How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Hank F. Your Own Question
Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 14620
Experience:  Certified on Onan and Generac generators
19372534
Type Your Small Engine Question Here...
Hank F. is online now
A new question is answered every 9 seconds

Onan generator mobile genset problem

Customer Question

onan generator mobile genset problem
Submitted: 1 year ago.
Category: Small Engine
Customer: replied 1 year ago.
I have performed all brush checks/stator/rotor/ everything ohms out correctly. I think my unit has an all in one computer module but not sure if there are diodes or something somewhere else. my computer is(###) ###-####and I don't see an external regulator or anything. I am getting 12 v to to the brushes. motor runs fine. Just don't get but 20V ac out
Expert:  Hank F. replied 1 year ago.
What is the model number of the genset?Where did you get the specs for the resistances?
Customer: replied 1 year ago.
I have the service manual for my model hgjab.If I have 12 v dc going into the brushes then 120 should come out the stator windings correct? I am getting 60hz at low voltage so I assume my rpm is correct.
Customer: replied 1 year ago.
Can the rotor magnet become de magnetized?
Customer: replied 1 year ago.
I am actually getting 20v out of t1-t2 and 3 v out of t3-t4 with the connections open from their grounds or spade connections which is also how I ohmed out the rotor/stator
Expert:  Hank F. replied 1 year ago.
1. There are 2 different service manuals for the HGJAB. Are you sure you have the correct one? 2. If you only apply 12V to the brushes, the AC output will not be anywhere near 120V - it will be in the 30VAC - 50VAC range. 3. If you are getting 60 Hz, yes, the rpm is correct. 4. The rotor does not retain residual magnetism. This is an externally excited unit, not a self excited unit. 5. Testing for voltage is not an ohm test - it is a voltage test. Resistance testing is done with a meter set to the appropriate ohm scale, with the circuit completely de-energized. This means having the unit not running..
Customer: replied 1 year ago.
OK, just to give you background - I have an electronics engineering degree and currently repair MRI machines. Now is my rotor/stator not the problem and my computer is? I have all the correct resistance on all stator / rotor/ quad outputs and brushes are good. So far you havn't told me anything other than magnetism is not my problem. So that statement alone tells me my computer should be putting out more than 12v dc to the brushes even though I havn't found any documentation on that anywhere. and if you read all my posts you would of seen that I am getting 20v ac on 1 transformer winding and 3 v on the other. So if you could explain further that would be great
Expert:  Hank F. replied 1 year ago.
There is no transformer on an HGJAB. I believe what you are probably referring to are the power windings in the stator.Both power windings should be putting out the same voltage. Since you have 20V on one, and only 3V on the other, you have a problem in the stator.But the controller should be putting out more than 12V. With no load on the genset, the DC output to the brushes should be somewhere between 24 and 30 VDC. With a full load on the genset, it can go as high as 80 - 100 VDC.If you are only getting 12VDC, it sounds like your controller is also bad.There is not direct test for the controller. It must be connected to a special tool from Onan called a Break Out tool to test it..
Customer: replied 1 year ago.
OK, and yes you are correct the stator is what I was referring to. I have no load on generator as its disassembled on bench. I found an old post from 2 years ago with someone with a similar problem but I don't think he was getting 12v or any volts to the gen. So stator really? I am def only getting 3v on 1 circuit and 20 on the other. but they ohm out correctly. can a stator be returned? boy thats a lot of work. lol.
Customer: replied 1 year ago.
by having no load would 12 v not be correct?
Expert:  Hank F. replied 1 year ago.
I'm not sure what you mean by 'can a stator be returned'. These units are very difficult to work on without a Break Out tool. It actually bypasses the controller, so the engine can be kept running, and all circuits tested separately. Without it, you can not keep the engine running to do any testing. Even with no load on the genset, the controller should be putting out much more than 12V. As I stated above, to get 120V output, there should be between 24 and 30VDC to the brushes.One test that can be done is to connect 12V directly to the brushes, observing proper polarity, and measuring the AC output.Then apply 24VDC directly to the brushes, and measuring AC output.Then the readings are compared. 12VDC should give around 30 - 50VAC output, while 24 VDC should give a minimum of 100VAC output. Both power windings should read the same. If there is that much variation between them on either test, there is a problem in the stator..
Customer: replied 1 year ago.
OK, I can do that. I have plenty of batteries to make 24 V or I'll bring my bench power supply home from work. Is there any way to put the generator into ERO mode? which I read is engine run only mode so computer won't shut it down?
Customer: replied 1 year ago.
also can you explain to me what the quad stator outputs going into the computer do?
Customer: replied 1 year ago.
Thanks for help
Expert:  Hank F. replied 1 year ago.
Some HGJ's don't have ERO.The quad winding has several functions. It is where the controller reads voltage and frequency inputs from, and it is what powers the run relay on the controller to keep the run relay active.
Customer: replied 1 year ago.
hmm ok well thats why if I disconnect those the gen won't run at all. kills motor as soon as I let off start. Ok, I will do 12V/24V test tomorrow. What is the max voltage that I should try? Being that I have a power supply that will go up pretty high and at around 10 amps.
Expert:  Hank F. replied 1 year ago.
Don't go over 3 amps - this is only a 3A rotor.I've never gone over 24V, as 24 is all that is required to get the output to 100V..
Customer: replied 1 year ago.
OK,thanks!!
Customer: replied 1 year ago.
OK, just measured output with 24V going into brushes with proper polarity. Both outputs after separating are 40 V out or actually 39 V ac 60hz output. they are equal. Sooooooo does that mean my computer is bad not outputing enough dc volts to brushes and my stator is bad?
Customer: replied 1 year ago.
And i put ammeter in line and I get .3 amps with no load on output and 40V AC out on both outputs.
Customer: replied 1 year ago.
.3 amps DC for the brush voltage. If that wasn't clear.
Expert:  Hank F. replied 1 year ago.
By the book, with 12V applied to the brushes, you should have an AC output of 39VAC, +/- 10%.
With 24VDC applied to the brushes, you should have an AC output of 78VAC, +/- 10%

If you are only getting 40VAC with 24VDC excitement, you have a problem in the brushes, slip rings, field winding or stator.

At this point I would meg the field and stator. Spec is 600V for 1 second, with a minimum of 1M-ohm.
.
Customer: replied 1 year ago.
i dont have a megger and im sure that would help as my multi meter wont read like that. . any other tricks for narrowing down rotor or stator?
Expert:  Hank F. replied 1 year ago.
There are only 2 tests for a rotor or stator - the resistance test, and the meg.
For the cost, I don't have a megger, either. The few times I run into needing it done (it is extremely rare), I run the rotor and stator to the electric shop down the road and pay them $30 to meg for me.

The only other thing you can do is to pull the stator and visually inspect it. Sometimes you can see damage, sometimes you can't.
.
Customer: replied 1 year ago.
OK thanks. Do I have to separate the upper area from the lower area to pull stator just like getting to the drive belt?
Expert:  Hank F. replied 1 year ago.
No.
The stator is completely accessible/removable from the top of the unit.
.
Customer: replied 1 year ago.
OH thank GOD
Customer: replied 1 year ago.
Does one or the other normally have the problem before the other? meaning stators usually or rotors usually
Expert:  Hank F. replied 1 year ago.
Not really. Sometimes its a rotor, sometimes its a stator.
.
Customer: replied 1 year ago.
thanks
Expert:  Hank F. replied 1 year ago.
Ok
.