ONAN MDKC running 220vac issue

[HeartWarrior]

My generator runs fine with a 110vac load. Recently I added a 220vac Compressor to fill my scuba tanks. When I turn in the generator and run the compressor only the generator start to throw a bunch of black smoke out the exhaust. It’s not oil as it dissipates quickly in the water. When I run the compressor on shore power everything runs fine. Any thoughts on what this could be? Would it have anything to do with the governor or kicking into a higher speed to support the 220vac requirement?

 

[Arc]

Fist question , what is the load from the compressor? Preferably tested with an amp meter

 

[HeartWarrior]

Each leg is pulling 14.2 amps. I verified this while it was running under AC power.

 

[Arc]

Is your Mdkc a 4k ?

 

[HeartWarrior]

Yes it is.

 

[Arc]

Did you read 14 amps from leg to ground or from hot legs?

 

[HeartWarrior]

From the hot legs

 

[Arc]

Ok so you should get about 18 amps @220v if you gen is running nicely. Does the smoke clear up or is it constant?

 

[HeartWarrior]

Right now it is constant, I have only let it run for about 10 minutes on the generator.

 

[Arc]

Here are some possibilities
Air filter,
Low fuel pressure
Governor adjustment [ check hertz under load ]
Armature shorting. [ check for voltage drop ]

You could try bringing on load slowly up to 35 amps and see if it happens

 

[tiltrider1]

Assuming you have the 4K generator. It appears that the diesel engine is not getting enough air. The black smoke is essentially unburnt fuel do to a lack of air. Usually as the engine gains RPM’s it gets enough air and the smoke goes away. With a generator you run at a constant RPM.

So this leads uS to one of two conclusions. obstructive air flow, like a dirty air filter or there is more load on the generator than the generator is rated for.

When running the air compressor on the generator, are you sure there are no other loads. Is it possible that some house loads in conjunction with the air compressor are maxing out the generator?

 

[diver dave]

Each leg is pulling 14.2 amps. I verified this while it was running under AC power.

I am very surprised a 4kVA gen will start a 240/14A compressor. My 3HP air compressor is barely started with a 5.5 kVA genset with a softstart.
Something suspicious here. Is the compressor running at rated speed?

 

[C lectric]

Yes, check/change the air filter. If it cannot get air easily then the engine cannot produce power and it needs all it can get in this case.
I can think of some other problems although I am not a generator mechanic.

Motors often draw 3 to 5 times the running current to get started. Check the
start up current both on the generator AND the shore power and compare the two.
Check the running current AND the voltage on both also and compare.

Does the generator slow down. THe note will change of course with the load but does it actually slow. It will slow a few rpms but maybe to much. THis can be checked by reading the HZ.
Or get and use a laser pointer digital phototach or borrow one. Both Ebay and Amazon offer these tools for $20 to $40.

Voltage and HZ are intimately tied together. Low HZ will offer low voltage.
If the HZ are down to much at the generator the voltage will be low so the compressor cannot get the power it needs meaning the generator is overloaded.


Read the manual. It should also show unloaded HZ and then full load HZ.
THey will be different with full load lower. You may be able to turn the loaded HZ up a few Hz to regain some voltage. But not over the unloaded HZ or you may cause other problems. Once the compressor is off then turn the HZ back to it normal position.

Often small gens are rated at peak output which is a short term condition . But they cannot sustain that long term.

Talk to the compressor supplier. Depending upon the motor you may be able to install a HARD START capacitor. Intended to help with starting on RV air conditioners it MAY be a help here.

Another possibility would be to change compressor speed so although it pumps less air per minute it will unload the generator. If the compressor is driven by belts then a smaller motor side pulley and belt would help.

 

[Greg QS]

Just like you main engines on a boat. If you are getting black smoke you are overloading the engine. Black smoke = Not enough air for the diesel injected.

You have
bad air cleaner or too much load. ( too small of a genset for the load)

Watts = volts x amps
4000/220=18 amps
a little math and you only have available 18 amps of power at 220. Most likely you have an unsized genset. What is the start up amps on the compressor.

I have a dive compressor on a 220 circuit and on a breaker of 20 amps.

