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Purposely running a 3600rpm generator at lower speed

K-Tron

Registered
My brother needs a portable generator to run the block heater for his 7.3L ford diesel for about an hour every now and then during the winter months. I have a like new 7 horsepower 170432 Briggs powered 3kw Pincor generator that I would feel no real remorse if it was tossed about in the back of his pickup. The unit has about 30 hours of run time on it, and it like most 3600rpm generators run at the brink of destruction. Since the only thing that will be plugged into the generator is a 1000watt block heater, would there be any detrimental issues if I set the governed speed down to 3200rpm? That would save extra wear and tear on the engine, keep the noise down and shouldnt really effect the block heater performance one way or another. I do not think these Pincor contractor portable generators have any automatic voltage regulators in them. I will have to add a car muffler to this unit to keep the noise down. My brothers college will not allow him to run an extension cord to the trucks block heater, nor provide a power outlet. What do you guys think about this idea? Id hate to run it wide open with a big muffler, these engines already had trouble pushing the heat out of them.

Chris
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
If the block heater is 120 volt, and the generator can output 120/240, you could slow it down until it outputs 60/120. :brows:
 

K-Tron

Registered
I am aware of the different style engine heaters on the market. I have a Webasto plumbed into the coolant on my truck with the Detroit Diesel in it. There is no reason to install such a unit on my brothers pickup, as this is the only winter he will need a generator to run the block heater, as he will be graduating in May. The last I ran this generator, about 5 months ago, it was running 3560rpm producing 59.2hz and 132.2volts. It reached 60hz right around 3660rpm, a little too fast for me! I can certainly test it out with the said load around thanksgiving and drop the governed speed until 120volt is met.

Chris
 

Thaumaturge

In Memory Of
Age
68
Last Subscription Date
07/12/2019
No problem if a brush style generator. I typically back mine off 5% or so just to save on fuel and for the most part my loads are resistive heater elements or switching supplies good down to 90VAC input. Not so rosey with non-brush cap regulated gens. Their output falls off sharply too far from optimum frequency.

My suggestion is plug in killawatt meter or DMM and dial governor back to output reading ~112 VAC. (Note: Lower voltage will reduce heater WATT output. But not damage it. Watts = Volts x Amps. If 120 = 1000W then 112 = ~877W)
Doc
 

armandh

Sponsor
Last Subscription Date
09/02/2010
a friend had an older 190 ish diesel and a second key for the door
he just let it run [small town]
 

enginenut2

Registered
Age
78
I think the life of a briggs or most other small air cooled engines is not compromised at 3600 R/M. If you look at the engines design/advertised HP rating you will find it to be almost always at 3600. The HP will be reduced below that speed and the cooling could be less adequate. The generator designer selected the engine size to build his generator set to, claiming a realistic output that would provide a reasonable life span-his company's ongoing success depends on that. Running lower speed with lower output might extend the lifespan a little but I think more would be gained by careful attention and periodical maintenance. I think there would be no compromise in adding a premium, quieter muffler, then it could sound like a Honda.
 

richard.bessey

Registered
I will chime in, yes I think you can run a resistor heater at a reduced RPM without much trouble.
I did this same thing except I was running a fan. I learned the hardway that the burned out, turns out the fan motor wants a certain frequency of AC power, even if it is 120 volts.
 

Power

Registered
I believe that Briggs engine is splash lubricated. The owner's manual on some engines say not to idle engine or run it at less than full throttle. I suspect reason for this is splash lubed engine does not get proper lubrication at lower speeds.

In my opinion, he will probably be fine running engine at slightly lower speed. Since it will only operate in very cold temperatures, reduced cooling airflow should not be an issue.

If it is just powering a resistance heater, gen end should be fine.

Eons ago, when I was in Marine Engineering school, we had formulas which I have long forgotten for calculating anticipated life of a diesel engine. Formulas were based on horsepower hours. RPM was one of the variables. If the RPM was reduced while holding load constant, engine life increased. Per formula, a 10% reduction in RPM at same load substantially increased engine life.

I suspect a 10% reduction on a gasoline engine would also substantially increase engine life. Without specific manufacturer's advice, 10% is the most I would consider on a splash lubricated engine.
 

Kevin K

Subscriber
Last Subscription Date
07/12/2019
I suspect cooling would be more of an issue than splash lube by running a small engine less than 3600RPM. The small fins on the flywheel can't push enough air for effective cooling with the engine running at low speed. I typically run my lawn tractors at 1/2 to 3/4 throttle (but not under a heavy load!) and have never had an issue.

