Mattyman
The description below is long but wanted to provide detail. Thanks.

Generator is an 11kw model G0070331 on natural gas. This particular unit is backing up a 3T AC unit which is being controlled using the load shed board in the transfer switch. Owner had an outage this summer and when the AC came on it caused the generator to shut down.

Arrived on site and observed the behavior. After simulating an outage and 5 minute timeout, the AC attempts to start. You hear a slight change in the generator speed for about 1 second and then it is back up to speed. After about 4 seconds the unit shuts down. It then automatically restarts and does this process 2 more times and then faults out due to under voltage. At the time of the AC unit attempting to start, the load shed board never opens contacts to drop the load.

During this time we observed the output voltage and frequency. When the AC attempts to kick on, the frequency drops to about 58hz for a very short period and then right back up to 60hz. During the approximately 4 seconds when the unit is running at 60hz, the voltage output across the 2 hots is about 90vAC. This is strange because the frequency is fine.

A new load shed board in the switch was tried but as expected did nothing to change the behavior because the frequency doesn't drop to trigger the shed but the voltage. We tried a new evolution controller with the same results.

 The AC unit was tested on utility power and it is rated at 95LRA. When it first attempts to start, measured a max of 110A on my current meter and then back to about 12A running. There is not much else load on the system. About 5A per leg without the AC.

I'm stumped with why the generator output voltage is dropping when the AC unit is attempting to start and the frequency is fine. All other systems I have seen bog down if overloaded and either fault out or drop the load if being controlled.  Any suggestions on what is going on here? Thanks 
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murphy
Check the capacitor in the A/C compressor.  They go bad which greatly increases the start current.  The last one I changed had dropped from 45 mfd to 0.7 mfd.
If that isn't the problem install a soft start controller in the A/C compressor.  I did NOT say hard start, that would make it worse.
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Peddler
In my opinion a 3 ton unit is too large for an 11KW, not enough reserve power.  May work with a soft start setup which Ziller sells but very marginal especially if there are other loads.
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Mattyman
Peddler wrote:
In my opinion a 3 ton unit is too large for an 11KW, not enough reserve power.  May work with a soft start setup which Ziller sells but very marginal especially if there are other loads.


Understood. However shouldn't the engine bog and the load management drop the load? All other units I have dealt with worked this way. This one does not. On a side note, I met a guy yesterday that has his 2.5T unit connected to his 8kw on natural gas. I told him that with the startup load, that is pushing it for the 8kw but it still starts the compressor.
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murphy
I was able to reliably start a 3 ton A/C with my old 7 kW generator after I installed a soft start controller.
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78buckshot
I agree with Murphy and Peddler, 11kw and 3 tons don't go together very well, check the run capacitor in the condensing unit and replace it if it is 10% or more out of spec, add a SOFT START capacitor, have the A/C refrigerant charge checked by a competent company to make sure it's not overcharged, you could also add a head pressure switch to delay the condenser fan until after the compressor starts, yes the load shed should kick it out but your asking a lot from the set-up. I would also double-check all wiring and connections to confirm they are correct. My old 4390 15kw feels the inrush of my 3 ton but it's less sensitive to alarm than the new stuff. 
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Peddler
The whole problem with load shedding is that you have to overload the generator in order for the load to shed but the engines are so strong, especially when new, that they don't pull down and are able to maintain frequency so you don't get the load shed.  No one should be relying on load shed in a new installation in my opinion.  The reason all the generator companies put load shedding on their 200 amp switches was an end run around the NEMA code that says the the power source has to be equal to the main breaker.  This would require a 45 KW generator to switch a 200 amp service which of course in most cases is idiotic.  The real problem is both NEMA and the local authorities who hide behind the code to camouflage their own incompetence.   The utility companies are exempt from the code but that doesn't help the homeowner.  For more than 40 years my store was served by a 10KW transformer(45 amps) on the pole.  We have a 200 amp panel that is heavily loaded and should be a 400 amp service.  When a car hit the pole and threw the old transformer across the road an old guy from the utility company said he couldn't believe that can had held up all those years and they replaced it with a 25KW (100 amps).  I never rely on load shedding when sizing a generator you are only asking for trouble if you do but in order to switch lightly loaded 200 amp panels we have to have it there because the local authority can read the code but can't think for themselves.
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Cobranut

Peddler,

You're right about the engines not pulling down.
My 30KW Protector diesel shut down on overload once.
We had a couple day's outage, and I was working in my shop, which the genset is adjacent to. 
My wife was doing some housework, and I could hear when the load changed as she turned stuff on and off, as the tone would change a bit, and I'd hear the turbo wind up, but the RPM never dropped enough to detect. 
Eventually I heard it load up and stay loaded about 10 seconds, then the lights went out and it shut down.
I walked outside and it had a "Shutdown on Overload" on the display.

