PTO generator

[Rob-D]

Guys, give it a rest. You are way past convincing the other guy of anything and we ate not going to choose a winner between you. Jeesh.

Good point, I'm done.

 

[wrooster]

day 9. :hissyfit:

 

[SPYDERLK]

That's locked rotor amps! What happens when you lock a rotor on a motor? You blow the breaker! It's not the same as starting amps.

Motor Calculations | NEC content from Electrical Construction and Maintenance (EC and M) Magazine

" Motor locked-rotor current. If the rotating part of the motor winding becomes jammed so it can稚 rotate, no counter-electromotive force will be produced in the motor winding. This results in a decrease in conductor impedance to the point that it痴 effectively a short circuit. What痴 the result? The motor operates at locked-rotor current, often six times the full-load ampere rating, depending on the motor Code Letter rating [430.7(B)]. This will cause the motor winding to overheat and be destroyed if the current isn稚 quickly reduced or removed."

Electrical Motor Calculator

Plugging in 1 hp , 90% efficiency, 85 PF (power factor) and 230V to the calculator in this link.

You get:

1 phase = 4a.
FLA = 5A

What does FLA stand for? Full Load Amps! At full load the motor draws 5A, 230V * 5A = 1150W; 1 HP = 746W FLA for a 1 HP motor = 1150/746 or 1.54 times.

Now look at the chart for starting amps, what does the FLC state? 150% to 300% of running current and what's 300%? It's 3 times, exactly what I said. I work with this stuff everyday my systems are built on these calculations. Are you off the grid running several inverters on a daily basis?

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)

Im glad this is finally being straightened out. All motors draw locked rotor current at startup because the rotor is stationary at that time. This is a very short term event that you will need a peak holding meter or an oscilloscope setup to capture because the current drops quickly as the motor starts to turn and continues to drop until it reaches speed. It is on the order of 4x or more of full load current. If this current times the Voltage exceeds the gen rating the gen voltage will buckle immediately. If it drops too much a motor with load wont start. A well motor starts without load , but immediately begins to develop an increasing load as it speeds up. This situation allows reliable starts at only 3x and lets you get by on a smaller gen.

Also, but not in your post, any suggestion to use 746 input Watts as equivalent to a 1HP motor will mislead the unwary. Virtually all motors use more than 1KW per HP. Actually it is a very good motor that can achieve that efficiency. And, as above, even a lightly loaded 1HP will still exhibit the high locked rotor initial current 4x above nameplate. Should this cause gen V to buckle it will cause big problems with other motors that may be running. Refrig or AC compressors may stop and take down the system. Multiple motors running simultaneously and cycling on gen power are going to cause an issue with systems not having ample surge capacity. Thats why its so good to oversize the gen while sizing the engine to supply the running load. The gen doesnt buckle and inertia controls V drop thru the surges of startup loads.
larry

Good, now tell me how much starting motor reserve generator manufacturers recommend? How about 3 times. Now let's take a motor with 90% efficiency, 746/.9=828.9 watts. OK, now let's take some empirical measurements:

3/4 HP motor FLA 11.4 amps. Running a grinder: actual watts used 362! (only load is the grinding wheel) Now let's put a heavy load on that grinder with a chunk of steel: 850 watts and that's with the steel getting to hot to hold.
So what exactly are we talking about here?
You want to put a scope on it to see the instantaneous power? Are you kidding? I'll tell you what, I have instruments here that can measure down to nanoamps, don't let me embarrass anyone with more real life tests.
And a locked rotor is not starting inertia from zero rpms, go back and read my post on how locked rotors blow fuses.

How about 3 times is all anyone needs to know.

