# Correct Generator RPM

simagic
Solar Expert Posts:

**100**✭✭✭✭
Boy is this forum helpful. So I got a Yamaha ef1000is (1000watt) generator (with a propane conversion) to power my 40 amp battery charger http://sterling-power-usa.com/sterlingpowerprochargeultra12volt40amp-marine-battery-charger.aspx

to supplement my solar system for when there are days of cloudiness. Although I was concerned if the genny would power this 40amp charger , it does ( It has a good power factor correction). Anyway, I also was running a 600w space heater (not at the same time). The genny's rpm with eco off is 5000 rpm (full speed) .......with eco on and no load, approx. 3000 rpm. I read somewhere on this forum that running the genny at approx 75% is most efficient. (75% of 5000 is 3750rpm). With the space heater on, the genny runs at approx. 4400 rpm which is approx 90% of full rpm. Soooooo, the question is, is it ok to run the genny for extended periods at 90% (4400rpm). OR EVEN AT 75% (3750 rpm) AND, what exactly is an extended period. Can these genny's be run for 72 hrs (non stop).-- 48 hours (non stop). -- 24 hrs. (non stop). How long are the designed to run for?. Will they overheat or some other issue. I sometime see vendor trucks (with larger genny's) running all day. How long can they be safely run?????..

to supplement my solar system for when there are days of cloudiness. Although I was concerned if the genny would power this 40amp charger , it does ( It has a good power factor correction). Anyway, I also was running a 600w space heater (not at the same time). The genny's rpm with eco off is 5000 rpm (full speed) .......with eco on and no load, approx. 3000 rpm. I read somewhere on this forum that running the genny at approx 75% is most efficient. (75% of 5000 is 3750rpm). With the space heater on, the genny runs at approx. 4400 rpm which is approx 90% of full rpm. Soooooo, the question is, is it ok to run the genny for extended periods at 90% (4400rpm). OR EVEN AT 75% (3750 rpm) AND, what exactly is an extended period. Can these genny's be run for 72 hrs (non stop).-- 48 hours (non stop). -- 24 hrs. (non stop). How long are the designed to run for?. Will they overheat or some other issue. I sometime see vendor trucks (with larger genny's) running all day. How long can they be safely run?????..

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## Comments

5,433✭✭✭✭An inverter genny is somewhat different than a conventional. While it is probably true to is most efficient under ~75% load, that does not directly translate to rpm.

As for longevity, these ( and the Honda eu series) are pretty well built units. It old probably run them for ~ 6 hours betwee oil check shut downs. If find that the oil level does not drop, then extend it as suggested. The biggest issues as I see it is making sure it has lots of cooling air.

Tony

100✭✭✭✭Thanks for the reply. So if the ~75% doesn't relate to rpm, can I assume it relates to watts. If that's the case and the 1000w genny is rated at 900watts, then 75% of 900 is 675 watts. Soooo what I am understanding from your reply is that I can run ~675 for about 6 hours between shutdowns. Well, can I run 850-900-950 watts (more than the 75%) those same 6 hours??? ( without harming the genny). Also, ,,,So these genny's I see vendors using for 12-14 hours a day (day after day)......are they doing their genny's harm???

Also, you say "making sure the oil level does not drop". I ass-u-me you mean making sure the oil level light doesn't come on and not stopping the engine to check the level

5,433✭✭✭✭In short, running at 75% load is not going to hurt the genny. Running for long run times isn't going to hurt it either. Personally I would not wait for the low oil shut down,, I would shut down and check it every ~6 hours until it becomes clear that a longer interval is ok.

If you look at it this way, an engine has X amount of rotations until it wears out. It doesn't really matter if you run those in short burst or long run times. (in fact long run times would probably be better due to fewer heating and cooling cycles,, the biggest wear time for engines.). There is a member here who has over 6000 hours on a Honda eu 1000. Personally, I wouldn't run the genny at over ~75% load very often if I was looking for good longevity.

Please explain what you are trying to achieve with continuous duty run times. It is possible that another genny might be better suited for longer run times.

