Onan NH Problems Revisited

[DavidInWilmNC]

Some of you helped me try and get my old Onan NH 6.5 genset running at Timmonsville.  NCBob suggested new points, condenser, and plug wires as a place to start.  I replaced those and along with the battery cables.  Still, it turned over very slowly and wouldn't start.  I disconnected the output wires M1-M4 in the control panel just in case there was a short that was loading down the generator.  Amazingly enough, it started fine.   I rechecked my AC connections and found a couple of mistakes.  I corrected those and it wouldn't start again.  As it turns out, if I disconnect M2 or M4, it starts and generates fine (but at half capacity).  Basically, if both M2 and M4 are connected, it's loaded down and won't start.  If either are connected singly, it starts. 

I bought this used back in March.  It worked fine then.  Since, I've replaced the output leads and then must have screwed up those connections then.  Now that it's connected properly, I don't understand why it won't run.  If anybody's got any input or suggestions, I'd love to hear them.  I'm ready to move on from this generator and tackle some other projects!  Thanks, as always.

David

[DrivingMissLazy]

David, are the output leads numbered, and what are the numbers, and how are they connected together?
Richard

[pete81eaglefanasty]

     Hi Dave
 Have you checked the brushes to see if they are to short. if so, replace them and use a seating stone to seat them. also see if your commentator is clean.
 I replaced the points with electronic ignition in my apollo and had great success, it would start pretty fast.
 
     Pete
 Fantasy

[DrivingMissLazy]

Pete, based on his description, the unit is outputting voltage when he disconnects some of the leads, so I do not think it would be brush problems. Rather a misconnection problem.
Richard

     Hi Dave
 Have you checked the brushes to see if they are to short. if so, replace them and use a seating stone to seat them. also see if your commentator is clean.
 I replaced the points with electronic ignition in my apollo and had great success, it would start pretty fast.
 
     Pete
 Fantasy

[DavidInWilmNC]

I have leads M1 M2 M3 and M4.  For 120 Volts, I connect M1 & M3.  I've connected these together after the built-in circuit breakers.  M2 is shown to be internally grounded (I can verify this by the ground lead inside the generator) and connects with M4 to form the common of the single 120 volt leg.  For 240 volts, one connects M2 & M3 for the common and uses M1 & M4 as the two hot legs.  I haven't tried this yet, as I'd rather have 120 volts at 50 amps.  I might try this, though, just as a test.  I'd think that something was internally shorted, but the fact that I can get 120 volts with either M2 or M4 makes me think that this is not the case.  Help!

David

[DrivingMissLazy]

M1 and M3 are connected together to form one leg of the 120 volt circuit. M2 and M4 are connected together to form the other leg.  Therefore you should have 120 volts between the M1/M3 connection and the M2/M4 connection. The M2/M4 leg would be the neutral/ground leg of the circuit.
Richard

[DavidInWilmNC]

M1 and M3 are connected together to form one leg of the 120 volt circuit. M2 and M4 are connected together to form the other leg.  Therefore you should have 120 volts between the M1/M3 connection and the M2/M4 connection. The M2/M4 leg would be the neutral/ground leg of the circuit.
Richard

Richard, that is correct.  When M2 AND M4 are connected together, the generator is shorted and will barely spin when the start button is pressed.  If M2 OR M4 are connected alone, the genset will start normally and put out 125 +/- volts, but I assume with half capacity.

David

[RTS/Daytona]

Hi

I believe your connecting the 2 outputs "out of phase"

for 220 the windings are in series m2-m3 together  and m1 m4 are 220 output

but because of the phases of the windings to get double amps and 120 volts

I believe they have to be in true "phased Parallel" - m1 to m4  and m2 to m3   (to invert the phase)

same as my Chinese Honda 3000KW

see attached

[DrivingMissLazy]

The standard NEMA alternator connections are as I described them.
I recall someone in the past posting about the Onan being an unconventional connection with one 30 amp circuit and one 20 amp circuit but they could not be combined for one 50 amp circuit.
As I recall M1 and M3 was the 30 amp circuit and M2 and M4 was the 20 amp circuit. These outputs went to two different circuit breakers and were not tied together.
Richard

[DavidInWilmNC]

This is an older 80's Onan that has two equal outputs.  It's not one of the 20-30 amp models.  The connections I mention (M1 to M3, M2 to M4) are from the wiring diagram inside the cover.  The wires from the windings have numbers printed on them to keep from screwing them up.  This generator worked fine in June when I last ran it, so I'm at a loss as to what the problem could be.  I'll try connecting it for 120/240 tomorrow and see how that works.  Thanks.