With the shock load or anything else running at all you will probally not be able to get it started. Do the lights dim too? if so genset too small if running correctly

 

[ronobrien]

My generator runs fine with a 110vac load. Recently I added a 220vac Compressor to fill my scuba tanks. When I turn in the generator and run the compressor only the generator start to throw a bunch of black smoke out the exhaust. It’s not oil as it dissipates quickly in the water. When I run the compressor on shore power everything runs fine. Any thoughts on what this could be? Would it have anything to do with the governor or kicking into a higher speed to support the 220vac requirement?

Black smoke is unburned fuel. Oil smoke would be blueish/white. Poor combustion due to low compression would also be blueish/white but wouldn't be intermittent. It sounds like over fueling or air restriction. How old is the mix loop? also how does the coolant temp behave when this happens? A bad/clogged mix loop would restrict breathing and also contribute to overheating. My instinct would be to look at the mix loop first. Always go for the simplest thing first.

 

[HeartWarrior]

And winner is........low voltage caused by low RPM's. Adjustment of rpm to move the Voltage from 195 to 220 fixed the problem and the Generator is running fine. Thanks for everyone's input.

 

[diver dave]

Running a 3HP compressor now? Sometimes it takes 2 or 3 tries to get mine going.

 

[HeartWarrior]

It starts and runs fine all the way to 3000 psi no problem. To be safe it’s the only thing I have running when I fill a tank. We did some testing and it still had room for a couple of other ac things but it only takes 20 minutes to fill a tank and since I have a good house setup with 800 ah and 630watt solar I’m good with the other stuff.

Run amps on the motor is 14.2. The unit is built by American Air products in SC. It is there Fast 35 unit. Great machine and nice to know know we can ho dive while anchoring out as we are just starting a 3 year cruise of the Bahama Islands.

 

[diver dave]

Thats awesome. We have charged around 20 fills, half on nitrox32, during our current bahamas cruise. Have a 20’ fill whip so we dont need to carry tanks from dink.
Its liberating to have HP air and RO too. [emoji106]

 

[ronobrien]

And winner is........low voltage caused by low RPM's. Adjustment of rpm to move the Voltage from 195 to 220 fixed the problem and the Generator is running fine. Thanks for everyone's input.

Something to be aware of regarding RPM is HZ. There is less flexibility with cycles than voltage. Might be worth checking. 60HZ is your target.

 

[Mischief Managed]

My Onan MDKD seems to be getting weaker and weaker voltage-wise as time goes on. I have the regulator settings maxed out and I'm getting around 118VAC at 61ish Hz. I bought a generic voltage regulator on Amazon that is supposed to allow more adjustment, that I intend to install this Summer. Was only 24 bucks. https://www.amazon.com/gp/product/B078XFCX3J/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[ronobrien]

My Onan MDKD seems to be getting weaker and weaker voltage-wise as time goes on. I have the regulator settings maxed out and I'm getting around 118VAC at 61ish Hz. I bought a generic voltage regulator on Amazon that is supposed to allow more adjustment, that I intend to install this Summer. Was only 24 bucks. https://www.amazon.com/gp/product/B078XFCX3J/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

SWAG here, but I wonder if a compromised capacitor would do something like that. At 61 HZ the RPM is correct. Just a guess.

 

[Mischief Managed]

SWAG here, but I wonder if a compromised capacitor would do something like that. At 61 HZ the RPM is correct. Just a guess.

Certainly a reasonable theory, but there's no capacitor in the design.



To adjust the output voltage, you simply move jumpers around on a 6 tap transformer (T21) secondary coil. The different taps provide different voltages to the feedback loop in the voltage regulator circuit. The primary coil of this transformer is inline with the L1 genset output. The jumpers feed a bridge rectifier (CR21) that feeds "DC" to a field coil that' used to excite a small three phase alternator(EXCITER) that's spinning on the same shaft as the generator rotor. This alternator provides "DC" to the genset field coil. It's pretty archaic, but it works and it eliminates the need for any brushes and slip rings.


Mine has the jumpers connected to the taps that provide the highest possible voltage right now.



The new voltage regulator I bought uses the actual output voltage of the genset to determine the correct input voltage to the field coil in the EXCITER described above. Because it puts out "purer DC" (not bridge rectified and unfiltered "DC") it should be easier to control and because the system uses two stages to get the final field coil current, DC output of the voltage regulator does not need to be very high current. This is why I can get away with a small and inexpensive voltage regulator that I bought on Amazon.



My guess is that the insulation in T21 is getting weak and shorting out adjacent coils here and there, lowering the voltage.