A lot would depend on the generator. A generator with a voltage regulator or a brushless capacitor regulated generator would never tolerate this. A brush type generator with a saturated pole or harmonic winding design may produce usable power at a lower speed. It would be an interesting test at any rate.
 

DustyBar

Subscriber
Last Subscription Date
01/14/2020
Watts = Volts x Amps. If 120 = 1000W then 112 = ~877W)
Doc
Actually it will be something less than 877 I think. From the rated wattage you can calculate the heater's resistance. Then apply that resistance to the lower voltage and I'm pretty sure it will be less than 877. Sorry I'm not sure of the formula but from experience I've seen that. Not that it matters...just a note that is the way it works.
 

Fred M.

Registered
I think the problem with running the generator slow, is that the power available may be proportional to the square of the speed. If this is correct, running a 1000 Watt generator at half speed, you would only have 250 Watts available.

For a specific load, you could probably find an ideal speed, so power available is equal to the demand.

Fred
 

gnucklehead

Subscriber
Last Subscription Date
07/13/2019
Dropping to 3200RPM shouldn't be a problem for the engine.. what voltage comes out of it depends on the type of voltage regulation.

This is the perfect use for a smaller generator, say 3HP Briggs or Tecumseh, or dinky 80cc Champion, just stick to a 4-stroke.. cheap, lightweight, quiet, expendable.. For teeny-weeny engines, 3600RPM really isn't that big of a problem :shrug:
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
Engine cooling should not be a problem at lower speeds,, nor should be oiling. Starting an air cooled splash engine in cold weather is. You will need to use a 5W-30 or even a 0W-20 if extremely cold, to prevent bearing starvation. SAE 30 will not work in a splash system under 40F. Certainly, running the gen end at lower speeds will cause cooling issues, especially if loaded. As noted, running at lower speeds will lower voltage and available amps. The heater will not put out the rated heat either, with the reduced power. In that case, reducing speed of the engine is self defeating - the block will not be heated as needed, and the gen set will not do as designed - make enough power to make the heater work as designed.
 

turtmaster

Subscriber
Last Subscription Date
10/03/2019
If the voltage is a bit too low at 3200rpm, why not use a ~24v ac step down transformer wired in "boost" configuration.
 

Newoldstock

Registered
Just make sure there are no motors involved.

Years back I remember the parking lot at Pamour mine had big warning signs not to use a car heater only plug in your block heater.
But up in the near north there was always someone trying to run a 60hz interior car heater on 25 cycle and things would go wrong and burn up
 

csareb

Registered
You can definitely safety idle it down to 3000 rpm for a resistance load like a heater. The exact same engine is used on generators sold for 50hz countries and those run at 3000rpm for 40hz. Granted the generator head is wired different for 240v 220v etc but the engine is identical. You will have low voltage at 50hz but the engine will be fine.
I wouldn't go lower than that for cooling and oiling reasons
Set governor for 50.0hz with. A 1500w heater connected. That way it will have headroom to run his 1000w heater without getting under 50hz or 3000rpm
 

M.Canute

Registered
Engine cooling should not be a problem at lower speeds,, nor should be oiling. Starting an air cooled splash engine in cold weather is. You will need to use a 5W-30 or even a 0W-20 if extremely cold, to prevent bearing starvation. SAE 30 will not work in a splash system under 40F. Certainly, running the gen end at lower speeds will cause cooling issues, especially if loaded. As noted, running at lower speeds will lower voltage and available amps. The heater will not put out the rated heat either, with the reduced power. In that case, reducing speed of the engine is self defeating - the block will not be heated as needed, and the gen set will not do as designed - make enough power to make the heater work as designed.
You are certainly correct about what oil viscosity to use.

Years ago I had a small Toro Snow Hound snow blower and one not too cold winter day I decided I should do an oil change on it so after using it for a half hour I whipped it inside my shop and pulled the drain plug. I was expecting hot oil but to my surprise it was just lukewarm and you know how cool it had to be to feel lukewarm after I'd been outside blowing snow! This was a replacement B&S engine designed for operating in the summer months so it had the normal type of cooling fins you'd expect to see on a small air cooled engine, but because they were designed to cool an engine on a hot summer day, they actually over cooled the engine on a cool winter day. Long story short, I think you won't have to worry about a generator engine overheating in the winter time but you might have to worry about it over cooling!
 

ulgydog56

Registered
don' forget the dynamo they are also rated to turn at designed speed, you can burn it out running it at lower speed, stay within spec, that's how it was engineered...….:bonk:
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
If it has no AVR it can be run at any slower speed you like. Too slow and it won't build up though. 1800 rpm would probably be too slow, but I'd bet it would work beautifully at ~2400 rpm.
 
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