[DrivingMissLazy]

Dave, you do have the output circuit breakers off when you are testing?

Without the leads connected together, as we have discussed, I believe there should be 120 volts between M1 and M2 and 120 volts between M3 and M4. Then when you connect for 240 volt output you should still have 120 volts between the points just mentioned as well as 240 volts between M1 and M4.
Richard

This is an older 80's Onan that has two equal outputs.  It's not one of the 20-30 amp models.  The connections I mention (M1 to M3, M2 to M4) are from the wiring diagram inside the cover.  The wires from the windings have numbers printed on them to keep from screwing them up.  This generator worked fine in June when I last ran it, so I'm at a loss as to what the problem could be.  I'll try connecting it for 120/240 tomorrow and see how that works.  Thanks.

[Ncbob]

David,

We looked at your connections and the diagram in T'ville and since you've added more info I'd like to suggest this:

You need to verify that M2 and M4 are generator ground....they probably are.  With the genererator not running you need to check M1 and M3 to ground...they should be isolated and above ground.  Since all these leads go to the brush holders for the slip rings it's entirely possible that some insulation for the M1 or M3 leads might have chafed through and that is grounding one of the slip rings and that would appear as a load to the generator end. And it is within the realm of possibility that a slip ring ( which shouldn't be) is also has ground potential.

The only way I could know for certain is to slide the generator out and remove the drawband protector, which was off when I looked at the unit.  Recall, if you will, that somone had substitued 3 smaller guage wires from the brush holder to the M terminal in the controller.  I assume you've corrected this...but there's a good possibility that since someone (who didn''t know what they were doing) has also mucked around in there with wire of lesser insulation qualities than original.

With a meter you should be able to determine is one of the so called + leads is leaking or grounded...that would account for the slow exciter cranking...somewhere there's a load on the generator while it's attempting to start...you proved that when  you disconnected all the M leads and isolated them.

You're doing fine guy...now we're just into the nitty gritty...and that's what's going to make you a top-notch Onan tech.

Best of luck and if I can help...you know the button to push.....

NCbob

[DavidInWilmNC]

Hi Bob, 
It's funny you mention what you did, as this is pretty much what I just checked (I think)!  I also posted at an Onan forum and here's part of my reply to a suggestion there:

Knowing that M2 is supposed to be grounded, I checked all terminals (M1 - M4) referenced to ground with the meter set to Ω. Here are the readings:
 M1 to ground = .1Ω
 M2 to ground = 3.5Ω
 M3 to ground = infinite Ω
 M4 to ground = infinite Ω

I would expect M3 and M4 to read as they do, but it appears that M1 and M2 are reversed. The lables on the wires show that they are connected properly, but evidently they are connected incorrectly at the brushes. Also, I do get 125 volts between M1 & M2 and M3 & M4. This is with nothing connected to the terminal block except for the wires coming from the brushes, ie. not load at all.

Another thing I noticed and I'm not sure if this is correct or not, is how the brushes are arranged. There are 4 slip rings for the brushes. If the engine ring is 1 and the generator fan is 4, here's how they are arranged:
Top - 1 & 3
Front - 2 & 4
Bottom - 2 & 4
Rear - 1 & 3

If the brushes are oriented correctly to the rings, I'm thinking that if I reverse M1 and M2, all should be well.  Now that I think back, when I got the generator there were four wires coming out of the box - two white and two black leading to the original breaker panel.  I needed longer wires and wanted them protected, so I added new ones in armored flex conduit.  I remember having a hard time figuring out how the thing was wired 'cause it didn't seem to follow the diagram.  It would make sense that whoever last worked on this ran into the same problem I did, switched the wires on the terminal block, and didn't bother to relable them. 

Besides this, after the little tune up you suggested, the things runs great.  I had to adjust the choke a bit, as it wasn't opening completely when the engine was hot.  I tried the genset under load yesterday (on one leg only) with a 15k btu A/C and an 1100 watt microwave and assorted lights.  There was a noticeable change in sound when I turned on the microwave, but the voltage didn't drop much at all.  Thanks for all the help!

David

[Ncbob]

The only thing that I might add, David (and I'm going from memory only) is the the slip rings are numbered 1, 2, 3, and 4 from the back to the front (back being the baloney slicer fan, end).

You should really check the E place for a Service Manual for the NH, Spec "P".  I must have sold every manual I had there some years ago...but they're out there.

Again, best...

Bob

[Stan]

David: I have the wiring dagram for the NH if it is any help. Send me an email to stan at shaw dot ca

I wiill scan it and send as a jpeg if you want it.