Wrong on virtually everything:
1] Find a single phase motor with 90%. Perhaps impossible but at best extremely unlikely,
2] 11.4 X 120 = ~1400 W ~ 2 HP ... 0.75/2 = 38% efficiency. But maybe thats a hi service factor motor and the quoted amps are at its top capability. If SF 1.5 [dont think youll find higher], then motor will output 1.5 x 0.75 = 1.125HP MAX. It probably would do this at SF x FLA = 17.1A ... but to give your scenario the best credence possible lets say it puts out 1.125HP on the ~2HP wattage input represented by 11.4A. Then the motor efficiency is 1.125/2 ~ 56%,
3] Hot steel is irrelevant. Even a small piece of pre hot steel, big enuf to lean on will stop a 3/4HP grinder ... but since grinder motors start slow they dont draw hi in rush - maybe only 15A. Youre saved here, accidently,
4] Are you kidding? is a diversion. Its nice that you can measure nano A, but we can assess this close enuf with one billionth of that resolution. Lets stay on subject. Locked rotor current is the current drawn when the rotor is stationary. It occurs almost instantaneously when power is applied and subsides as the motor starts. With a hard supply Voltage such as available on AC mains, yes, the locked rotor current will blow a breaker ... eventually. Breakers will normally sustain several times their rating for at least a second. By that time the motor should be started,
4] Hard start motors such as used on compressors, washing machines, pumps, general utility, etc, pull 4x+ their FLA in the 1st instant of starting. If this resulting current is above the capability of the gen its voltage drops instantly reducing start torque, perhaps below what is needed. And at the same time reducing run torque available on any motor already running. Such motor slows a little and draws more current. In such a case its likely everything will stall.
5] 3x is not all one needs to know because some motors are employed at their HP rating and start under load. Those motors require full voltage during start. This is trivial on AC mains. A much different situation with a genny sized at 3x. NOT enuf. 4X or more is needed there or the motor does not start.
larry

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?

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

Great, denigrating the conversation to insults. Why don't people realize that ad hominem attacks just reflect on their own character? Thats fine, however I would fall into a recriminatory stance were I to answer you on this. I try not to recriminate.

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.

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

1) Depends on the house and what he is going to run.

2) For only about +-30 years. I hook up my house generator every time I lose power. I also hook up emergency generators industrially ranging in size from a 30KW that I hooked up today (very nice Cat unit) to a 1.2 Megawatt generators that is powered by a V16 Cummins diesel engine(my biggest so far and I hated the cables they were heavy).

Rob-D Quote> "every ounce of fuel saved means longer run times. I say get the smallest gen set you can to do the job."

We have some smart, experienced guys here, hats off to all. Without getting in the debate, I personally beleive smaller is better, as in more ecconomical in a crisis. So how do I size for a 25hp hydraulic elevator rated at 63.5amps/208volts? My electric usage averages 8kW, but my maximum demand can be between 14kW and 21kW measured and billed monthly, including lots of constant on flourescent lighting, a big circulating pump and a forced draft boiler. An electrician told me 1.5X RkW would be enough to start the elevator, or about 20kW, although often my max demand is only 14kW in a given month. If historical max demand has been 21kW, could I get by with a 22kW 3phase genset? Fuel is precious in a crisis.

What condition does the elevator motor start under. Is it just initially driving a pump pumping unpressurized fluid, then once started the fluid is valved to drive the elevator? Or does the elevator move as soon as the motor does? ... Essentially, does the motor start under load?
larry

You're a question behind:

What motor starting value would you use to size a generator in your home? 6 times? Overkill, you're wasting fuel!

I would size it according to what kind of loads it was running because that would give me an idea of what surge combinations may come into play and how sensitive the composite setup would be to any brownout caused by a surge. The question for fred was to help pin down info in this vein. It should be noted that this is a 3phase setup and may also be driving 120V or 208V single phase loads of unknown torque demand. The 3phase loads have less surge. The others, proportionately higher but not known from his post.
larry

 

[fred239]

The question for fred was to help pin down info in this vein. It should be noted that this is a 3phase setup and may also be driving 120V or 208V single phase loads of unknown torque demand. The 3phase loads have less surge. The others, proportionately higher but not known from his post.
larry

I am assuming the elevator has an unloader valve to operate at start-up. The circ pump runs constantly, the only other surge would be the boiler burner draft fan, minimal in my guess. While I personally do not have the equipment to measure surge current, I am going by what demand(surge) I am billed for, so far a max of 21kW. Whille the measured demand has been as low as 14kW, I am also assuming the elevator start up would draw less than 14kW. So, I am hoping that a 22-25kW generator will suffice. Your expert advice is greatly appreciated. Thank you...

