Power Loss, Step Up Ratios

Power is inversely prportional to Torque with the Rpm being constant.Every increase in Torque Results to a proportionate decrese in power and vice versa.

Interesting. If you really get 4Kw out of your generator, it is pretty efficient, for 6HP. You should be able to easily run 2hp motors, one at a time. I wonder if the governor on your generator is not working right. As you apply load there should be little change in speed, but the generator will get louder and sound a bit labored. If the governor has a lot of slop, maybe you could set it so that you get 60 HZ power under load (and allow the generator to run a little fast when off load.)

Is the wiring from generator to saw adequate? Could there be shorted coils in the generator field? I'd investigate the generator first before putting in a shaft system. Could the diesel be low on HP? If so, a mechanical system won't help. You could make a dynamometer, and measure the actual torque and rpm of the engine, without the generator attached, and eliminate the engine as a cause. (If you were certain that the generator itself was OK, you could load it with a known resistive load, and use that arrangement as a dynamometer.)

If you have to go with the shaft system, you could figure about 90% efficiency for each belt, so if there are three on the way to the lathe, you'd still have 4.3 hp left. But I'd try to get it to work electrically first -- for safety, among other things -- in a panic stop, the less inertia the better.

Ken I have the motor set at 61 cycles with the florecent lights as a load 127v. The govoner on these motors are lazzy . Im thinkin because I have a step up from 650 rpm to 3600 rpm as the cycles drop much faster on the gen in comparison to the motor. Thus the problem . These motors work great for smaller loads and I was warned by a fellow who repaires generators this would happen and not to push it as far as the voltage drop. I run the motor on waste veg oil. and live off the grid. This motor generates power to charge my battery bank in the dark and cold winter days of a north Canadian winter.

Its funny, this motor is turn of the century tecknolegy but yet when I threaten to build a complimentary turn of the century power system most people try to diswade me. I have a fairly sophisticated system designed with a air ram to operate a belt clutch, which stops with a brake.

As far as the cable to the saw its 30 ft #12 . The saw runs fairly good for most things but if you try to push heavier stuff you can hear its on the edge . The lathe starts in a few gears but not in others so higher speed is out. It wont start.

Greg

I admire your self-sufficiency. A turn of the century motor is not too old -- 1999, 2000, somewhere in there, right? (Actually I'm an old fart, so I knew exactly what you meant!)

Have you tried running the speed up to maybe 70 cycles? Is there any slippage possibility between the diesel and the generator?

I like the idea of the mechanical drive, from a nostalgia point of view. Just seems like a lot of work if you can get the generator up to close to its rating.

I think your idea is sound. Emphasize safety and adequate guarding for your shaft and belts, etc. I suspect medical care is distant, but any injury is painful and costs time. If you get your hands on an electric motor with a high-slip design, this may allow your system to power the saw or lathe. Otherwise, you idea of a shaft-driven system is probably best, because you trade the efficiency losses of the generator and motor for efficiency losses in belts and drive chains and completely avoid the problems of frequency, RPM, and motor design. Your existing motor, with its substantial slow-down to provide maximum power will work well with a mechanical connection to the heavy load.

I'm with Ken on this one. There's some very good reasons why the old fashioned overhead shaft systems gave way to electricity. You should be able to run a 2hp motor off a 6hp prime mover. If you can't I'd suggest investigating until you establish why not before spending time and $'s on another arrangement. Really, taking a broad viewpoint, electricity is the best and most flexible way to move power from one source to another. Obviously not on a par with direct coupling but over all....

If you can't make it work I'd suggest asking questions until it does. Surely CR4 is here for these sorts of questions?!

In the UK a large factory was bombed out in the blitz and boarded up. Us kids found ways inside the boards and I saw the exact factory described. Recalling it now, it seems there was a flywheel of some mass. This flywheel would supply the start torque surge for high gear ratios, along with belt slip clutch action.

A flywheel may help, but you would need a starter that applied voltage to the motor in stages as the flywheel sped up to minimize low speed motor currents to a safe level.

