Generator KW

[kwired]

In answer to the original post, the maximum kW output of the generator is determined by the engine. The kVA is determined by the alternator. The engine is limited by its ability to drive a physical load. The alternator is limited by the amount of current it can carry without overheating.

The 100 kW generator has a rated power factor of 80%, which gives the genset a rating of 125 kVA. But if you place a load of 125 kVA at 95% power factor on the engine, it will stall because 125 kVA @ 95% power factor is 119 kW and exceeds the rating of the engine. The alternator can handle it, but the engine can't.

That may depend on the "service factor" or whatever equivalent name that internal combustion engines have. If the motor is rated 100 kW and you put additional load on it, will it attempt to drive the additional load or will it immediately stall if loaded to 101kW? I would like to think it can have some overload for at least a short time without much harm. Or do they commonly size the prime movers that close to the alternator capabilities and run them at/near their absolute peak capabilities? I understand that voltage and frequency may not remain all that constant during a surge in load, but if prime mover has the ability to recover it will still ride through instead of stalling.

On the other extreme, I once saw what happens when a farmer mis-wires a motor and creates a line to line fault in the process, then tries to run it with his tractor PTO driven 20 KVA generator, but uses a tractor that can deliver 125+ HP to the PTO shaft. Lets just say the tractor did not stall but the generator did.

 

[Sahib]

Do you mean a numerically lower power factor?

Yes.

 

[Sahib]

That may depend on the "service factor" or whatever equivalent name that internal combustion engines have. If the motor is rated 100 kW and you put additional load on it, will it attempt to drive the additional load or will it immediately stall if loaded to 101kW? I would like to think it can have some overload for at least a short time without much harm. Or do they commonly size the prime movers that close to the alternator capabilities and run them at/near their absolute peak capabilities? I understand that voltage and frequency may not remain all that constant during a surge in load, but if prime mover has the ability to recover it will still ride through instead of stalling.

Usually the engine hp is two times the alternator KW or something like that.
The diesel engine has no inherent overload capacity. However one hour overload capacity is designed into it.

On the other extreme, I once saw what happens when a farmer mis-wires a motor and creates a line to line fault in the process, then tries to run it with his tractor PTO driven 20 KVA generator, but uses a tractor that can deliver 125+ HP to the PTO shaft. Lets just say the tractor did not stall but the generator did.

Stalling is a specific term applicable to engines and motors.

 

[JoeStillman]

Stalling is a specific term applicable to engines and motors.

And alternators with a sheared-off coupling.

I did an experiment once with a 1500 kW load bank connected to a Cat 1500 kW 3516. We increased to voltage gradually and watched the running load. The load bank kW increased with the square of the voltage as you would expect, but you could hear the engine lugging and the frequency started to drop at less than 105% of rated kW. I was surprised at how little overload the engine could handle.

I suppose the limiting factor for a genset operating within it's kVA rating, but over it's kW rating is temperature. The radiator becomes the weakest link.

 

[kwired]

And alternators with a sheared-off coupling.

I did an experiment once with a 1500 kW load bank connected to a Cat 1500 kW 3516. We increased to voltage gradually and watched the running load. The load bank kW increased with the square of the voltage as you would expect, but you could hear the engine lugging and the frequency started to drop at less than 105% of rated kW. I was surprised at how little overload the engine could handle.

I suppose the limiting factor for a genset operating within it's kVA rating, but over it's kW rating is temperature. The radiator becomes the weakest link.

An electric motor when overloaded simply demands more current from the supply, as long as the supply is capable of delivering it gets what is demanded. There is still a certain level where it can not supply enough torque and begins to slow down.

With a internal combustion engine, more fuel is demanded, and usually granted, but unless more combustion air is also introduced this fuel will not give its full output capability. Turbo chargers and other similar devices do help in this area, but still may have limitations.

 

[Sahib]

And alternators with a sheared-off coupling.

Weak coupling?!

I did an experiment once with a 1500 kW load bank connected to a Cat 1500 kW 3516. We increased to voltage gradually and watched the running load. The load bank kW increased with the square of the voltage as you would expect, but you could hear the engine lugging and the frequency started to drop at less than 105% of rated kW.

.

Worn out old gen set ?

 

[GoldDigger]

I was surprised at how little overload the engine could handle.

Often overlooked is that the maximum motor horsepower can vary with altitude and with intake air temperature.