Tony

100✭✭✭✭Well, the one issue of a continuous run time would be for a space heater, that I mentioned in my initial post. It runs the genny at ~4400rpm for ~600 watts. That's probably the item I would want to run the longest. Other than that, I'm trying to find out if it's Ok or not to run it for a prolonged time at the higher rpm..thanks

100✭✭✭✭Oh. So can I assume the ~75% is wattage since you said it wasn't rpm???

5,433✭✭✭✭75% of load,, is watts,, not RPM. Try this test. Fill the tank with a given quantity of fuel, run it until it runs out with a 75% watt load and time the run. Then put the same fuel amount in, and run it at 50% and time that.

There is a series of posts where a member did exactly that.

Is your goal to use the least fuel over all, or the least fuel per kwh, or make the genny last the longest?

Tony

17,615✭✭Correct. The percent load rating is relative to a generator's total Watt capacity.

32,026adminRunning a genset to run a space heater is probably not the most cost efficient way to make heat...

Either a fuel fired space heater (wood, propane, or oil burning stove) or a heat pump mini-split "A/C" unit would be more efficient for long term needs...

Also, lots of insulation?

-Bill

100✭✭✭✭Goal is to let my genny last the longest.

I have a propane conversion so the fuel test might be difficult.

I'll believe the others if they say ~75% is best for fuel consumption.

So, the 600 watt heater (600 is less than 75% of 900), is good with respect to falling in with the 75%, so the 4400rpm of a max of 5000

I guess would be OK

17,615✭✭Sometime electric heat is the right solution, even if not strictly efficient.

For instance for occasional heating of a small space that doesn't justify the capital expense of another type of heating unit.

Insulation only retains heat; it doesn't produce it. No source = no retention.

And I can't believe I'm actually defending electric heaters!

10,300✭✭✭✭i'm going to agree with bb here that you may want another source of heat like propane or wood if on a constant basis along with insulation.(not sure of circumstances there and what steps may be possible for you) this would save running a generator 24/7 too.

i'll confirm they are talking about the % of its full power wattage rating. this is more of a design standpoint of your loads or to buy the right sized generator. in normal use loads will vary widely and it would be idiotic to throw a redundant load on it using more fuel just to have percentages inline with best efficiency. on the other hand one doesn't want to just run say 200w of lights with a max of about 800w total load due to another appliance that is occasionally used on a 2kw genny either, if they can avoid it. a 1kw genny would be better for that circumstance and it is unlikely you will get loads to be perfect.

tony,

i'm confused here about what you say about the oil consumption on the genny. do these gennies use that much oil in a short time period? if so, i would not opt to have one of them that consumed that much as it is burning oil then.

100✭✭✭✭The heater use is for the anticipated power outage scenario. (Prepare for the worst-hope for the best). The 600 watts would run down my 220ah battery in no time. So if there were a power outage, I could run my "usual" items ( the ones that run well with my panel and battery limitations)( tv-computer-a few lites, etc) and if it were friggin freezing in here (let's say overnite), at least I could turn on my genny, plug in the 600w heater and sit or sleep in front of it until the next morning. These are the circumstances that initiated this thread

So..running at 4400-4600rpm (out of a 5000 available) for 6 or more hours would be ok......right

17,615✭✭Well my EU1000i with 6000+ hours on it uses a bit of oil, but I've yet to see any consumption on the EU2000i which has a mere 2000 hours on it. One key element with these little generators is to follow the maintenance instructions and change the oil regularly; every 50 hours. This probably accounts for the long lifespan I've seen with mine.

5,433✭✭✭✭I am not sure that OP was going to use the genny for heat on a regular basis. My guess is that he used the heater as a dummy load.

NIel,

These gennies don't use much oil,, but they only hold a tiny amount (a failing in my opinion ) so checking more often than less often is just good practice. A Honda Eu 1000 holds ~ 1/2 quart, so it gets hot! And the difference between enough, and not enough is pretty small. My 10 kw Onan by contrast holds 14 quarts, if memory serves. If I could change anything about the EU series I t would be to add oil capacity since the oil does a substantial amount of the cooling.

