generator output

[dugw]

i have a 4200 watt craftsman with 7.8 hp generac engine. what should the output in volts be with no load on the generator? i had 104 volts ( the auto idle feature quit working). i bumped up the idle until i have 112.6 volts now without a load. i am replacing this gen. with a bigger one and thinking about selling this one or taking it to keep at a parents home to use. it is 13 years old and i figure it has around 120 hours on it. does anyone know the output at no load or do all gen. vary on this. i won't be able to test my new one until later on in the week when it arrives. sorry if i posted this in the wrong forum.

 

[Quest]

The proper way to test is to put a load (e.g. a 60watt filament light bulb) in order to get the output voltage reading properly. Try getting a voltage reading off of a no-load generator is downright silly. Q.

 

[dakota99]

My 5000 watt generator varies. With no load, right around 125. 1/2 load brings it to 120 and full load if i remember correctly is between 115-120. Like the others said, put a load on it first.

 

[eljefino]

I'd put a couple hundred watts of incandescent bulbs on and check again.

 

[tig1]

The volts may vary a bit but, 60 cycles can't vary much.

 

[boraticus]

I'm with dakota99. Look for 125 volts at no load. 104 volts with no load is far too low. You had better find the cause for the low voltage before you put it to work.

 

[AandPDan]

Don't worry so much about the volts yet. It needs to run at 3600rpm for a 60 hertz output. Check that first then check the voltage.

 

[dugw]

well i have to get the carb. reset to where it was. i plugged in a light and it stayed at 112 volts. i plugged in a heat lamp and the engine started to sputter like it was flooding. too cold and dark to mess with it anymore tonight. where is a good starting point for the screw on the carb.? the only other adjustment looks like a gov. which i turned a bit to idle a little more. guess i should have left that alone.

 

[Panzerman]

I replaced the governor in my 5000 watt generator, it has a 10hp Briggs on it. I adjusted the governor to put out 125 at no load. I didnt even test it with a load. I plugged two electric heaters into it and they should have loaded it to about 4000 and it didnt miss a beat. I would think you want to get as close to 120, out of the plug as you can, 104 is way to low.

 

[Cujet]

I don't know the design of the generator head. But some were designed with a Z winding that produces more current/voltage to excite the field, as the load increases. It's a "classic" design feature and often eliminates the need for a voltage regulator. However, 104V seems very low for any design at zero load. You may have a rectifier going bad, the RPM may be too low (not 60HZ) or a voltage regulator going south.

 

[1 FMF]

do you have a model # and manufacturer of the generator head? Can you look up a wiring diagram or manual to the generator head? For 13 years old, I assume it does not have fancy electronics and I'm kinda surprised you said it has an auto-idler. Tuning or adjusting engine rpm will have no affect on AC voltage, this is taken care of by the voltage regulator internal to the generator if it has one. There are various generator head designs, I would guess yours has a voltage regulator and is the cause of the low voltage problem, assuming the engine is running at 3000 rpm or better. If it's running less than 2000 rpm then that's your first problem- most generators run at 3600 rpm all the time. The crankshaft to generator is geared in half so the generator armature spins at 1800 which results in 60HZ frequency of alternating current.

 

[JimPghPA]

( the auto idle feature quit working) is probably a good clue about why the voltage is low at no load. If it idles back with no load, it could be that with the idle backed down both voltage and frequency are NOT expected to be proper.

 

[boraticus]

"most generators run at 3600 rpm all the time. The crankshaft to generator is geared in half so the generator armature spins at 1800" Is this correct? Most generators I've seen appear to be coupled directly to the engine output shaft. Not arguing, just curious how the gearing would work. Do they use a planetary gear to step the armature speed down?

 

[Quest]

The formula as follows: N =120* (freq / poles) where N is the number of rotations. So, in order for a direct-coupled 2-pole generator to provide 60Hz, you will have to run N = 120 * (60 / 2), which is 3600RPM. 4 poles are more expensive to produce, but will lower the engine's RPM by half (1800rpm, direct-coupling). Additional gearing will introduce cost and mechanical complexity (thus reliability issues). Q.

 

[1 FMF]

Originally Posted By: boraticus

"most generators run at 3600 rpm all the time. The crankshaft to generator is geared in half so the generator armature spins at 1800" Is this correct? Most generators I've seen appear to be coupled directly to the engine output shaft. Not arguing, just curious how the gearing would work. Do they use a planetary gear to step the armature speed down?

sorry, i had a brain [censored] while i was typing. I was looking up generator heads at the time and was reading about diesel engines connected to generator heads via belt pulleys, more of a diy setup. A diesel motor would run at full power at 1800 rpm, and the pulley ratio in this case would be 1:2 to get the generator head spinning at 3600 rpm. the generator heads when they are 2-pole (single phase) need to spin at 3600 rpm per the equation Quest posted above, that gets you 60Hz frequency which you need for AC power. and they usually are because they are cheap and easy to produce, and also why they are direct coupled to a gas engine rated to run at 3600 rpm. So basically disregard everything i previously posted, except for checking the make/model of generator head that's being used so you know what you got and what's required.