1800 Rpm vs 3600 rpm Same HP?

[Shadow]

I have a 5000 watt Generac belt driven generator, while getting 4 inch pulley made a fellow commented it will take a few hp to run this generator. I never thought about it at the time, but could it be it takes more power to run 3600 rpm vs 1800 ?
        With my 6/1 running at 650 -652 rpm (using a laser tach) when I plug in a 500 watt halogen light(560 watts according to the watt meter) I immediately lose 10 rpm.
      By plugging in another 1000 watt load (heater) the engine slows and really labours. 1600 watts and I'm down to 600 rpm. In order to maintain 650 RPM I have to start unloaded at 690 rpm!
      Unloaded the generator is putting out 130 volts at 62-64 hz. With light plugged in it lowers to about 127 volts 60-62 hz.In order to lower those numbers  I'd have to slow the engine down.
     Sooo... Engine issue? or generator issue?  With 560 watt load the engine ran at 650 rpm for 3 hrs tonite, temperature right about 170, no thermostat. Thermosyhponing into a 20 gal tank of water.

[oliver90owner]

Hi, you wrote: 'I never thought about it at the time, but could it be it takes more power to run 3600 rpm vs 1800 ?'

5kW is 5Kw whether it is DC, AC, mechanical or heat.  The speed will make no difference to that.  What might make a small difference is the bearing and belt losses, etc. 

Your machine:

An overly large generator may only be at it's most efficient somewhere near full load - advertising specs often don't give too much info on this.  Think about it; no load at all, you are still driving the machine so efficiency is zero.  Full load, you still have the same drive losses but they are relatively insignificant as a %compared to low load. Remember, for instance, the cooling fan on that generator is designed for 5kW output, so that will always be oversized. 

All these things are relatively small but are all additive: 4 lots of 5% mean a whacking 20% in total - that's 1.2 of your 6 .5 horses! Now 20% of 5kW is a 1kW loss ; that same IkW lost out of 3kW is 33%.  See my drift? Figures may not be right for your situation/installation but the similarity will hold true.  Remember, we only have power losses, no gains from input to output!

it is generally accepted that one needs about 10 HP to drive a 5kW generator (2HP/kW).

Hope this helps.
regards, RAB

[ronmar]

Yes, it will take more energy to spin that generator at 3600 RPM.  But probably your biggest killer will be the reduction in available torque to maintain RPM.  650 engine RPM to 3600 RPM is about 6 to 1. You have divided the torque output of the engine by 6, but you have also shortened the time in which it is applied to the generator by a factor of 6 (RPM multiplied by 6) so the net HP applied to the generator should be about the same 6HP minus the losses of the drive system(and that absorbed by the faster spinning rotor).  But with a power stroke once every 12 generator revolutions, you are probably way behind the power curve and the engines ability to make up RPM under load. Kinda like a truck with a 5 speed gearbox trying to climb a steep hill in 5th gear.  You have a certain ammount of HP, but because fo the gearing, you are behind the engines ability to deliver it, even at full throttle and RPM's fall off.

Your engine may also not be making full HP.  How old is it?  How is it setup(squish and injector timing)?  Is it broke in?  Are you at any appreciable altitude? You loose about 3% per thousand feet above 1000 Feet.  What type belt are you using? "V" belts use a large ammount of energy, especially at high speeds.   

There are reasons you don't see 650 RPM singles driving many 3600 RPM genheads...

Ron

[xyzer]

Don't forget the govenor! A lazy one will make you think you have a big load! It should maintain RPM till it doesn't have the power anymore.
Dave

[lowspeedlife]

I agree with xyzer, your dropping rpm immedeatley upon introduction of load, have you made the changes in govenor linkage most everybody feels is required on these engines? if not i would suggest a search to find these threads. scott.

[gpkull]

you have no chioce but to turn the generac head with a shaft speed of 3600. you can hook up any combo you want you stiil have a 2pole head that has to turn 3600 to maintain 60 hz.  to get 60 hz from a real gen head turning 1800 shaft speed it needs to be a 4 pole. the more poles the less shaft rpm to keep freq in line. also you usually see the lower shaft speed (more poles) grows more heavy duty. lots more iron and made to generate long after the cows come home and at max rate or above for long periods of time.

[gpkull]

sorry i have another bit to throw in slip rings and brushes last longer on slower turning rotors

[rcavictim]

When that 3600 RPM Generac sheds the armature wire from centrifugal force and makes a fuzz bass inside it is going to be very hard to turn and the efficiency will drop to zero.  I wouldn`t waste my time with a generac.  I have one here did that and have seen two others.  Slow and steady wins this race.