T

10,300✭✭✭✭ok, but in an outage these can go on for more than 2 days or so and 50hrs is roughly 2 days of constant use. for a 6 day outage he'd have to change the oil 3 times. i'd keep looking for a different genny in that case as he's limited to 2 days of use.

as to the oil consumption one would not have to worry if it needs changed so often.

5,433✭✭✭✭I'm not suggesting that he change the oil every 6 hours,, merely that he check the oil level every six hours or so!

Tony

17,615✭✭Yes; with only 14 oz. of oil in the crankcase a small reduction can give you a gen that starts then immediately stops as the oil splashes up and the level goes down too low. My plan has always been to check it daily, which in the EU1000's history has been up to 10 hours continuous operating and a change every 5 days. Not likely most people will use their gen that extensively!

Two things I'd change in the Honda's design:

1). Fuel tank is too small and too hard to see the level in.

2). Oil capacity is too small and too difficult to change.

For the OP: I'd suggest using an oil-filled electric radiator if you're not already. They're relatively inexpensive, have multiple Watt settings (I have a tiny one that has one setting @ 550 Watts), and have a good "comfort" rating as the mass of the radiator evens out the thermal changes. No fan means it takes a while for them to heat up an area, but there's not any fan noise either. Hmm. I ought to try it with one of those eco fans like we use on the wood stove.

5,433✭✭✭✭Fel tank capacity is easy to augment with an aux tank. I can't think of a way to add oil capacity. I suppose you could fashion a nipple in the drain plug to add a bigger sump. I know many of the EX series Honda engines have drain plugs on two sides, one could, I suppose build an aux sump using both drain plugs to set up some circ.

Tony

1,372✭✭✭The correct way to state this is: The more you load a genset above 50% (up to its continuous rating), the more efficient it will be. If it is rated to run at X watts, you will get your best effiency loading it up to X, not just 75%. My 12 kw Generac is rated for 12 kw continuous. It burns 1.5 gallons of LPG per hour to output anywhere from 0 to 6000 watts, and just 0.5 gallon more per hour to output 12000 watts (2 gallons total per hour). I'll get the most kw per $ by putting 12000 watts on it, not just 9000. That said, when you take

kvainto consideration, you may be limited to 75% of your kw rating.Also keep in mind that smaller gensets often list their maximum wattage as part of their model number, but that is not their continuous rating. The Honda EU6500is is rated for 5500 watts continuous (6500 watts for surge only).

100✭✭✭✭Great. so that makes me feel like i'm not hurting it running it hard but actually getting better effiency...

BUT.......I'm not sure what you mean when saying "That said, when you take

kvainto consideration, you may be limited to 75% of your kw rating."17,615✭✭The evil Power Factor rears its ugly head!

Some loads are inductive not resistive. As such their "Wattage" rating is inaccurate as far as the generator is concerned. When the gen's rating is 2000 Watts, it means 2000 Watts: power factor of 1:1, so that is also 2000 Volt * Amps. If the load is 1500 "Watts" but has a power factor of 0.75 then it draws 2000 Volt * Amps. No "room left over" for additional loading.

It's all to do with the difference between the supply (generator) power curve and the load demand curve, which can be "at odds" with each other. The load can demand maximum current at a time which is not "in synch" with the supply's maximum Voltage. (Not a perfect explanation but I hope you get the idea.)

32,026adminkVA... Gets into AC math... The difference between DC and AC math is the difference between Edison and Tesla (very interesting piece of history and the conflicts between experimentation vs math based analysis).

Let me put it in physical terms first...

Imagine you have a car on rails. If you stand in front of the car, your 100 lb pull will pull the car forward with 100 lbs of pull on the car.