Well guys I have a confession to make. I have allready purchased a 22 ft long piece of 1'' shaft and have half the needed sheaves as well as the angle drive compressor, ram and valve in my stash. I figure it would cost me about 500 additional bucks to fabricate my overhead power. As I said before with this system I could shove stuff through the saw till the smoke boiled off the blade and still keep going. I intend to still run the gen at the same time but for smaller stuff and lighting. It confused me when I did some math to find the power Id have at the buissness end . I dont want to go through the effort to build this if it is a gauranteed failure. The motor and gen are only a year old. with small hr on both. Im sure it is the combination of the slow prime mover and high gear ratio of the gen that is the problem. In real life when you look at store bought gens the prime mover and gen are a matched set weather 1800 or 3600 rpm

Here is what I figure It aint the perfect combination. If the motor is spinning 650 rpm with a step up ratio of 5.5:1 then 650-:-60=10.83 motor rpm/ cycle. So say with a heavy load I drop to 55 cycles x 10.83=595 motor/rpm that means 595x5.5:1=3272 rpm/gen. Now that sucks. Its the gear ratio I cant do anything about this except replace the gen wit an 1800 rpm gen, which I have one of. But I bought it from a repair shop and not shur if it needs work or not, so cost and effort to mount in place.The kicker is it will still do simaler things just not so bad the ideal situation would be to winde a 600 or 1200 rpm gen.

Ill go out and fire it up today I have a hot water 1500 watt element Ill let you know the voltage drop and cycles later.

What you are saying is making sense. Having leapt in above in favour of the electrical solution I'm now not so sure. I have vaque recollections of similar problems running an induction motor from a generator. I can't recall the figures but it was a similar thing. On paper there should have been ample capacity but in practice the compressor couldn't get up to pressure. Generator capacity has to be twice the load from an induction motor is a rule of thumb I recollect.

I'll have a look today and see if I can dig up the figures from my little problem. Twenty years ago but I've still got the compressor so that half's easy!

Just to keep you guys up on my insanity. I made a dynomameater or what u ma call it. It consisted of a 2x4 Only one. I took it and placed it on the ground partially under one of the two large 150 lb fly wheel on the motor. And pryed like I was trying to move the motor. I also placed the cycle meter within easy reach. When I pryed on the wheel I could really drag it down, below 50 cycles. Ok so I tried on the shaft, I could bearly get much movement from the cycle meter. The wood smoked. Now I put it on the gen with the large gear ratio, it smoked very little meter movement. I was really reafing on this puppy. Good enough for me the gen shaft is the same ratio as the shaft Im building, so if it can take that tratment then my creation should work. And yes its a lot of work to build but it will be only one time.

Greg

I'd guess your flywheels are close to 24" diameter, or 1' radius. Let's assume the coefficient of friction between the wood and steel is about .4. Then if you press against the flywheel with 100# of force, the drag at the rim will be 40 lbs. It will require 40 lb-ft torque to overcome that drag. Your engine produces 48 lb-ft maximum, so the 40 lb-ft will heavily load it. On the shaft, however, at 1" diameter, the radius is only 1/2 inch, so 100# force will only produce a torque of 1/24 of 40 lb-ft -- less than 2 lb-ft -- not enough to noticeably slow the motor. Even on the shaft of the generator, you'd have 1/24 x 5.5 x 40 or 9.2 lb-ft at the motor shaft -- still not enough to seriously load your motor.

Your 2x4 experiment went as expected, but doesn't show what the actual output of the engine is: (Was the force really 100#, Was the friction coefficient really 4?) It also doesn't say how the generator is working. It you hook up your 1500 watt element (make sure it is in water) you will have a better idea of how it performs with about the same load as a loaded 2 hp electric motor. (2 hp = 1491 watts) If your generator/motor combination works hard with the heater, then your 4k generator is not really a 4k generator.

