Brushless alternator fault

[AnotherTom]

Hi folks,

Not been on here for a few years. see some familiar names though.

Can anyone help me troubleshoot a 3.5 KVa capacitor regulated brushless alternator? It produces almost no voltage on the outputs (about 4 Vac open circuit at stator winding)

So far I have:

1: checked the excitation capacitor. It shows 20 Vac when running. When disconnected and charged to 12 V it then holds that charge for several minutes.
2: removed and checked the rotor diodes. They test OK (450 mV forward, no result reverse)
3: checked the winding resistances. They are reasonably close to nominal, given my cheap multimeter, except that excitation seems a bit low. (stators 1.1 ohm each, 1.3 nominal; rotor 3.6 ohm each, 2.6 nominal; exciter 4.8 ohm, 7.9 nominal)

I can't think of an easy way of testing the varistors, but they can't be short closed or I wouldn't get a resistance reading on the rotor windings.

The only thing I can think to try is to "flash" the exciter, but I don't think it's that or I wouldn't get any excitation voltage, plus output faded away when running rather than refusing to excite after storage.

can I find a better way to test the cap to a higher voltage? am I at the point of replacing the cap and hope it helps?

anyone any ideas?

The alternator is a mecc alte s16w90, manual here: http://www.meccalte.com/downloads/man_s16w_rev04.pdf It's a 3000 rpm single phase machine with two stator windings that can be connected series (for 230V) or parallel (115v)

I only have a cheap multimeter at the moment so can’t measure capacitance or AC current, but I might try and borrow a better one

I'm looking at it for a charity and would prefer not to have to tell them to buy an new generator, even if this takes up a bit of my time.

Tom

 

[SimonH2]

One way of testing the cap is to wire it in series with a mains lamp and see if the lamp works. The manual does list faulty cap as a cause for lack of excitation.

 

[AnotherTom]

Thanks simon!

after searching through my supply of modern LED and fluorescent bulbs, I found a GU50. Inefficient old halogen spot lights to the rescue

DSC_0021

• AnotherTom
• 10 Dec 2017

lamp lights through capacitor

I think it's dimmer than usual, but still putting out plenty of light, so I think the capacitor is probably OK.

anyone any other ideas?

 

[ericmark]

Interesting, I have found generators without any AVR but these are always brushed, I look at the circuit diagram on page 13 and I would have expected to see an AVR feeding - and + terminals. The down load area lists a few regulator, but as to which one is fitted to your generator not a clue.

What is connected to the - + terminals

 

[AnotherTom]

Hi Ericmark,

I don't fully understand the voltage regulation but I think it is achieved by some combination of a) driving the iron cores to the edge of saturation, b) changes in the current and/or power factor of the excitation winding and c) possibly the varistors have a role in limiting rotor field strength. It's a bit rough and ready and I expect voltage does droop a bit under load. There isn't an AVR.

If you are looking at the dotted box at the top of the wiring diagrams, that's an optional 12 V DC winding and rectifier that's not fitted to my alternator. It's used to charge a starter battery, for electric start applications, but I have a pull-start motor.

I think voltage output is also going to depend on shaft speed, and I'm arranging to borrow a better multimeter with frequency function so that I can confirm it's spinning fast enough. It will also have AC current and capacitance measurement so I can do those if it's useful.

Tom

 

[SimonH2]

I don't fully understand the voltage regulation but I think it is achieved by some combination of a) driving the iron cores to the edge of saturation

I believe that is the primary method.

I found this article which explains how the machine works - but it doesn't mention what the cap is for
Then this video shows how to test it.

Ah, I found a description of how the capacitor/winding combination works - though I don't find it very clear.

 

[AdrianUK]

Meccalte have a UK office/workshop in Oakham. They do have a technical guy there who is very helpful & knowledgeable, its not just a Sales office. Suggest you give them a call.

Voltage output will depend on shaft speed, but also, more importantly, the frequency of the output will depend upon the shaft speed. Do you have a stroboscope handy?

 

[AnotherTom]

I don't have a stroboscope, but a friend is going to lend me a fluke on Wednesday that has a frequency mode, so I should be able get shaft speed from the exciter frequency. I'm hoping it will give me a better measure of the winding's resistance and the capacitor's capacitance too.

Thus armed, I will probably give Mecc Alte a ring and see what they have to say. knowing where to find a knowledgeable tech is a great life skill

Simon, thanks for the links, I'll take a look. The last one has a good amount of technical detail, though still not enough for me to fully understand what happens as load goes up and down etc.

Tom

 

[AnotherTom]

update:

when running cold, it's at about 49.5 Hz, so speed is OK

The capacitor only reads 11 uF when it should be 14, and the windings are all a bit off from spec.

I called mecc alte. the technical guy seems nice enough, but not very hopeful. He said a tolerance of 20% was fairly normal on winding resistance, no comment on the fact that mine are further off than that. He said the exciter should run at about 300V, so mine at 10 isn't right. He said they don't normally supply spare windings for this sort of small alternator. Basically, troubleshooting consists of try a new capacitor, and then if no joy, bin the lot.

I'm waiting on them for a price for a cap and a whole alternator.

 

[WhydidIstart]

Unloaded engine speed should be about 3,160 rpm = 52Hz for most generators like these, but 49.5 is nowhere near low enough to cause such a low output.


I would change the cap, did you also check the cap for DC resistance? You should get in the mega ohm range (once the reading has stabilised due to it charging up to the meter's test voltage). Anything in the kilo ohm range and the cap is dead.


Also you generally don't need their specific capacitor, you need to match the capacitance, have a voltage rating that is the same or exceeds your existing one and it needs to physically fit, plus must be suitable as a 'run' capacitor and not a 'start' one. Generally something like this would be suitable www.ebay.co.uk/itm/Universal-14-UF-Microfarad-Motor-Run-Start-Capacitor/302014004895 which is £4.99 free delivery, sent from the UK.

Once you have your new cap fitted and if it still doesn't work, try the flash that you mentioned in your original post. I find a 12V car battery works well for that.

 

[AnotherTom]

Final update:

I did get a cheapo replacement Cap, it got lost in the Christmas post. Once found and fitted I had ~110v on the stator windings as I should. It then sat about for a while until I had a chance to load test it and adjust the speed.

I had that chance today. Sped it up slightly to get 251V at 51Hz unloaded, 234V and 48.8Hz at ~2.5KW, seems good enough to me.

Obviously the capacitor was bad, even though it measured a reasonable capacitance, held a charge, passed the bulb test and showed open circuit when tested with an ohm-meter. My suspicion is that it had dielectric breakdown but that this wasn't evident at the low test voltage of my ohm meter, or the 12v charge I applied from a car battery. As for the bulb test, a shorting capacitor would pass that too. If I remember I might pass the old cap to a mate with a megger to see how it likes that.

The machine will be off to the highlands shortly, where it will be back in use providing power for refurbishing a mountaineer's refuge

Thanks to all who contributed

Tom