If you stand off to the side of the car and pull on the rope, your 100 lb pull will be the Cosine of the angle towards the forward motion of the car. Say you are standing 60 degrees off of to the side, then the part of force pulling the car forward would be (i.e., the part of the effort actually doing work):

- Force pulling car forward = Force * Cosine (angle) = 100 lbs * Cosine 60 degrees = 50 lbs

So, in this case, the factor of the force pulling forward is 50% or 0.50 ... So, if you wanted to still pull the car forward with the same force (say pulling up a hill) but you are 60 degrees to the side, you would have to pull 200 lbs on the rope to move the car forwards/up hill vs 100 lbs if stood in front of the car.Another analogy... Say you now pulled on the rope 200 lbs for 0.5 seconds and 0 lbs for 0.5 second... The average force is 100 lbs. However, the rope will have to withstand 200 lbs of force because of the "non-linear" force on the rope.

So, back to DC math... Power for steady state voltage/current (i.e., DC) is:

- power = Volts * Current = Watts

However, for AC, you have a sine wave of voltage and current... Turns out that for resistive loads, the current exactly follows the AC voltage wave form. For inductive loads, the current "lags" the AC wave form (i.e., voltage rises, then x Time later, the current rises). The "phase difference" between voltage and current can be measured in degrees... So, the power equation is:- Power = Volts * Current * Cosine (phase angle)

The more inductive the load is, the farther the current lags the voltage. So, resistive load, phase angle = zero degrees; AC induction motor phase angle ~ 48 degree lag; "perfect inductor" phase angle = 90 degrees:- Power = V*I*Cos = 120 volts * 6 amps * Cos (0 degrees) = 720 watts (i.e., resistive heater)
- Power of motor = 120 volts * 6 amps * Cos (48 degrees) = 482 watts (of useful work)
- Power of inductor = 120 volts * 6 amps * Cos (90 degrees) = 0 watts (inductor does no work)

So, you can see if you want a motor to do 720 watts of "real work", then you need around 1/0.67 times more current or:- 6 amps * 1/0.67 "power factor" = 9.0 amps

And that leads us to the the "VA" / "kVA" ratings of generators, transformers, etc...Basically with "poor power factor" (less than 1.0 Cosine(phase angle)), the wiring has to be heavier to carry the additional current that is not doing any real work... So that 720 watt motor would have a VA/kVA rating of:

- 120 volts * 9 amps = 1,080 VA = 1.08 kVA

Those "extra" 3 amps are not doing anything at all except heating up the wiring and generator windings.Sometimes we will talk about Power Factor as the generic term to discuss the "inefficiency" of various loads like motors and computers... PF = Cos(x) for motors, but for computer power supplies and other equipment (CFL, LED type lamps), the current wave form may not be a pure sine wave, but a series of "peaks" near the voltage peak (see jerking on rope as an example). This peaks are mathematically more complex, but can be simplified to "Power Factor" too... So, sometimes you will see a PF=0.6 or even PF=0.5 on a CFL lamp... Again, the lamp may need only 23 watts of "power", but with a PF of 0.5, it will have a VA rating of 46 VA... I.e., the generator wiring will see a 46 VA load (heating of wires, etc.), but the gas motor will only see ~23 watts of load (+any heating losses).

You have to size the wiring/genset to the total VA ratings of the load--Just like one would have to do with my "rope" analogies... If you pull from the side, you need a stronger rope to move that same car up that hill because you must pull harder if standing on the side/jerking the rope.

I hope this helps... AC Math is a highly complex and confusing subject. I did learn some of it in Engineering Classes--But have pretty much forgotten most of it by now from non-use.

Here is a good thread that discusses VA vs Watts vs PF and why it matters with generators (and inverters):

Question about

battery chargerselection withEU2000generator-Bill

100✭✭✭✭AHHHHHHHHHHHHHHHHHH. I"ll have to read that,

oh, 10-20 times to see if I can make heads or tails of that and to make it logical to me. Where did 2000 volts come from. I'll completely understand if you can't go into that "easily" within this forum. That wasn't the English that i speak every day. What I think you said was if my genny can put out 900 watts, but I'm trying to power something that's 600 watts ( but has a power factor of .50, then it really needs 1200 watts to work . Am I close???..Even if I'm right about that, then how does one calculate if 75% is most efficient or something higher. But still, where did 2000 volts come from to enter the calculation. If it's to much to type, I'll understand. I don't expect a private lesson here on this forum

17,615✭✭Not 2000 Volts, 2000 Volt * Amps (or Volt Amps if you prefer). It comes from the standard power formula of Watts = Volts times Amps. Except that the power factor throws this for a loop, giving us "real" power and "apparent" power. Your electric company charges you for "apparent" power (straightforward Watts) but charges the big manufacturing company for "real" power (Volt * Amps - the Watts with the power factor figured in).

