Brushless alternator capacitor size - which way's up?

pmrobert

I own and operate a generator/small engine service and repair business. I have a situation where a customer (potentially a very large volume customer) has brought me a 6KW CH6000LN diesel generator where the complaint is "low voltage" - which is indeed true upon evaluation. It is outputting 86/174 volts AC as opposed to 120/240 as is nominal. Usually it's the excitation capacitor so that was my first target. I found a fairly new 35 uF cap installed, windings all ohm out where they should, megger indicates no insulation/short issues. The machine only has 120 hours (hard hours, they're a concrete contracting firm that uses these to run chipping hammers. core drills, etc. in south Florida US - not very cool usually). I notice a few loose bolts, when examining things, call the customer and inquire as to whether anyone has attempted a repair prior to current day. He then informs me that another shop had declared the OEM cap defective and replaced it. The machine then showed even less voltage and they didn't have an explanation, hence why I have it. Cut to the chase: I'm unable to find the OEM specs for said cap, I think 35 uF is incorrect - I might have to do this empirically. Does anyone have any idea on which way I need to go? More? Less? These caps come in a multitude of values. I'd prefer not to purchase a bunch of caps that I'll likely never use. Any input is appreciated.

Mike R...

PS: Also have a 2 x Prop 1 prototype engine management system for steady load IC engines working well on test stand. Waiting for P2 for future development, really would appreciate those smart pins...

evanh

Get the customer to help out by allowing you to inspect a fully functioning generator.

pmrobert

evanh wrote: »

Get the customer to help out by allowing you to inspect a fully functioning generator.

That is an option, probably the correct one though it will cost him at least $200 in labor. Reliability of equipment is extremely important to him, the labor cost shouldn't be a problem. Trying numerous values of caps is a fool's errand the more I think about it. Thank you for making my mind up! :-)

Mike R...

lardom

I would look online for specs. You have to know the test points. In addition to 120/240 volts it has to output 60Hz. If the frequency is also out of spec then I would look for a pcb. If the output power works by switched mode then you can only find out if you have a bad IC by looking at the datasheet.

pmrobert

@lardom, it does output 60Hz, engine speed is correct. I'm in the US, though I also service machines that are 50 Hz from relatively local UK and NL associated islands; I am acutely aware of the output frequency. I've scoured the net looking for specific info on this machine to no avail. There is no AVR or logic board with this particular generator - it's totally analog voltage regulation. It uses "Capacitor Excitation" as in machineryequipmentonline.com/electrical-power-generation/generatorsbrushless-excitation/ I'm certain there's a better description of the VR scheme, my apologies. The customer does not have any more of that particular generator so I'm back to square one, the empirical attempt. Most of these types of machines typically utilize a 20uF cap so at this point I'll start with that. The caps are relatively inexpensive (~$10-15) so no big deal doing the trial component attempt. I'm fairly sure I can't burn a winding, etc., if there's no load on the machine. I do appreciate your input and any opinions or suggestions are well taken.

Mike R...

lardom

@pmrobert, please don't see my post as anything more than an attempt to try to find a solution. It's tough to at times to communicate with text. Actually I would have no problem telling a customer that I think the cap should be upgraded.
You're not obligated to re engineer the machine if replacing the cap gets it working again.

pmrobert

@lardom, I most certainly don't see your post as anything more than assistance! I apologize if I presented as anything other than that. The customer is perfectly OK with whatever I do. Going forward I'm going to obtain a 20 uF cap and see where that puts the machine. Updates to follow!

Mike R...

LA6WNA

Hi. This reminds me about a similar problem I had with a Ruggerini 6kw diesel generator. It had no AVR, just capacitor exitation. I also tried changing the cap, but still the voltage was to low at the right RPM. Should be 230volts @ 50 Hz. At last, I found that one of the two rectifiers on the rotor was defective. Ended up changing both rectifiers, and then the generator worked perfect again. Think it was schottky rectifiers with reverse voltage at 1000 volts and maybe 5 or 8 amps forward current. Do not remember exactly. I dont know if it helps you in this case, but I felt I could share my experience with this exitation system.