Onan MDKD Generator low voltage

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Baypoint

Baypoint

Veteran Member
Joined
Oct 8, 2010
Messages
75
Location
USA
Vessel Name
Parra-dise
Vessel Make
Gulfstar 44 MY
We purchased a boat with an 8.0 Kw MDKD generator (240V). The generator had a low voltage issue when we purchased the boat. We reduced the price by what the Onan tech gave us as a repair estimate. Six Weeks later and over 100 hrs of troubleshooting and no solution. These are the readings and symptoms. I am hoping someone may have an idea out there in trawler world.

Thanks in advance

History: Problem is low output voltage. It is transformer regulated. Output
voltage is 94VAC, 62Hz with load and about 60VAC, 63Hz unloaded
Genset is configured for 120/240 output @ 60Hz

Unit has a new Broward Armature rewound stator and rotor. Also a new exciter stator and T21 regulation transformer P/N 315-0572.

Current status: Troubleshooting is complete. Fuel transfer pump has been
replaced. Exciter rotor has not been replaced.


Onan model: 8.0mdkd-3cr/1a
Onan spec: A


Fluke 179 digital voltmeter was used for all tests.


The following tests have been done with these results:

[A] AC Residual voltage: 8VAC

[C] CR21 and RV21: Diode check with a Fluke digital meter shows all four
diodes in bridge are good with forward resistance being .5VDC on diode check
and 0 VDC for reverse. RV21 shows infinite resistance in both directions.

[D] Flash the field: Flashed with 9 volt battery saw slight increase in
output voltage, flashed with two 9 volt batteries in series for 18VDC,
voltage increased to 126VAC.

[G] Rotating rectifiers: F1+ to CR1,CR2,CR3 Showed .5VDC forward and 0VDC in
reverse. F2- to CR4, CR5, CR6 showed .5VDC forward and 0VDC reverse.

[H] Exciter stator: 12.5 ohms between F1+ and F2-. over 1Mohm with megger to
ground.

[J] Exciter rotor: T1-T2= .7 OHMS, T2-T3= .7 OHMS, T1-T3= .7 OHMS. Over 1
Mohm to ground on megger test.


[k] Generator stator: T1-T2=.2 OHMS, T3-T4= .2 OHMS. Over 1 Mohm to ground
with megger.

[L] Generator rotor: F1+ - F2- = 3.1 OHMS. Over 1 Mohm to ground with
megger.

[M] Wiring harness: No opens or corroded connections noted.

[N] T21 regulating transformer: H1-H2= .2 OHMS, H3-H4= .2 OHMS, X1-X2=.2
OHMS, X1-X4=1.2 OHMS, X1-X3=1.2 OHMS. Megger shows open between primary and
secondary.
 
Register (free) and post on SmokStak.com. There's some really sharp Onan guys over there.
 
Don't know about the Onan, but we have a Westerbeke with the transformer regulation. There is an airgap in the transformer core which causes the transformer to do the regulating. The size of the airgap determines the voltage. If there was a shock to the transformer this airgap may have changed thereby changing the regulated voltage. One of the troubleshooting hints in the Westerbeke manual is to tap the core with a hammer and see what happens. Adjusting the voltage involves inserting nonconducting shims to control airgap.

Bob
 
You might also post as a Guest on Boatdiesel. There are a couple guys that regularily work on generators and have helped people often.

Give them everything as you have done here.
 
Baypoint said:
Unit has a new Broward Armature rewound stator and rotor. Also a new exciter stator and T21 regulation transformer P/N 315-0572.

Current status: Exciter rotor has not been replaced.

[D] Flash the field: Flashed with 9 volt battery saw slight increase in
output voltage, flashed with two 9 volt batteries in series for 18VDC,
voltage increased to 126VAC.

[H] Exciter stator: 12.5 ohms between F1+ and F2-. over 1Mohm with megger to ground.

[J] Exciter rotor: T1-T2= .7 OHMS, T2-T3= .7 OHMS, T1-T3= .7 OHMS. Over 1 Mohm to ground on megger test.
Click to expand...

Other than troubleshooting it myself, I believe the issue has all to do with your excitation circuit. By increasing your DC exciter voltage you will get your AC output.

Elwin
 
Is the regulator capable of voltage adjustment by moving tx & s1 on the x taps?
Do you have the service manual?
 
The transformer is new. It was just replaced. No change
 
It is at max voltage using the taps
 
I would think the trouble is in your voltage regulation circuit.

The reason is that when you apply a DC current to your excitor winding you get good AC voltage out.

I believe ths isolates your trouble to the transformer, the bridge rectifier, the feed to the transformer, or the feed from the bridge rectifier to the exciter winding.

I've seen similar problems with Onan units and if I were to guess (not being there) I'd swap out the bridge rectifier.
 
When we apply a DC current with 2 x 9 volt batteries we get 126 VAC. This is only half of the voltage. It is a 240V generator. The transformer is brand new. Bridge rectifier was replaced and rechecked yesterday and diodes good. I will check the feeds to the exciter windings. Maybe a bad wire. This one is being a tough one.
 
The numbers are perfect. This is something dumb. When you flash are you reading L1 to L2 or to L0? Do you have a good ground? Are the standoffs good?Remove the bridge rectifier and with jumpers plug the ac side into the wall. What does the dc side read?
 
This is a dumb question: What does not work properly because of the low voltage and/or what problems could is cause?
 
Everyone says the generator runs perfect just wont put out the correct voltage. I will try the rectifier trick. The 4 different mechanics have checked the wiring and no one has found anything. Maybe time to call the witchdoctor.
 
Baypoint said:
When we apply a DC current with 2 x 9 volt batteries we get 126 VAC. This is only half of the voltage. It is a 240V generator.
Click to expand...

Oops missed the 240 volt part.

I wish I still had my books for that one...

I thought there was a spec for X excitation current = Y output voltage.

In order to troubleshoot this, you might need to separate the excitation from the regulation circuits. I do not remember if 2X 9 volt batteries will produce enough excitation current for full output voltage.
 
One other thought. When the stator was slid back over the studs was there any chance it was nicked?
 
anything is possible the windings tested good, I would think it would show up.

Generator stator: T1-T2=.2 OHMS, T3-T4= .2 OHMS. Over 1 Mohm to ground
with megger.
 
Well without being there I'm stumped! I think I would like to see more alot more than 1 meg at 500 volts. If you were a customer this would give me the excuse to buy that Simpson 270! Good Luck. When you find it please let us know.
 
Onan MDKD

Just replaced the Exciter Rotor so now all parts have been replaced with no change.
 
Onan MDKD

1800 rpm
 
60 hz is the us standard 120 or 240 volts. Just like your house
 
Did you plug the bridge rectifier into the wall.
 
This thread makes me chuckle. Hire a good electrician!

Elwin
 
definitely a possibility
 
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