 

[joshuabardwell]

While I personally do not have the equipment to measure surge current, I am going by what demand(surge) I am billed for, so far a max of 21kW.

I'm surprised to hear that your electric bill includes your max surge current. Mine just bills based on total kWh consumed.

 

[Rob-D]

Good point, I'm done.

Larry,
Apparently you missed this post!

 

[aczlan]

I'm surprised to hear that your electric bill includes your max surge current. Mine just bills based on total kWh consumed.

If you have a "Smart Meter" the electrical company can get that kind of info.

Aaron Z

 

[fred239]

I'm surprised to hear that your electric bill includes your max surge current. Mine just bills based on total kWh consumed.

It's called a "demand" meter, for commercial accounts, bills for kWh usage as well as kW demand.

 

[Rob-D]

Rob-D Quote> "every ounce of fuel saved means longer run times. I say get the smallest gen set you can to do the job."

We have some smart, experienced guys here, hats off to all. Without getting in the debate, I personally beleive smaller is better, as in more ecconomical in a crisis. So how do I size for a 25hp hydraulic elevator rated at 63.5amps/208volts? My electric usage averages 8kW, but my maximum demand can be between 14kW and 21kW measured and billed monthly, including lots of constant on flourescent lighting, a big circulating pump and a forced draft boiler. An electrician told me 1.5X RkW would be enough to start the elevator, or about 20kW, although often my max demand is only 14kW in a given month. If historical max demand has been 21kW, could I get by with a 22kW 3phase genset? Fuel is precious in a crisis.

Fred,
I won't give you that answer and here's why:

First, you should pay an engineer to do a work sheet for you. Taking my advice or anyone's on the net is a bad idea because if the calculations are wrong the responsibility for the failure is yours not theirs especially in a situation where we are talking sizable motors like in your case.

It's the same as if someone on the net asked me about charge pumps, the Miller affect and capacitor loading on high side 'N' type MOSFETS in DC to AC inverter applications. You want someone to be responsible for their calculations and I would say PM me and I'll give you an invoice that you can rely on if those inverters start blowing up.

In the case here the individual just asked about PTO gens for his home. I gave a straight forward answer that is accepted by the industry. Simply get a P3, monitor your house and appliances and with motors use 3 times their hp. Good enough, efficiencies, inrush current, etc. etc. all get covered and weren't necessary info for the question. IF he had brought in 25 hp motors, well above the average home, I would have given the same answer to him I just gave to you.

Now there may indeed be people here who can analyze your specs but, again, it's not the best path to take in my view.

 

[fred239]

Fred,
I won't give you that answer and here's why:
First, you should pay an engineer to do a work sheet for you. ... You want someone to be responsible for their calculations and I would say PM me and I'll give you an invoice..."<end quote>

OK, thanks Rob. My reason for asking was to get some free advice, without any professional liability. Most engineers overdesign at the expense of their clients to cover their own Errors and Omissions. I'm a risk taker, and I accept that. The generator salesman wants to sell me a unit three times bigger than I need. The fuel salesman the same. I'm a survivor, just trying to get by. When the goin gets tough, every ounce of fuel really counts. That's why I originally posted your quote. We live in a wasteful society and we don't even know it, until we run out what we need. Just like our country has run out of affordable health care, because doctors order a lot of unnecessary proceedures to cover their own liability, and guess who pays. So, back to the drawing board I go...(BTW, what would you charge me to design the smallest, minimalist, survivalist back-up system that only gets used a few days a year, with a discount for waived professional liability?)