Edit,  You are probably wondering what the hell a `FUZZ BASS` is.  It is apparently a brain fart.  I meant to type FUZZ BALL.  My apologies.

[Shadow]

Thanks for all the comments. Yep going to get a different generator,but this one is working temporarily to break in my engine and get a few bugs out.Its been about a 600 watt load on the engine while we get our cooling system working.I plan to move that 5000 watts about 350 feet to the house, so need all the watts possible to start out as I'm sure I'll have some losses along the way.

[gpkull]

a 6/1 should be good for 4k or a little less you also have to figure powerloss. this maybe the innefficentcy of the gen plus the drive to th gen head and maybe more. the generac head is a good example of a home owners special. it is only that no more. dont get me wrong its a good thing when they work but they are not designed for long life at a work hard rate (duty cycle) and the price of said gen reflects this directly. if this set up is working for you keep on keepin on wattch your load and carry on smartly thats all folks beam me up scotty

[BruceM]

Shadow, I concur with XYZers assessment.  It sounds more like you need to try some different governor springs to keep your engine speed better regulated.  A softer spring, stretched longer, will perform  better.  If it's too soft then you get "hunting".  Then try another spring or add a second even softer one.

As for your 350 foot distance...you're going to need some big wire (maybe 4 guage or larger for 120V)  unless you're going 240 volt all the way (then 6 guage would do it), and stepping down to 120 via step down transformer.  A bigger generator won't help with the voltage drop in the wire.  If you adjust the voltage up for a heavy load, compensating for voltage drop in too small wire, then you'll have a dangerous over voltage situation in a light load.  There are some regulating step down transformers, but I've never tried them.

My setup has the "House of Lister" about 400 foot total from my shop, about 370 to the step down transformer, with a 1/2 hp well pump (240V) en route.  6 guage wire all the way to the transfomer, then 8 guage to the shop.  Minimal voltage drop.  Sure glad I bought all that copper 18 months ago.

Best Wishes,
Bruce M
Metro 6/1
Snowflake, AZ

[oliver90owner]

Bruce M,

I like your step-up and step-down transformer set-up.  However, we are into cost effectiveness again.  You will certainly have reduced your line losses with a smaller transmission cable cross sectional area.  However, as efficient as they may be, your two transformers are going to lose you power.  It may only be a few percent but it is there.  Yes, you have the correct voltage but you have a little less power!

Without details of capital costs, actual resistances, current values, labour costs and incidentals it is impossible to calculate the better way to go.  You obviously got your transformer pair at a good price.

For a short run, a much thicker cable is the best answer - nearly same labour costs, simpler installation and no transformer losses.  Where the break even point occurs is another matter and each installation needs to be properly assessed.  As you say, an excessively long run with a high loss cable is going to be a problem at low- or high-power!

Another alternative might be running more than one cable in parallel - you can often get reclaimed lighter duty cable at a good price from a recycler.  Again just depends on the situation....and may be subject to local rules and regulations.

We don't have as much of a problem with a 230-240 volt AC supply, on power losses - but we need to keep separated from the mains better than with a 120V supply!!

Regards, RAB

[Doug]

I have considered a pair of 5 kva transformers to boost my voltage from 240 to 600 then back down to 120-240.

Its easy for me get a pair of very common Hammond  transformers, its also easy for me to get several hundred feet of tech 90 #8 or #6. Choices are based on availability of wire and or parts and how horrified your insurance adjustor and local inspector would be to find a 600 volts system in a residential home.

The back of the electrical code books are formuals and table to calculate line loss and wire size for very long runs of cable.

Doug

[BruceM]

RAB, Of course a transformer represents a loss, but I'm only using one, to step down 240 to 120. I don't run the neural.  My step down transformer is a toroid (surplus bargain), though efficiency is as high as transformers get (this one something like 88%).  Stepping up and then down again would be twice the transformer losses and only makes sence for a very long run, and then 480 volts or more would be more cost effective.

If you will price copper for long runs (>300 feet), you'll quickly see why 240V for a long run makes sence.  Multiple smaller strands wouldn't pass inspection here, but I'd do it if I found a surplus bargain.  There's nothing wrong at all with the "fire hose" approach (BIG wire) if you can afford it or can find a bargain.

[Holter]

Just get aluminium cables. They cost only half of copper cables. of course they are more difficult to install, but once in the ground you cannot tell the difference
As for the voltage drop, it might be interesting to let the AVR regulate the voltage not directly at the generator but in the house with a thin second cable.
But I would not want to bury a small cable, if you want to have more Power later with a big cable you have the option to work with a transformator later instead of digging the trench a second time.
And just calculate your electricity costs, even with free wvo you will save a lot if you do not lose some % on the cable/transformator.


Christian