Help with an AC motor

[simonedwards]

I have a problem with a set of mains Horse Clippers which I hope someone can help me with.

The problem is that, intermittently, when I switch the clippers on, or switch them off it blows the fuse.

So, I've taken the beast to bits and I have found it is an AC series motor but it has a "condenser" which I suspect has gone faulty. So I called the service centre and they quote me £23 to replce the condenser !!!!

No way am I paying that much so I thought I'd fix it myself. The only problem is the "condenser" actually comprises 3 capacitors and I dont know how they are connected withing the condenser unit.

The condenser unit has 2 wires going in (the mains supply is connected to these) and 3 wires coming out. Two of these wires go to the Stator windings and then to the brushes, and the one remaining wire is soldered to laminated stator itself (kind of like and earth wire?). One of the mains input wires (the neutral) is common to one of the output wires which is connected to one of the stator windings / brushes.

The condenser has printed on it 0.1uf and 2x2500pf 250v. I therefore assume there are 3 caps in the condensor unit but I have no idea how they are connected up.

Does anyone have any idea? The caps themselves will cost a fraction of the unit so I'm hoping to fix this myself. Any help would be greatly appreciated.

Cheers

Simon

 

[colin55]

The possibility that the caps are faulty is remote. They are a "short circuit" or "open." I hardly think they would "flash over" and "heal themselves" as the caps are potted. I have never had a cap flash-over intermittently. I think its more likely the switch. I have had hundreds of switches blow the fuse, just like you are saying. If it is double-pole switch, this is your problem.
Trust me, I am a crook TV serviceman.



The 0.1u is across the mains. Then you connect the two 2500p in series and place them across the 0.1u Now take the two outside wires out - and the join of the 2500p's out to make three wires.

 

[simonedwards]

Hi Colin,

Ah, but if the caps were connected like this then both the mains wires (live and neutral) would be common to the i/p and the o/p. As it is only the neutral is common, so one of the caps (I suspect) must be in series with the live.

I was thinking that at the point of swiching the thing on or off there is a surge, which is blowing the fuse. If one of the caps is there to filter out the surge, and it is faulty, then this would explain why the fuse keeps blowing, but once the thing is running, it runs quite happily.

Would the caps also provide some kind of (necessary) phase shift, or is my electronics too rusty and I'm thinking of a different kind of motor?!

Cheers

Simon

 

[colin55]

Is the Sw double-pole?

 

[simonedwards]

It is indeed a double pole switch. However, it blew the fuse also when I switched the mains on and off from the wall socket. In fact I dont think I used the double pole switch at all. (I didnt want to give myself a mains belt!)

 

[colin55]

It takes an enormous amount of energy to blow a fuse.
Is it a circuit breaker or fuse?

 

[simonedwards]

Ah, now this might help. Originally it was fitted with what looked like a circuit breaker. (On the manufacturers web site it is shown as a reset switch). When I tried to press the little button in to reset it, it just wouldnt latch and kept popping straight out again. So I took it apart and it was prettry burnt out inside. On the outside it was labled 500ma so I thought I'd just replace it with a 500 ma fuse. Well I did, and this blew so I put a 1A fuse in and it started up fine, but blew the second time I switched it on. So I put another 1A fuse in and this time it switched on ok, but then blew when I switched it off.

Does this help?

 

[colin55]

Anything will blow a 1 amp fuse.

 

[simonedwards]

Well, I can put a bigger fuse in if you think that will work, but I dont want to blow the motor up if the condenser unit is faulty.

It is a 120w motor, and as we have a 240v mains supply here, then it shouldnt be drawing much more than an amp or so.

What size fuse do you think I should try?

 

[colin55]

I have 240v mains too.

Put in at least 2 amp. I have never heard of 1 amp!!!!!!

And make it "slo-blow."

 

[simonedwards]

Hi Colin,

Well, I think I've found the problem. I stuck a 3.15A fuse in, but removed the brushes so I could measure a few voltages, without me damaging the windings.

As expected there was 240v across the brush connectors, and also as expected there was about 110 - 115v between the stator itself and each of the brushes, which confirms the connections with 2 of the caps as we expected.

At this point, while I am sitting there stroking my chin and having a think ...... BANG ! the condensor unit produces a nice blue spark and the fuse pops. Now the condensor has a nice black burn at one end.

So, I take out a knife and start to dissect the condensor. This surprised me. I was expecting some discrete components which had been potted, but no. I peel off the thin plastic outer cover, and I'm imediately into peeling away paper / metal layers like peeling an onion. Yes, it looks like a bespoke capacitor arrangement. No wonder the replacement is so expensive.

Eventually, I find a metal strip between the mains i/p and o/p on both the live and neutral connections so I'm pretty sure the connections were just as you suggested earlier on.

So, I am going to get myself some discrete caps and connect them up and give it a whirl. Hopefully I wont blow it (or myself) up !

Thanks very much for your guidance on this. Much appreciated. And it was nice to blow off my electronics cobwebs and get back on the tools.

Cheers

Simon

 

[colin55]

You need "X-2" capacitor 0.1u 275v rating

 

[Boncuk]

AC motors have the bad habit to draw six to eight times the nominal current when starting.

If you use a 10m long extension cable with a cross section of 0.75 square mm the fuse won't blow anymore.

Using a 1.5KW milling machine a 16A fuse blew right away. After using the extension cable nothing more happened like this.

Boncuk

 

[Hero999]

AC motors normally have run capacitors but 0.1:mu:F and 2500pF sound too small, normally they're >1:mu:F even on small motors.

 

[Boncuk]

If the motor is brushed you might as well kick out the caps. Their only purpose is spike suppression back into mains.

As already mentioned by hero999, run caps are normally dimensioned 2.2µF to any higher value depending on motor power.

You'll probably get some interference on your TV when operating the motor.

Boncuk

 

[colin55]

It is obvious the switch "on the wall" is at fault. Is is a double-pole switch?