PTO generator

[DarkBlack]

Correct (for a 2 pole generator head, a 4 pole head would only need to be spun at 1800RPMs), but I was talking about switching the tractor to use the 1000RPM PTO setting (if equipped) with a lower engine speed to power the generator at 540PTO RPMs. When you do that, you lose 50% of your PTO HP (+/- the difference in the torque curve between ~2600 engine RPMs and ~1300 engine RPMs.

Aaron Z

More poles =more cost. An 8 pole genny would run off the gearbox with half the engine rpm's, but there's a big problem. Tractor rpm's are not regulated very tight. Everytime you halve the rpm's of the diesel you double the frequency variation of the 60hz coming out. That would be compounded by the effect of having less HP at those rpm's. You would likely end up with an unacceptable wandering generator output.

 

[fred239]

And just one more thing to consider, folks: As of right now, there is no gasoline in the disaster area and the natural gas lines are blowed out.

Here in Michigan the past few days, the residual Sandy winds took down a lot of electric lines (including my district). The one station that is open has diesel only and I'm running my 50kw Winpower off of my diesel F-935 lawnmower (60hz., 126.2V at the moment). U-verse is up so I have TV, Internet and landline phones. I even vacuumed the first floor, laundered the hockey uniforms and the new fridge made some fresh ice cubes. And, it snowed yesterday! No gas, no juice, no heat, no water, no flush. Mamma's NOT happy.

Wow, 50kW is a big unit! How many horsepower does your F-935 put out? I assume you are not using 50kW of energy, any idea what your average load is and how much fuel you use? At $4/gallon deisel gets spendy. Has anyone compared the economy of propane vs. deisel, dollars per kW hour?

 

[GPintheMitten]

Maybe its a typo...5kw?

 

[Rob-D]

Rob look at your pic of the starting current notice the first peak is 600% the FLA. Here is a quote from your link. "a. For full voltage (DOL) starting, starting kVA= 6*rated kVAod the motor . Use the nameplate LRA data or Code letter where available."

Two things:

First the illustration shown on the drawing is not linear. The starting impedance the generator sees is for a much shorter time than the load. Getting the generator up to speed from zero rpms is more of an aspect of inertia that represents a load to the motor:

Newton's First Law:
A body at rest tends to stay at rest.

Secondly, the generator voltage dips when the motor starts so it never sees that full 600%. (that chart represents a constant voltage) That's why generator manufacturers state the 300% or 3 times, below the drawing it states "150 to 300%" reserve. Even the statement above says "for full voltage starting".

 

[Rob-D]

While understandable, I'm afraid this is somewhat incorrect. A number of motors draw their locked rotor amps for a short time period at start up. Classic and common example is heat pumps. It's also true of some table saws, which has caused me joy over the years. My sawstop wired at 120 regularly trips the 20a breaker - surge is briefly 80 amps, per the engineers at the factory. Milliseconds, but still would stall out with a little 5k generator.

You can get around LRA with thoughtful staging of equipment starts - I have 3 HVAC units, and I bring them up big to small. But LRA is written on the plates for a reason (note, this is a purported advantage of inverter drive HVAC, no LRA issues)

You're overlooking the fact that generator output dips at motor start up. The question here is how much overhead do we need our generator to be able to produce? 3 times max, ask any generator manufacturer or any PV installer. (inverters dip at start up also). locked amps is not an issue, in fact read the definition in my post; "This will cause the motor winding to overheat and be destroyed if the current isn't quickly reduced or removed."
A locked rotor is not instantaneous, instantaneous is absorbed by generator dip at start up. Watch the lights dim when a motor comes on. What do you think is happening? Even your house power drops on a heavy initial load. Do you think that motor is seeing locked amps? I don't!

I'm trying to keep this basic for those here who want to know how much overhead they need in their generators for motor starts.

 

[PineRidge]

Do any of you know if these PTO generators need to be started occasionally? I have heard conversations elsewhere that say something to the effect of long storage without starting causes the generator to fail to make power.

 

[Rob-D]

Probably not, a lot of farmers use them here for milking machines and they just sit under a wood overhead outside until they're needed.

 

[PineRidge]

Thanks Rob......

 

[aczlan]

Do any of you know if these PTO generators need to be started occasionally? I have heard conversations elsewhere that say something to the effect of long storage without starting causes the generator to fail to make power.

It can. If that happens you need to "flash the field", see: PTO Generator Instructions or http://www.voltmasteramerica.com/service/troubleshooting.pdf for directions.

Aaron Z

 

[PineRidge]

It can. If that happens you need to "flash the field"

Aaron Z

Last time I did that Aaron was at a football game when I was 17. Almost got arrested. :laughing: