PTO generator

[techman]

The motor is a 3/4 hp motor. 3/4 of 746 = 559 watts. It was drawing 362 watts running with just a grinding wheel. The hot steel was due to the fact that I was pressing on the grinding stone hard in an attempt to load it down (it did slow) to show the current under load. The 3/4 hp motor I tested came in at 362 watts running and under load it came in at 850.

You have completely missed the point of the test.

We don't care what the motor efficiency is, we are talking about generators starting motors and generators dip when motors start. What are we talking about here, an industrial application with high hp motors running at 440 volts? No people are using PTO gens to run their homes.

Sizing Gen-Sets For Large Motor Starting | Basics content from Electrical Construction and Maintenance (EC and M) Magazine

"Let's look more closely at a motor start. Induction motors have typical starting characteristics. The curve of motor current versus speed shows that during starting, the motor draws approximately six times its full load current; this current remains high until the motor reaches about 80% of speed. This high inrush current causes a dip in generator voltage. The electric power initially required by the motor (with the motor at standstill) is about 150% of rated power. The power required by the motor peaks at about 300% of rated power and 80% of speed with full voltage applied. But, the generator set supplies less than 300% power because starting voltage is lower than full voltage during acceleration, and because the generator set's rotating inertia transfers energy to the motor."

So you're wrong, 3x is all we need. It's an accepted standard. I employ it here and I'm off the grid running inverters and calculating using 3x for years. I haven't blown a fuse or stalled a motor. So if you think that you need 100x overhead than build it into your system, I don't give a fiddle. But tell me how long you've been running off the grid?

Geeze, how many links do I have to post showing this?

You want to be argumentative, find someone else, this is getting silly, the question is how much overhead do we need to start motors.

3 times! How many more links do you need to see?

By the quote referenced, 300% at 80% of speed does not support your argument. At the initial startup, from a dead stop, the current can indeed exceed 300% of FLA. Also one is not interested in total power, it is instantaneous current that can be the limiting factor. Above a certain current, the generator field will saturate, limiting available current. The worst case scenario is with split phase motors. The high starting current limits acceleration and at the same time lugs the generator voltage. The motor never accelerates enough to trip the start switch so the motor chuggs at low speed and high current, the gen set is pulled down and if an overload does not save the day, you could fry the generator or the motor or both.

Bottom line is that 3x FLA is a good RULE OF THUMB, but peal currents at initial startup can definitely exceed that value. Just look at the motor specs I posted earlier. Will the motor start if less than the typical peak is available, most likely it will, but a few may not.

paul

 

[Rob-D]

By the quote referenced, 300% at 80% of speed does not support your argument. At the initial startup, from a dead stop, the current can indeed exceed 300% of FLA. Also one is not interested in total power, it is instantaneous current that can be the limiting factor. Above a certain current, the generator field will saturate, limiting available current. The worst case scenario is with split phase motors. The high starting current limits acceleration and at the same time lugs the generator voltage. The motor never accelerates enough to trip the start switch so the motor chuggs at low speed and high current, the gen set is pulled down and if an overload does not save the day, you could fry the generator or the motor or both.

Bottom line is that 3x FLA is a good RULE OF THUMB, but peal currents at initial startup can definitely exceed that value. Just look at the motor specs I posted earlier. Will the motor start if less than the typical peak is available, most likely it will, but a few may not.

paul

I looked Paul, but empirically I have never had a motor or generator problem in several years using the 3x ratio, nor have I heard of anyone else having a problem. As the gen set drops the motor rises enough to start. I've never seen it come even close, I've seen lights dim but the motor always starts. You could use a gen set with 6x overheard but now you have a system that uses more fuel. I just heard that there are people stil without power from Sandy, every ounce of fuel saved means longer run times. I say get the smallest gen set you can to do the job.

OK, how much overhead do you and Larry think is needed?

 

[Rob-D]

Well put and I agree fully.

Really?

Than how much generator overhead do you think the average homeowner needs to run the motors, appliances, etc. in his or her house?

Next question, how long have you actually been doing it on a continuous basis?

 

[Shimon]

A PTO generator was one of the first implements I purchased as we lose power often. I mounted it in a corner of my shop and put in a removable hatch so I can back up the tractor to the shop and put the PTO shaft through the hatch. I also put in a multimeter on the wall so I can adjust the throttle and read the frequency (and amps/watts/volts) while standing near the tractor. The generator has served us well and will run the whole house (hot tub included) at 13kW. Plus we have a 500 gallon tank of diesel since we have a diesel furnace so fuel is never really an issue.

 

[sdkubota]

Sweet set up.

 

[DarkBlack]

A PTO generator was one of the first implements I purchased as we lose power often. I mounted it in a corner of my shop and put in a removable hatch so I can back up the tractor to the shop and put the PTO shaft through the hatch. I also put in a multimeter on the wall so I can adjust the throttle and read the frequency (and amps/watts/volts) while standing near the tractor. The generator has served us well and will run the whole house (hot tub included) at 13kW. Plus we have a 500 gallon tank of diesel since we have a diesel furnace so fuel is never really an issue.

QUOTE]

A tractor "glory hole"!:laughing:

 

[DarkBlack]

Well, Im less confused about where youre wrong than you are. You seem to lack appropriate attention span on pertinent parameters. This is going nowhere.
larry

Early in this thread I quized him on some of his posted 'expert' material to feel out, if he really understood the very text he was posting. Questions anyone, who has been in the businees, would know.
Let's just say... that he didn't even know that was, what I was doing...

 

[SPYDERLK]

Great, denigrating the conversation to insults. Why don't people realize that ad hominem attacks just reflect on their own character?

As for my attention span, I'm bored, to busy to look back and the issue is a moot subject to me after link after link showing that 3 times overhead is the accepted standard.

I try not to recriminate.
...larry

 

[GPintheMitten]

My 50kw Winpower is no typo. It could punch out 50kw continuosly with the JD 4020. My F-935 is/was 22 hp when new. But, my demand is not 50kw, its probably less that 6kw. No matter, the F-935's motor can deliver enough kinetic energy for the Winpower to convert it to electricity for my use. If my demand was for a higher electric power than is equivalent to the 22 hp on the mower, then it would brown down. Or I could throw my JD 1070 back on (35 hp) and see if that's enough motor (18kw load). But the alternator is wired to handle the current equivalent of 50kw service in case I need to run a small town. Then I would need the 4020 (100 hp?) to deliver the equivalent engine hp.

When I bought the Winpower at a farm auction, all the buyers were told it would take 100+ hp to run it. But I was not going to dry corn, I wanted a furnace, well, lights, fridge, TV/radio and a battery charger. So I was the only one who bid after a lot of discussion and opinions by the know-it-alls : $600. I can still hear the auctioneer yelling: "SOLD. The auction is today folks!"

I stand corrected. I didn't know you could run such a large generator on a lower HP pto.

 

[RoyJackson]

A PTO generator was one of the first implements I purchased as we lose power often. I mounted it in a corner of my shop and put in a removable hatch so I can back up the tractor to the shop and put the PTO shaft through the hatch. I also put in a multimeter on the wall so I can adjust the throttle and read the frequency (and amps/watts/volts) while standing near the tractor. The generator has served us well and will run the whole house (hot tub included) at 13kW. Plus we have a 500 gallon tank of diesel since we have a diesel furnace so fuel is never really an issue.

Nicely done!! And has given me some ideas!