 

[steve66]

I suppose the limiting factor for a genset operating within it's kVA rating, but over it's kW rating is temperature. The radiator becomes the weakest link.

I always try to keep generators operating below both the KW and the KVA ratings.

And not all alternators are rated for a 0.8 PF. If a generator is specified with an oversized alternator, it will have a lower PF rating. I once ran across a 75KVA generator, but it was only rated for 25 KW (0.33 PF).

Steve

 

[JoeStillman]

Weak coupling?!

I was just trying to imagine what happend to kwired's farmer who had the 125 kW tractor and 20 kW PTO generator. He said the tractor didn't stall. If he was lucky, he had an ablative coupling.

Worn out old gen set ?

It was practically new, maybe a couple of hundred hours on it. It was part of a plant with three 1500 kW gensets in parallel. It had closed-transition-transfer with the utility co. Whenever they did CTT, the paralleling gear would leave the voltage setpoint at whatever it last saw from the utiltiy. When utilty had been a little hot, a single engine in island mode couldn't drive the 1500 kW load bank. The client thought there was something wrong with the gensets because he didn't understand the relation between kW and Voltage. When we dialed the voltage back down to nominal, it was fine.

 

[JoeStillman]

Often overlooked is that the maximum motor horsepower can vary with altitude and with intake air temperature.

This was in Philadelphia, which is about 50 feet above sea level. (Although the gensets were on the 12th floor of the buliding:/)

 

[eHunter]

This was in Philadelphia, which is about 50 feet above sea level. (Although the gensets were on the 12th floor of the buliding:/)

Sounds like you were on Broad Street or Market Street in Philadelphia.

 

[kwired]

I was just trying to imagine what happend to kwired's farmer who had the 125 kW tractor and 20 kW PTO generator. He said the tractor didn't stall. If he was lucky, he had an ablative coupling.

Universal joint in the drive shaft suffered the consequences.

 

[GoldDigger]

If he was lucky, he had an ablative coupling.

Trying to figure out just what word you were aiming for, Joe.
Ablative (as in heat shield) involves boiling or evaporating away to carry away excess (sometimes frictional) heat.
I have heard of frangible, breakaway, and shear couplings before, and there are probably a lot of other words I don't know for that purpose.

 

[JoeStillman]

Trying to figure out just what word you were aiming for, Joe.
Ablative (as in heat shield) involves boiling or evaporating away to carry away excess (sometimes frictional) heat.
I have heard of frangible, breakaway, and shear couplings before, and there are probably a lot of other words I don't know for that purpose.

James A. Michener had a whole riff on the word "ablative" in his novel "Space".

I think the correct term is a "shear coupling" - one that fails on purpose in shear mode. Genset manufacturers use them so there is a link between the engine and alternator that is weaker than the drive shaft of either.

"Frangible" is a new one for me. I like it.

 

[GoldDigger]

"Frangible" is a new one for me. I like it.

The two contexts that I regularly see "frangible" in are freeze plugs and similar relief mechanisms and lock shackles for fire-safety related equipment.
When I first heard of frangible-shackle locks (designed to be easily broken with a hammer, crowbar or fire axe), my thought was "Why bother with a lock, then?", but it turns out to be similar to the "break glass" on fire alarms and the alarmed panic bars, just one step harder to open accidentally.

 

[JoeStillman]

Sounds like you were on Broad Street or Market Street in Philadelphia.

9th and Race. You can see the power plant from space.

 

[eHunter]

9th and Race. You can see the power plant from space.

I had to get a look from google maps.
Looks like a telco colo hotel with 4 rooftop mounted gensets among other communications equipment.

 

[Sahib]

I was just trying to imagine what happend to kwired's farmer who had the 125 kW tractor and 20 kW PTO generator. He said the tractor didn't stall. If he was lucky, he had an ablative coupling.

If it was a self excited generator, its field would have collapsed under short circuit and the engine would have run on load with no damage to the coupling.

It was practically new, maybe a couple of hundred hours on it. It was part of a plant with three 1500 kW gensets in parallel. It had closed-transition-transfer with the utility co. Whenever they did CTT, the paralleling gear would leave the voltage setpoint at whatever it last saw from the utiltiy. When utilty had been a little hot, a single engine in island mode couldn't drive the 1500 kW load bank. The client thought there was something wrong with the gensets because he didn't understand the relation between kW and Voltage. When we dialed the voltage back down to nominal, it was fine.

The gen set usually has 10% over load capacity for one hour. I wonder why this was not so in your case.