How to fault find DC motor speed controller?

I have an older cowells CW with 200v DC shunt wound parvalux motor and variable DC drive.

The motor has 4 wires with separate armature and field windings.

I guess the controller is doing PWM

It runs OK for 10 mins then something trips in the main house fuse-box ( (either the ring-main trip or the RCD depending where in house I plug it in). If I reset the trip then it only takes another 30seconds or so to trip.

So I guess something is getting warm and leaking current. I don't know what. Any idea how to fault find?

I have a 40v variable DC supply which maybe I can use to test the motor? I can probably find an oscilloscope and multimeter.

Any suggestions?

Is a schematic diagram available for the controller?

I'd start with meggering the motor.....windings and armature....are there any RFI filters associated? They may be leaky?

Then look at the speed controller.....in fact you could disconnect the motor altogether from the controller and substitute it with a 100 watt lamp as a load....if the trip doesnt go after a period, then it looks like earth leakage...( a much safer approach rather than lifting rhe earth wire)

Edited By John Rudd on 30/10/2016 16:57:13

When an RCD trips its an earth fault. Does the motor have that motor burnt varnish aroma ? After checking the controller which John has already said using a 100 watt bulb. then you will have to check the motor. As you know which is field and which is armature, "Megger" both, assuming you have access to one, one lead to frame other to brush pig tail and for the field, one to the frame other to field wire.Very unlikely to have fault between armature and field but check anyway. You may find three capacitors connected to the brushes, they are part of the filter arrangement and they do fail, some times you can see a split or bulge along one side. If you have a soldering iron you can temporarily disconnect them, the motor will run without the capacitors. You may just have got a build up of carbon from the brush to the earth frame tripping things out. If it is carbon dust use a clean paint brush and vacuum cleaner to clear it out, at worst you may have to dismantle the motor.John

Hi Is it like this ? I used to have a cowell that used this type of controller , i dont know whats causing your problem with rcd tripping ,but the two paper film capacitors arrowed in the photo were the culprits on mine when the drive was playing up..

Edited By mark smith 20 on 30/10/2016 19:48:53

Edited By mark smith 20 on 30/10/2016 20:07:20

Edited By mark smith 20 on 30/10/2016 20:09:14

If the unit is like above and it turns out the caps are the cause, should you replace them, ensure their replacements are X rated...(designed for connection across the mains..)

I have similar but newer controller, housed under cowells base casting:

If it is those two capacitors, you can snip one lead on each and try running the motor, a fully functional motor will run without them.These capacitors are fitted to Kenwood food mixers and the like and are available on ebay or your domestic appliance repairer, must be X rated.

You only seem to have one larger value capacitor (or is that another different make , middle far right?), is the capacitor made by RIFA, according the radio enthusiast forum i got help with mine from, they are reknowned for being unreliable and blowing(often with quite a bang).

One of mine was visibly deteriorating but i changed both and it worked fine afterwards. I think i used kemet ones possibly plastic film and rated X2 275V on the advice i was given.

I suppose they could be the source of the RCD problem as they are to do with noise suppression.

X caps are fitted across L-N or sometimes after the bridge rectifier. It's unlikely they would trip the RCD if they went leaky. On the other hand, Y caps are fitted from L and N to ground and could cause tripping. Generally when X caps fail, they will make a lot of noise and a filthy rank smell, blow their sides off and most likely blow the fuse. Evox RIFA (now part of Kemet) probably made the majority of RFI caps out there. Probably a bit of a generalisation to say they are crap, as they have made billions of them and I've not heard of any quality concerns. However, most electronic components have lifetimes and wearout mechanisms, so if it's getting on a bit you might have to forgive the odd failure.

As suggested, you could snip them out and the drive should still work fine but I doubt that's the issue here.

These drives generate a DC current in the field and a variable DC current in the armature (I think that's the right way round). It's possible the motor insulation is breaking down after all these years. If it hasn't seen much use, you might find that leaving it to dry out in a hot place (on a radiator, on a hot water cylinder, inside an old boiler) will help. If you have access to a Megger insulation tester, you could test the insulation between the windings and the frame.

As you have no test equipmet the tests that you can do are very limited. The first thing you can do is to test the controller without the motor connected as John Rudd has already suggested. I would suggest a 60 or 100 watt bulb be connected in place of the armature and a 15 or 25 watt bulb connected in place of the field winding. The bulbs should be the old fashoned filiment bulbs. (NOT halogen bulbs NOT LED bulbs NOT CFL bulbs.) (I would do this myself as a first step even though I have all the reqired test equipment.) I do not think it is a PWM controller as the board looks very simple. I think it will use phase angle control with an SCR or triac. I do not beleive in just replacing components without evidence that the component could be faulty. I could not find any schematics for the controller on the web so it would help a lot if you could trace out the circuit of the board. As it only uses discreet through hole components it should not be difficult to trace out.

Les

Actually it's the other way round, they feed the field with a constant current and in effect apply a variable voltage to the rotor. If you increase the field current the motor runs slower.

There are reasonably good complete replacement drives available on eBay for less than 30 squids that people have found good, look for HX - PWM in the type number.

I wouldn't mind having a way of testing if old motor windings insulation is breaking down... but I'm cheap.

If I put 'insulation tester' into ebay and click 'buy it now' on generic chinese tester for less than £30 delivered, would that do the job? e.g. this

Well it looks the part, the specs seem ok.....

But I'm not in the habit of spending other peoples money....

Dont you have a local motor rewinder in your area? How about a friendly electrician? These are the kind of people likely to have a megger.....

I'd be looking at all other possibilites that could cause the problems you are having before spending good money...( tight yorkshireman...😛 )

Whats the hp of your motor??

Heres the faulty capacitor that i removed. It didnt blow or explode with any smoke. Ive heard they are self healing but my controller worked when i replaced them.

Edited By mark smith 20 on 31/10/2016 12:39:35

Edited By mark smith 20 on 31/10/2016 12:41:36

My keyboard has been illuminated by the glow from the bottom of my cowells for 20 mins now... so I guess the controller is not the cause. This is a shame as I think it would have been the cheaper fix.

So the 200v DC shunt motor leaks to earth when it gets warm. New ones are expensive.

It could be a build up of carbon dust around the brush holders. It is worth dismantling the motor to find out.
I didn't understand at first about the illuminated keyboard. If you gave some idea of your location there may be someone close enough to you to check the insulation on your motor.

Les.

Location is 5 miles north of J6 M25.

I've taken a very similar motor apart before (AC though)... but this one I got stuck just trying to get the pulley off. Mind you, I was being very careful - now the motor is suspect I may be a bit more brave with a hammer.

Warm the pulley with a hot air gun often helps....rather than brute force.....or resort to using a puller if you need to or both in combination....

Is the pulley secured with a grub screw? If so, it's definitely worthwhile completely removing the screw and checking for a second one underneath it.

David