If you understand that AC power is a sine wave with the Voltage peaking above the relative Volts (higher than the 120/240 we read on meters) and apply Ohm's law with a

fixedresistance to this then you see the current follows the Voltage curve. (Note AC Voltage actually drops to zero within this sine wave, which is why it is "easier" to disconnect AC than DC even if Volts and Amps are equal; because the AC essentially shuts itself off 60 times per second.)Now toss in inductive loading where the resistance is not fixed but varies within the time frame of the AC sine wave so that its lowest value may be at a point other than the lowest Voltage point. Therefor the maximum Amps will occur at one point on the sine and maximum Volts at another. Multiplying these two gives you the Volt *Amps, or "real" power as opposed to multiplying the averaged numbers which gives you the Watts or "apparent" power.

32,026adminIt was not 2,000 Volts, it was 2,000 Volts*Amps or 2,000 VA generator rating. Yes, the generator is also rated for 2,000 Watts peak, but only with a Power Factor = 1.0

And, your example of 600 watt load with a PF = 0.5---Yes, you need 1,200 VA genset to run that load... It is not 1,200 Watts.

- Power = Volts * Amps * Power Factor = 120 Volts * 5 Amps * 1.0 PF = 600 Watts
- Power = V*A*PF = 120 Volts * 10 amps * 0.5 PF = 600 Watts

Or, the VA rating needed to run a 600 watt load with 0.5 PF:- VA = 600 watts * 1/0.5 PF = 1,200 VA

The generator windings and the electrical wiring will "see" 1,200 VA (120 volts at 10 amps as an example). But the gas engine will only consume gasoline based on running a 600 Watt load (actual power vs "apparent power").The generator needs to supply more current and the wiring needs to be sized to support that additional current when running poor power factor devices.

AC power - Wikipedia

The rise of "electronic loads" with poor power factor has caused laws to be written in the US and Europe to use "power factor corrected" PFC power supplies for desktop computers and such.

And industries that use large electric motors do pay (effectively) for kVAH used instead of kWatt*Hours (which residential customers are billed).

For motors, you can parallel capacitor banks and "bring the current back in phase with the voltage"--and this is done a lot... Both by industry and utilities (the utilities will switch in capacitor banks during summer A/C and well pumping season).

-Bill

1,372✭✭✭Yes! You are correct.

As for efficiency, the most efficient thing to do is to load it up just as much as possible up to its continuous rating - taking into account power factor, which may mean you can't load it up to that rating.

5,433✭✭✭✭Plug your loads into a Kill-a-watt meter. In addition to measuring power in watts, it will display PF as well as KVA (or VA).

Tony

1,372✭✭✭Good advice, the Kill-A-Watt is cheap and tells you a lot. There's also another point I haven't seen mentioned yet. You need to account for the surge (start-up) load which also may mean you can't load up a genset to its max, and may require you to do some load management. You can assume most motors need triple their running watts to start - unfortunately the Kill-A-Watt can't measure surge loads, one of the few things it can't do - so 3X as a rule of thumb usually works.

Using my father's Honda EU6500is as an example, he needs a few hundred watts to run his oil water heater, and 2000 watts to run his well pump. His genset can handle 6500 watts as a surge so if he wants to take a shower during a power outage he needs to turn off almost everything else in the house and then he'll have just enough overhead to run the water heater and well pump. It can handle 5500 watts continuous, but unfortunately he can only use around 2300 watts of that capacity due to the needs of that well pump as it cycles on multiple times during a shower. After the shower is done he can turn on all his other lights, a TV, etc.

19✭<==========================0======================================>

A great heater

>http://www.northerntool.com/shop/tools/product_200307957_200307957

GGK

1,372✭✭✭The Olympian Wave series is a good catalytic heater, too.