The motor's governor should not let you drag the speed down easily. As you apply more and more load it should remain at 650 rpm (shown as 60 Hz on a frequency meter, if there is no slip in the belt drive to the generator.) I am sure the torque curve of your engine is relatively flat, but if you slow the engine to 5/6th normal (540 rpm) the torque available at the at speed may have dropped to 40 lb-ft, in which case the HP will be 4.11, a significant drop.

Be careful with those smoking 2x4's.

Typically, AC generators are about 90% efficient and electric motors are about the same. The belt from your motor to generator should be about 95% effic. So your 6 hp diesel should supply 4.56 HP at either your saw blade or lathe spindle (if the motors could do so). A system of belts could do slightly better under ideal conditions... so if you have the shafting, the right angle gearbox, and the time...

Whoa Ken! What sort of 2hp motors are we talking about here, 2hp in or 2hp out?

Most 2hp motors I've come across pull somewhere in the vicinity of 3 kw+. I mentioned in a post above that I'd had a similar problem back in my contracting days. We were trying to run a compressor from a generator and couldn't get pressure. I've dug up the figures relating to that episode. The electric motor was ("is" actually, still going strong) a 2hp (11.8 amps @ 250 volt). The generator was a 6kva but the motor was a 7.5 hp petrol scavenged from a smaller generator. Possibly the motor was tired but it couldn't cut the mustard. Cure was a bigger motor, 10hp petrol.

I was thinking of motors that are about 2.22 hp in and 2.0 out:

Graph from Reliance electric site: http://www.reliance.com/mtr/b7087_5/b7087_5_7.htm

However, I'm being optimistic. Once you account for power factor, a typical motor is more like 75% in real efficiency. Here's a spec sheet from Baldor on a general purpose motor, with full load elec. effic. of 82,5% and a power factor of 90%, for real efficiency of 74%. Thus it takes 2kw (elec) to yield 2 hp (mechanical).

But even at 2 kw, or even 3kw, I'd expect a 4kw generator to work OK... However, as I think about, it the diesel is only 4.5 kw output max, so the generator is probably only putting out 3 kw, continuous. (I note that a modern Honda-powered 3.2kw generator is powered by 6.5 hp). But on the other hand, with 150 lb flywheels, this generator should be able to substantially exceed its rating for startup of motors. I am surprised the lathe won't even start in high speeds -- mine hardly draws any current at all until I am actually cutting.

So, I agree -- either a larger diesel (no fun even lifting the existing one, let alone the cost of replacing it with anything of close to the same quality) or more efficient transmission of power, and a belt and shaft setup could do that.

As long as greg remembers my story of a high school girl I knew who got her hair ripped out by a skiing rope tow, he should be ok -- keep all those shafts and belts guarded.

I intend to build in safety. The shaft will be on the ceiling 9' high. The belts on the wall will be caged the drive shaft to the equipment will have a shielded shaft. And I intend to make an air operated clutch. Which will follow the operator from one machine to the next. It will quick attach to the machine and be connected to the clutch with air lines. As far as swaping out motors this one is big enough for most stuff I have to do. It is hard to size a diesel to loads, most of the time Im using small amounts of power. When you have light loads you should load the diesel to 75% in order to keep it to temp or you can cause it over time to slobber oil. For this I use a dummy load electric heater. Till it burned out. But now I have loads that exceed the capabilities of the gen and maybe the motor. So overhead power it is.

Greg

I see your point Ken, however I think there must be a lot of variation in these things. I had a quick check around our motors this morning. The best is a new one we are installing on a kiln which is 3 kw and drawing (data plate) 8.2 amps of 3ph at 415v with a PF of 0.85. I'm rusty on these sums but I think that translates to about 5kw. The worst is an old single phase on a pump that draws a whopping 8 amps (240v) for 1 hp. Almost as much as the 2 hp you posted the link to!

These older motors tend to be physically much larger than the modern equivalent. The two mentioned above are about the same size although one's four times the output (and 3ph of course). Have motors got that much more efficient, or perhaps they used to be rated more conservatively?

