split system though. The faulty fan motor than runs off this PCB is for a 6kW cooling split is about 3x the size, watts and current rating. That shouldn't make a difference with the fuse blowing fault though.

[Magoo]

Hi Paul_H
Was the original fuse a slow blow type. They are often used for surge protection. Can sometimes be hard to determine what they are.
Just a thought.
magoo

[MikeHolm]

Just off the top of my head (which usually doesn't contain much) electrolytic caps sometimes can cause issues and they don't last forever.