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Generators & Motors General Discussion

Got 30 hp engine - which generator power - 15kw or 20kw?

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Old 05-06-2008, 02:30:43 AM
123weld 123weld is offline
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Default Got 30 hp engine - which generator power - 15kw or 20kw?

I've got a rated 37hp motor that would put out 32hp at my needed 1800 rpm when it was new. I'm thinking of buying a generator head for it. Factory calls for 30hp for 15kw and 40hp for 20kw. I'd like to buy the 20kw model, even though I only have enough hp for 15kw. My thoughts are the 20kw won't be exerted as much as 15 would be ( easier on duty cycle). However I don't know how much more energy it would take to turn the bigger gen head. It's not that I think that I'm going to draw more than 15kw from machine. ?? Is my thoughts right or wrong? Any words of advice?
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Old 05-06-2008, 08:52:51 AM
K D Redd K D Redd is offline
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Default Re: got 30 hp eng. , 15kw or 20kw?

I would rather have the bigger head. This way IF you do over load you will run out of engine before you run out of generator. If you have a bigger motor than generator you can burn-up the generator by over loading it as the engine can power what is an overload situation for the generator head.
Wth the bigger head, it can only convert the amount mechanical power it receives from the engine to electricity. If you put 32 hp into it you will get about 16 KW out. You put 20 HP in you get 10 KW out.
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Old 05-06-2008, 09:47:48 AM
Jim Rankin Jim Rankin is offline
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Default Re: Got 30 hp engine - which generator power - 15kw or 20kw?

I agree with Kent. Another reason for buying the larger head is that you will probably get better large motor starting performance out of it. A lot of the current that is required in the first milliseconds of starting a motor is not "real" power, but instead is reactive power. This doesn't require much engine work, it is created mostly by the generator's regulator and exciter. A larger generator end should be more capable.

The only big reason I can see to go with the smaller head is if mounting it up etc was a big problem because of the size.
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Old 05-07-2008, 10:22:03 AM
mooseye mooseye is offline
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Default Re: Got 30 hp engine - which generator power - 15kw or 20kw?

I would really like to hear more on this subject.
I was under the impression that output voltage is proportional to the rpm of the generator. If you under power a unit, wouldn't you loose rpm? If so, could you not up the running speed of the engine to compensate? Or, wouldn't the governor on the engine at least try to match the rpm to the load applied?
I have seen models of the same welding machine with different horse power engines, such as a Ranger8 with an 18hp and a 20hp. This makes me think that as long as the rated input rpm is not exceeded, it should not damage the generator just by having more hp.
Also, it is my understanding that even with the correct hp engine, overloading a generator can cause damage. This falls on the operator to consider the load applied.
This seems to be a complicated topic with many factors to consider, so with my limited understanding, I would rather have excess hp to minimize the load on the engine as an underpowered generator would create the chance of damaging both components due to lower generator output and excess engine load. The load on the generator would have to be controlled by the operator in either case.
All this is from logic with my limited knowledge so feel free to correct my thinking.
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Old 05-07-2008, 12:24:38 PM
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Talking Re: Got 30 hp engine - which generator power - 15kw or 20kw?

<I was under the impression that output voltage is proportional to the rpm
of the generator.>
This may be true for smaller, cheaper, AC Gensets, but is NOT true for any
Genset with an AVR. (Automatic Voltage Regulator) Most modern 10Kw and
larger genends will have an AVR, and ALL Brushless Genends will have AVR's.
<If you under power a unit, wouldn't you loose rpm?>
Not until you load the Genend up to the output capacity of the engine. That
is what the governor does, is keep the Rpm's constant, with respect to the
load, up to the capacity of the engine.
<it is my understanding that even with the correct hp engine, overloading a
generator can cause damage.>
Overloading a Genend, electrically, is ALWAYS a bad thing. Some Genends are
rated for 110% Capacity for a Specific Amount of Time, and or at a specific
Operating Temperature, but some are not.
Most commercial type Prime Power Gensets are designed with the Maximum
Capacity of the Engine and Genend matched. Backup and Standby Gensets
are spec'd differently, and may or may not be matched as close.
I would much rather have a underpowered engine with a BIG Radiator, so that
when an overload happens the engine just bogs down a bit, rather than
taking a chance on having the Stator Windings open due to excessive
amperage being drawn, but that is just "Me".
Bruce in alaska
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Old 05-07-2008, 02:34:19 PM
Jim Rankin Jim Rankin is offline
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Default Re: Got 30 hp engine - which generator power - 15kw or 20kw?