Nutwood,

A number of things have happened. Some of the real old motors had service factors greater than 1.5, which meant they could be "over"-loaded continuously without overheating, by that much more. With higher-temperature insulation systems, the physical size of the windings and the motor could be reduced for the same output.

Motor design involves many trade-offs--power factor, efficiency, temperature, size, life, inrush current, slip, base speed--are just some of them. Be careful when comparing current draw of 1-phase and 3-phase motors, as well as motors at different voltages, I've had to correct more than a few of my own mistakes there.

--John M.

Thanks John, I agree, it's a minefield. What're your thoughts in the context of the current debate on the running of a 2hp motor off a 6hp diesel generator. I'm sure that somewhere I came across a rule of thumb that reckoned you should take the plated input power to an induction motor and double it when sizing gen sets.

By the way, strictly for my own education; was I right about the the 3kw, 3ph motor pulling 8.2 amps@ 415v with .85 PF being equivalent to 5kw? If I'm way off track I'd like to know!

I calculate (Apparent Power) = 1.732*I*E, or 5.9 kW. (Real Power) = P.F.*(Apparent Power), or 5 kW. This is using your figures of 8.2A and 415V. I assume that the 3kW is the machine's requirement, not the motor's actual output, because this otherwise would mean your motor had an efficiency of only about 60%, which is very low!

That oversizing is needed because the inrush current to start the motor is typically 6 times the running current, when you are using "across-the-line" or "Direct-On-Line" starting. With very high-efficiency motors, the inrush current can be as high as 10 times the running current. That creates problems for unsuspecting electricians (and engineers) when they size motor starters, overload relays, and line fuses. If you had to start the large motor from the generator, a couple ideas may help--use a "soft starter" and start the motor without load, then apply the load with a clutch, or possibly have a smaller motor with an inertial load on line when you start the larger motor (its would act as a generator to contribute to the inrush current of the larger motor). John M.

Thanks John. That's the way I got there. The 3kw figure is the figure on the motor data plate. The kiln requirement is not much over 2kw but we oversized as it's being run through a VFD and that allows for a bit of over running should it ever be required.

I double checked the plate and must make a correction in that the amps are 8.0 not 8.2. Still only 61% efficient and this is a brand new, out of the box 3kw motor. I don't normally look hard at motor efficiency as they're only small motors and the running costs are negligible. Until of course you're running them from a generator!

Hi Nutwood,

It occurs to me that your nameplate standards might be different than ours. Here, in the US, if the nameplate reads 5 HP, that means actual shaft HP. (In other words it takes into account both PF and efficiency.) If it reads 4.14 kW, then that is "real" (or active) power, which takes into account power factor, but not efficiency. If it reads 5.3 kVa, then it is apparent power, with is simply I x E x 1.73 (for three phase).

So in this Baldor motor, apparent power rating would be 6.4 x 460 x 1.73 = 5.09 kVA. Real (or active) power would be 6.4 x 460 x 1.73 x .84 = 4.27 kW. The HP rating (most common on name plates) would be 6.4 x 460 x 1.73 x .84 x .875 = (3743 W) = 5.01 HP, So here, this is called a 5 HP motor, and is name-plated that way (usually).

Here, we would say that your motor has an apparent power of 8.0 x 415 x 1.73 = 5.74 kVA, and a real power of 8.0 x 415 x 1.73 x .85 = 4.88 kW. (So far these are the same as John's figures, but adjusted for 8.0 instead of 8.2 amps.) HP would be 8.0 x 415 x 1.73 x .85 x .615 for 3002 watts or 4 HP. Here, the nameplate would probably read 4 HP, but might read 5.74 kVA or 4.88 kW.

This may be one of those rare cases where what we do here actually seems more sensible than what is done elsewhere: above, you seemed to be referring to the same motor as both 3kW and 5kW -- potentially confusing and the cause, I'd assume, of John saying "I assume that the 3kW is the machine's requirement, not the motor's actual output". Here, by calling it a 4 hp motor, we are distinguishing between shaft output and electrical input. I suppose which ever one you're accustomed to makes sense.