I had about 3KW of lights on in the shop, plus the shop A/C was running.
I went in the house and asked my wife what was the last thing she turned on before it shut down.
Well, she had both washing machines running, and at least one dryer, the heat pump was running, she had the oven on, as well as 2 surface elements, and had started to fill the sink with hot water. (I have a 27KW tankless water heater).

I suspect one of the washers was filling, and when she turned on the sink faucet, the extra flow ramped up the power on the water heater and that's what put it in overload.
I had a little talk with her, and explained that the genset could run ANYTHING in the house, but not EVERYTHING in the house.  LOL

As far as Mattyman's problem, could it be the voltage regulator isn't ramping up the field current enough to maintain voltage, which would explain the rpm remaining constant but the voltage dropping.

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Mattyman
Cobranut wrote:
 

As far as Mattyman's problem, could it be the voltage regulator isn't ramping up the field current enough to maintain voltage, which would explain the rpm remaining constant but the voltage dropping.



I thought it may have been an issue with the voltage regulator, which is why I swapped the main controller, as the voltage regulator is integrated. Thought there may have been an issue with delayed field flash or losing the field flash.

This is on a 200A service and the only 240v load is the AC. The owner had her HVAC company out to check it and they said it is fine and it was 13a starting and 12a running. I tested with my current meter and had it set to max and saw an inrush of about 110A. I'm not very confident that the HVAC company is very good. I will check the main capacitor on the unit to see what it is now reading and if it is still in spec. This is an older compressor (manufactured in 2004).  I still suspect that the compressor is failing and drawing more than the generator can handle because from my experience an 11kw can start that size even though near the limit. I do agree with Peddler that it sounds like the engine can handle the load but the alternator cannot, so the frequency remains constant. In my 9 years of working with generators, this is the first time I have experienced this scenario.
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Mattyman
Also, I'm not an HVAC expert but know that there are different soft start kits for different compressor types. The model of the AC compressor is a RUDD UAKA-037JAZ. Does anyone know how to find out what type (reciprocating, scroll?) and what soft start kit would be compatible? Thanks.
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78buckshot
The RUUD model is probably 036JAZ - 3 ton, and would be a scroll compressor using R410A refrigerant, I don't think the compressor type is an issue, just the size. I can get info at the office today and report back this evening but someone will probably answer before that. If you are suspecting a compressor problem then I wouldn't put money into the machine, the compressor motor windings can be checked for resistance and shorts, I would think any problems would have shown up already, the scrolls are pretty fool-proof. I just re-read my post, the RUUD UAKA-036JAZ is probably R22 refrigerant and not R410A, won't make any difference in this case but I wanted to make the correction.
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patmurphey
Peddler wrote:
The whole problem with load shedding is that you have to overload the generator in order for the load to shed but the engines are so strong, especially when new, that they don't pull down and are able to maintain frequency so you don't get the load shed.  No one should be relying on load shed in a new installation in my opinion.  The reason all the generator companies put load shedding on their 200 amp switches was an end run around the NEMA code that says the the power source has to be equal to the main breaker.  This would require a 45 KW generator to switch a 200 amp service which of course in most cases is idiotic.  The real problem is both NEMA and the local authorities who hide behind the code to camouflage their own incompetence.   The utility companies are exempt from the code but that doesn't help the homeowner.  For more than 40 years my store was served by a 10KW transformer(45 amps) on the pole.  We have a 200 amp panel that is heavily loaded and should be a 400 amp service.  When a car hit the pole and threw the old transformer across the road an old guy from the utility company said he couldn't believe that can had held up all those years and they replaced it with a 25KW (100 amps).  I never rely on load shedding when sizing a generator you are only asking for trouble if you do but in order to switch lightly loaded 200 amp panels we have to have it there because the local authority can read the code but can't think for themselves.