Well lets see if we can complicate this a bit

The other part of loading a generator is the power factor of the load. Now don't shut down, power factor is a simple concept. All it means is the ratio of the REAL power ( measured in watts - takes engine work to make it and $ to buy it) to the VA (volts x amps). the difference between the two is called REACTIVE power and it circulates through the wires so it makes heat and is important in figuring out how much load can be put on a generator end before it burns up. Reactive power doesn't require much engine power to produce, it mainly is conserved and simply exchanged between the load and the generator.

The load can have a power factor of 1.0 in the case of a string of light bulbs or it can have a power factor of 0.20 for a few milliseconds during the starting of an electric motor. Most of the time, the power factor of a mix of electrical equipment will be somewhat less than 1, but who really knows without a watt meter on it. If you had to power a set of flood lights for an outdoor event, power factor might be 1.0 if they were quartz halogen or might be quite a bit lower if they were HID (metal halides or some such) with a ballast in each one.

It's even simpler if you consider the power factor of a generator set. It's engine has a certain power in kW (this is REAL) and the generator has a certain volts X amps rating (this is apparent power, it looks like the generator is making that much power, but it may not be).

So you connect up your loads to your generator, start it up and begin loading it.

If your load mix has a high power factor (a huge christmas tree light display for example) as you turn on things, your engine may begin to struggle and overheat before you even reach the amperage limit on the generator. You calculated that the amperage would be ok so what gives? Many/most generators are specified with a power factor of 0.8 assuming that most times there will be a mix of things running at all times and some will have low power factor.

If your load has a low power factor, (you have a bunch of electric motors running idle in a display, doing very little work), you may exceed the amperage rating of the generator end and burn it out while your engine loafs along barely coming up to operating temperature. This is the situation where it might be better to have a slightly underpowered set, so you can hear that something is going wrong as opposed to an over powered set that the engine can quietly drive the generator end into an overload.

Even with an AVR, output voltage IS generally related to engine speed, it's just that the AVR attempts to keep the voltage up as the engine speed falls under an overload. Some AVR/exciter systems are better at maintaining voltage under overload conditions than others, but once RPM begins to fall, usually it will continue to fall if the load doesn't decrease. Most modern generators have an option to allow the regulator to drop the voltage once the engine slows down more than a certain % of synchronous speed. This keeps voltage from varying under small load variations, but when a sudden overload is dropped on the generator and slows the engine down too much, allowing the AVR to drop the voltage (which usually sheds some load) may keep it from bogging down and dying and will speed recovery to full rpm/voltage. It's a trade-off, keep the voltage as high as possible through the dip or keep the dip as short as possible by sacrificing some voltage.

These settings (sometimes called underfrequency roll-off) also protect the regulator if someone inadvertently shuts the generator down under load or it runs out of fuel under load. Without the underfrequency roll-off, the regulator goes to full power as the rpm drops and may burn itself out attempting to maintain voltage in an impossible situation.
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Old 05-07-2008, 09:42:07 PM
armandh armandh is offline
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Default Re: Got 30 hp engine - which generator power - 15kw or 20kw?

there is always a weakest link
hopefully it is a circuit breaker.
but if not
I would rather stall the engine than burn up a generator
it is way cheaper to disconect load and restart.
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Old 05-08-2008, 12:05:28 AM
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J. McNugget J. McNugget is offline
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Default Re: Got 30 hp engine - which generator power - 15kw or 20kw?

I was watching this on Ebay, figuring that it would not last with the Buy it Now option. It didn't last past yesterday, but something similar may work very well for you. It was 20 kW and single phase 120/240

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