61%? That seems remarkably low. If you are running it 18 hours a day, 300 days a year, the extra kilowatt it is using would cost $54 (over here). A couple hundred of those in a plant, and the cost would add up.

I see what you mean Ken. I'm in Australia where hp are verboten. This is of course much simpler. Not!!! All it means is that you have to be bilingual in measurements, "a few inches more than a metre..."etc. The name plate figure that I quoted (3kw) is the rated output of the motor. It takes the place of your hp figure. The other figures on the plate are the amperage, the power factor and of course the voltage. Oh and the frequency, 50hz.

I'm not sure on all this comparitive efficiency. I would have thought that most electric motors doing similar tasks would have similar efficiencies. To take a simplistic view; where does the excess energy go otherwise. The poor performing motors would be running red hot and yet I've not noticed dramatic differences from one motor to another. If anything the older motors, like the old 1 hp I mentioned previously (2kva in, 1 hp out) run cooler than their modern counterparts.

I suppose I should worry about efficiency of the motors but I'm not convinced there are the differences the plates indicate. In this instance (our new 3kw) it really is academic as any waste heat simply saves the heating elements some work!

I agree... it is surprising that the inefficient motors are not really hot. And true -- often "waste" heat is not really such a waste.

I think the ratings are all over the place. The most obvious case is in consumer products where the HP quoted is sometimes "Peak HP" -- which can be the locked rotor current times the voltage. This is pretty much nonsensical, because if the rotor is locked, then no HP is produced -- and if the motor is running at any reasonable speed, then the locked rotor current cannot be achieved. I think industrial motors are more realistically rated, but there are huge variations in efficiency and power factor, depending upon intended use. I guess if a motor is designed to run most of the time under light load, but must also sometimes run under heavy load, then we can't reasonably expect it to be efficient under both conditions.

I think 415v, 84% efficiency, .85 PF calculates to 5.64 HP (4.2 KW). (In fact, I just verified this using this calculator.) But now I am confused. Using that same calculator, we get 3.0 kVA for apparent power. Seems backwards -- maybe we've finally created a perpetual motion machine. I think I'm just really fuzzy with three phase power.

Oddly enough, I'd started my last post before John posted his, went away from the computer, and then came back and finished it. Then, when I next checked my email, I saw a reference to John's post. His math seems to concur with my initial thoughts, but then I tried to come up with values that reflected both the nameplate data and also 3 kW. I'd assumed that maybe the 3 kW meant 3kVA. But then it gets confusing.

When I have a chance, I'll take another look at the calculator, to see why it doesn't seem to agree with common sense.

Yah I know the 2x4 dyno test was flawed but when the math fails me I work on instinct. It gives me an indication in the real world what to expect. Id guess that with the pressure I was using on the shaft of the gen 1.25'' shaft I would be happy with that performance at the table saw.

The flywheels are 24'' and the shaft is 2'' So your math is correct near as I can tell

The bottom line is. There is an over head shaft going in and IT should work. Probably only marginaly better then the electric motors but with no concerns of cycles, voltage drop and amperage. Ill keep you posted of sucess or failure.

Greg

Excellent, I like someone who acts on his opinions. I reckon that with those flywheels your saw is going to be fairly buzzing.

Please, please let us know the outcome!

I now have a line shaft. It works. really good. The details, I ran a 330'' v belt to a 17 foot overhead 1'' shaft. to a 200'' belt down the wall to an angle drive, and to a teloscopic drive shaft to either the lathe or table saw . Before with electricity I could only start the lathe in low gears. Now I tried it in all gears. There was no grunting or light dimming or even any major cycle drop. I use my cycle meter as a tack . 1 cycle represents 10.9 rpm on the motor. The worst cycle drop from 61 to 60 and held . So the moral of the story is dont always listen to the nay sayers. Some times old tecnology has good things too. I proved it. Now to put the finishing touches on it and voila.

Greg