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Old 05-07-2012, 03:36 PM   #1
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Onan MDKD Generator low voltage

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.
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Old 05-07-2012, 06:08 PM   #2
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Register (free) and post on SmokStak.com. There's some really sharp Onan guys over there.
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Old 05-07-2012, 09:13 PM   #3
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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.

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Old 05-07-2012, 09:34 PM   #4
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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.
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Old 05-07-2012, 11:00 PM   #5
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Quote:
Originally Posted by Baypoint View Post
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.
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
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Old 05-08-2012, 12:33 AM   #6
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Is the regulator capable of voltage adjustment by moving tx & s1 on the x taps?
Do you have the service manual?
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Old 05-08-2012, 10:12 AM   #7
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The transformer is new. It was just replaced. No change
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Old 05-08-2012, 10:13 AM   #8
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It is at max voltage using the taps
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Old 05-08-2012, 10:44 AM   #9
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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.
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Old 05-08-2012, 11:08 AM   #10
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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.
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Old 05-08-2012, 11:08 AM   #11
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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?
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Old 05-08-2012, 11:14 AM   #12
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This is a dumb question: What does not work properly because of the low voltage and/or what problems could is cause?
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Old 05-08-2012, 11:18 AM   #13
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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.
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Old 05-08-2012, 11:21 AM   #14
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Quote:
Originally Posted by Baypoint View Post
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.
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.
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Old 05-08-2012, 11:36 AM   #15
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One other thought. When the stator was slid back over the studs was there any chance it was nicked?
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Old 05-08-2012, 11:39 AM   #16
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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.
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Old 05-08-2012, 01:06 PM   #17
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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.
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Old 05-23-2012, 11:35 AM   #18
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Onan MDKD

Just replaced the Exciter Rotor so now all parts have been replaced with no change.
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Old 05-23-2012, 12:41 PM   #19
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Onan

So what rpm is it running at?
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Old 05-23-2012, 01:55 PM   #20
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Onan MDKD

1800 rpm
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