I think that your claim that load shedding doesn't work should be a topic for separate discussion. I can tell you that the system works just fine for me to justify a 10kw(9kw NG) generator for my lightly loaded 200amp service. Load shedding protects the generator on change over, meeting the requirement to be adequate for "all connected loads" and protecting me from stupidity after switching to generator power - like using my electric oven and dryer at the same time with any other significant loads. I can't buy your argument that they don't work and are a scam to sell smaller generators.
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Peddler
I AM NOT SAYING THEY ARE A SCAM AND THAT THEY DON'T DO WHAT THEY CLAIM!  The problem is that in order for them to work/shed you have to overload the generator so that the engine loses RPM and that means that you have overloaded the generator for some time just as you would if you waited for the MLCB to open on overload.  Has your unit ever shed the load after first taking it?  I will totally stand by my statement that the whole load shedding system development was to get around the NEMA and local codes.  Why would anyone competent to install generators who intends to stand behind them install a generator that was potentially run above 75-80% of the rated output of the generator for an extended period of time.  I have seen a 22KW unit run for hours at over 104 amps load on each leg yet I think the rating of that unit is around 94 amps, the generator had 4 load shed module on it and a heat pump tied to the ATS module and none of them shed because it was maintaining 60 hz.  There can be no other reason other then to get around the code or the modules would be measuring amps.  The problem isn't so much the overloaded units, it is the 90% of installs where there is virtually no load and someone who knows nothing about generators decides to enforce the source size rules in the code.
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78buckshot
I would start checking the symptoms of 'Undervoltage' without the A/C involved, load the generator with relatively steady loads and see if it can handle max current or close to it. Something is telling me you should check the rotor resistance to help diagnose if the brushes are not riding fully on the slip rings. Also it wouldn't hurt to check stator specs, I had one a few years ago that would run fine with up to 6 - 8 amps and then fall on it's face with the slightest increase in load. After checking all of the resistant values in stator and rotor Tech Support authorized a new controller, a few months later we had the same issue, this time Tech Support asked for a re-read of the stator and rotor resistance, they were suspicious of some very close stator test results, new stator took care of the problem.
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patmurphey
Peddler, I'm just saying it's worthy of a separate discussion. Your last post conflicts with itself. It may be simply that the generator kw/amp ratings are conservative. (BTW, I have not overloaded my generator yet during an outage.)
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Peddler
Happy to have a discussion on that topic.  I don't see the conflict my last post.  If load shedding is designed to protect against an over loaded source then we wouldn't we be measuring amps not HZ.  My example is a perfect case for why you should not expect the load shedding to protect the generator, 10 amps over max rated load for over an hour and not one load shed because the engine was strong enough to keep it's speed up.  The stator though can't take that kind of load for ever so what are the "load shedding" modules doing for you in this case?  No question that if the frequency drops they will shed but if it doesn't something has to give and therefore the only reason we use them is to satisfy the code authorities not to protect the generator.
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Cobranut
Don't all the controllers monitor the load current like my Protector series, or is that only a feature on the larger units?
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78buckshot
Cobra, when Generac started using the digital display controllers they also added current transformers to each leg of the alternator output, this is just a general observation of the air cooled and I can't say for certain if every model has them and I don't remember seeing separate CT's on the protector or the older liquids, but basically the newer ones are monitoring current. 
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ohmslaw
11 KW units are very bad at starting A/C. The reason it went into under voltage at restart is due to the load being applied at start up. You would need to open the Gen breaker to get it started. The load shed is useless in my opinion on an 11 KW for a/c unless you never want it to run. Then you should just lock it out. I always up size to a 16  minimum when whole house switch and A/C. The 11 KW have had a demand Regulator issue . You might want to check into that Load shed is useful for multiple ac units on large homes . Mainly for timing.
Tim
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Geoff Z
An A/C on an 11kw will almost certainly require a sure start and time delay. In rush has to be staggered sequentially when using a smaller generator. I'm all for saving money and using the smaller generator. However the application has to be designed properly to avoid nuisance problems. I'm sure it has been said elsewhere but my issue with relying on load shed is that every load on the generator sees that frequency drop prior to load shed being employed. Today's sensitive electronics do not want to see that. That is where proper design and sizing become critical IMO.
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BrianMartin
ohmslaw wrote:
11 KW units are very bad at starting A/C. The reason it went into under voltage at restart is due to the load being applied at start up. You would need to open the Gen breaker to get it started. The load shed is useless in my opinion on an 11 KW for a/c unless you never want it to run. Then you should just lock it out. I always up size to a 16  minimum when whole house switch and A/C. The 11 KW have had a demand Regulator issue . You might want to check into that Load shed is useful for multiple ac units on large homes . Mainly for timing.
Tim


This is what I normally quote customers as well. If they want the whole house switch and the AC to run, it's a 16 KW minimum no matter how small the AC. If they aren't concerned with the AC working on gen power, we will offer the 11 KW with a SMM and lock out the AC completely. We've installed a handful of Surestart's in combination with the 11KW, however it's normally easier and just as cost effective to just go with the 16KW.
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