please help, alternator with meccalte sr7 not working

[simplesimon]

hope someone may be able to advise. We are trying to set up a community hydro electric installation. the turbine will provide 28kW and seems all good.

the alterator is connected by belt drive, it has mecc alte sr7 avr - not sure if it is meccalte made or not. It has auxilliary and the AVR seems all properly connected. It has not been used before but is prob 10yrs old. Its not giving any output at all. I'm quite sure we're spinning it at the right speed, maybe 2% over at most.


First tests showed no voltage at output, aux to avr and 0.3vac at avr to excitor connections. We were able to measure freq at the output terminals although no voltage registering. The frequency jumped about - sometimes around 50hz sometimes 75hz and occasionally up to 90. We then ran it for a couple of weeks including 2-3days with gentle gas heater on it hoping to dry any moisture.

I read up and thought it must have lost residual magnetism needed to start it up. Coming back after the drying attempt we attemptied to flash it(?). We have applied 12vdc to the excitor stator with the alternator stationary for up to 2 minutes in attempt to put some residual magnetism in - we saw a 1.2A current which fits with measured resistance ~9ohms - assuming there is no extra resistance associated with induction? When we subsequently spun it the only effect was only to increase the small ac voltage measured at the excitor feed connections from AVR, 0.3V before 1.3 after.

We also tried connecting the 12vdc whilst it was spinning, (in attempt to excite it) for 2-3 seconds with no result. Measuring for output voltage on the aux connections to avr gives 0.07vac.

We could not measure any frequency at the output since the drying attempt. We did not see any slowing of the turbine as a result of electrical load.

Anybody got any ideas? Did we do the right thing with the battery? does it now point to the rectifier? - my suspicion but guess we'd need to pull rotor out to find and test it
Any help gratefully appreciated
thank you

Simon

[merryengineer]

OK here we go....

Lets start with the frequency. If you have a frequency that isn't 50Hz off load then:
1) The alternator isn't spinning at the correct speed (check you don't have a 4 pole, which should spin a 1500RPM)
2) You have test equipment that is buggered.


Is it 3 Phase or single phase? if single, you can reconnect to 3 phase and run the machine again. Do the phases give you a BALANCED (not more than 2v apart at no load) voltage off load. If so then the main stator should be OK. If not then the main stator has a problem and you will need to remove the machine for repair.

Check you have 8-15 Ohms on the yellow and blue leads going from the AVR to the excitor assembly. If you don't have this, then you have an excitor fault and you will need to replace it.

Remove the two red leads from the AVR, check that you have 0.3-4 ohms. If not then the MAUX winding has failed and you can reconnect it, by powering it from the sensing, but you will lose your short circuit capability of the machine. To do this on the SR7 2G you link 3A and 4.

You can also check and seperately excite it by 12V+ to yellow and 12V- to blue and run it, if you get the correct voltage here. If all the aboev check out OK you need a new AVR, a new SR7/2G can be bought here:

http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=170754959446

If you don't have any frequency or voltage output now, then you have a main stator failure and you will need a new machine. 0 volts = open circuit main stator.

What model of Mecc Alte alternator is it?

[merryengineer]

Sorry, just to say you might not "need a new machine" as you could replace the main stator, but as the main stator on most small Mecc Alte alternators is part of the main frame, you may find a new machine a better option.

Also worth noting if you go below 45hz on a 4 pole machine, then the alternator will cut the excitation (under-speed protection) and you will lose your voltage.

[simplesimon]

Many thanks merryengineer, very grateful for your help.

To answer a couple of points, it is a 3 phase alternator and I'm pretty sure the rotation speed is good.

The exciter stator (yellow-blue) measures 9ohms so looks good.

The aux resistance I can't remember the resistance of (away from home over xmas - back after new year) though I think it was something sensible - will check.

Using the sensing (green & black from main output) to power the AVR I understand and am tempted to try. However since we're seeing no voltage at the main ouput terminals I don't think this would do anything. Also our attempt with car battery on yellow-blue excitor did nothing.

So with no volt or freq measurable at main output connections, maybe this points to a main stator problem as you suggest. We have tested the continuity and isolation of 3 windings but only with 9v meter, not megameter. Not sure what type of fault you're imagining. Would it show up with the testing we've done or with a mega meter test?

Also can I just check that it is correct to run all these tests with no load connected ?(nor earth - no connections made to alternator at all)- just in case.

When i get home in a couple of days I'll check aux resistance. Double check rotation speed in case under-speed protection is disabling whole thing. Also get model/serial numbers though I suspect it is some kind of unbranded alternator built with meccalte avr.

Many thanks again.
Simon

[wellandpower]

Hi Simon, How did you get on with retesting the machine?

I would mega the windings, just do 250V to start with as more than this you could damage them further. Merry engineer is correct as in normally with everything disconnected, you would expect to get some kind of residual voltage around 50-70V from the stator.

[simplesimon]

Thanks Charlie & Merry,
I've not got any more testing done yet, sounds like I should borrow mega meter to do it. Have I understood you correctly that there should be 50-70V coming off the main stator to output terminals even without any power going to the exciter. So given that we have 0V at output terminals when spinning and providing speed is correct:

1- There could be problem with main stator. Given that I measure equal resistance through disconnected windings and no continuity to earth, the insulation could be damaged that would show up with mega test.

2- Alternatively could it be to do with a loss of residual magnetism (in the core of the main rotor?) as i had previously come to think?

Very gratefully
simon

[simplesimon]

Thank you both very much for your help. Mega test was not good throughout, so we've sent it off to be baked by local rewind co. who have oven. hopefully that will do it.
Simon

[wellandpower]

Simon,
OK let us know how you get on. If you need any replacement parts for it in the end, give spares a call and they will assist. sales@wellandpower.net

[simplesimon]

Thank you Charlie,
We found the problem... somebody had cut one of the cables on the rotor connecting the excitor to the main rotor...bewildering but it seems to be going now.

Hopefully it won't be for a while, but I will get in touch when we need spares.

Thankyou both for your help, greatly appreciated.
best wishes
simon