Assess ONAN 4.5 BGDxxx Spec C

One question: When you measured the slip rings, did you measure across the slip rings themselves or through the brushes? I have a friend with an Onan MicroQuiet 4kW (KY series) in his rig and the one of the brushes was completely worn away after only 250 hours of operation.

After replacing the brushes and cleaning up the slip rings with some crocus cloth, his runs like a champ now. He had the symptom that it would start but die immediately after releasing the start button. He had intermittent problems for several months prior to it "dying".

The slip rings had some sort of black, non-conductive build-up on them that was also somewhat abrasive that accelerated the wear. He bought the regulator unit thinking that was the problem, so now he has a ready spare.

BSR...

Tx 4 your query. The Onan fellow removed the brush block & cleaned the sliprings. After I removed the genset & the rotor, I also ran the rotor through the dishwasher & airdried inside for a few days, so that's about as good as it gets. Don't tell the wife on me! (It cleaned up well, but there was only about a half-ohm increase in measured ohms. I did note on the outside wrap of the windings wire, the insulation coating appears to be cracked in quite a few places. Heat? Old age? Vibration?)

On the matter of improving accuracy of stator measurements, tonight I figured out how to do it with my cheap meter. (Its voltage measurements are decently precise.) For each "circuit" test pair, if I add a 15-ohm resistor & use a 9-v battery, from the spec R plus 15-ohm added R, I calc the current (constant around the circuit), then measure the voltage drop across the part of the circuit that is of interest, & from that, calc the resistance. If the calc result is close to the spec'd R value (say 10%?), then the test is good. If not close enough, do another iteration of the same calcs, but substituting calc'd resistance value into the spec'd value. Repeat the calc iterations as required to get to within a reasonable tolerance (10%?). Compare the last iteration result R to the required spec value. Slow, but it will save me the bucks for a precise ohmmeter that I use once only.

No need for precision on the rotor measurements. It is difficult to test for shorted windings by resistance alone because the relative difference in resistance between a good rotor and bad rotor are not that great, not to mention you can have short with degraded "connections" that measure close to the resistance spec. 10% is certainly close enough for most rotor measurements.

A better measurement is an impedance measurement that will give you the inductance of the rotor, but I have yet to see that information associated with equipment at this level.

As for the cracking of the coating on the windings, you can expect to see some "crazing" of the lacquer on older machines and a quick check with an ohmmeter should give you an idea whether it is simply a cosmetic issue or a potential short path if the copper is exposed. The lacquer may be multiple layers so it's possible you are only losing an outer layer.

The rotors are not cheap, (upwards of $200 or so) so I understand your deliberate pace.

BSR,

Tks again. I have accepted the rotor is toast unless a well experienced person offers an innovative idea. The spec is very clear with upper & lower limits for ohms. My rotor is seriously outside the range, so the 'crazing' is likely through the full depth of the winding with many internal shorts.

My perceived need for precision is in the stator, where spec values for resistance (across given pairs of connectors) varies from about 0.058 ohms to 2.1 ohms. A range is not given for these values, so maybe they are not critical, but I'd like to measure them with 'reasonable' accuracy & then make a judgment about whether or not I call Onan about them.

You are most optimistic on costs. Up here, all pieces are very costly, so I want to check as much as I can before proceeding... if nothing else, it gives me a better estimate of my costs before I start spending.

Thx again.

Hey folks,

Surely someone has some thoughts on my queries in this post?... I am hoping to learn a bit & especially, to benefit from your knowledge & experience! (I am a civeng, will address hydraulics problems, esp. pumps, open channels, pipe flow, some soils, plus miscellaneous, but now I need electrical guidance!)

TKU!