where does the power go?

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Newz2000
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where does the power go?

Post by Newz2000 » Sun Sep 11, 2005 2:32 pm

My brother's VW has a generator. It produces up to ~18v which is regulated down to charge the battery. The generator turns at roughly 2 times the engine rpms. The voltage generated is RPM dependent.<p>When the battery is charged and the car is running at a high enough RPM to exceed the power requirements for the car's electrical system, what does the generator do with surplus energy?<p>My first thought was that the generator simply does not produce energy if its not needed. In this case the load on the engine would be less. Is this correct? Or does the generator instead produce a constant current (at a given RPM) and simply waste the electric energy if its not needed?<p>What happens if the electrical system wants more power?<p>The generator is rated at 30 amps. It uses about 5 hp (so the specs say). The engine, fwiw, is a 1500 cc '67 VW, which is the first year the VWs were on a 12v system (prior was 6v). And yes, the car has a generator, not an alternator, and it produces DC, not AC.

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Chris Smith
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Re: where does the power go?

Post by Chris Smith » Sun Sep 11, 2005 2:50 pm

Those big black boxes called the regulator are actually spring loaded and coil controlled relays, voltage and current dependent. <p>When the battery on a GENERATOR is no longer in need of power, these relays cut out the field windings. <p>In normal service they oscillate fairly fast at which ever rate [dwell] that is necessary to produce the correct voltage, and also for the current. <p>GENERATORS unlike alternators need a current regulator. <p>On the modern alternator, solid state relays cut in and out quite fast to regulate the voltage.

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Re: where does the power go?

Post by Robert Reed » Sun Sep 11, 2005 4:25 pm

Matt
As long as the engine is running you always need "12V" power. The ignition coil alone would need a constant 2-4 amps. Not to mention headlights (10-15 amp) and a whole host of other accessories you might be using. Although charging current varies its always there, carrying whatever the load required. The battery just floats along for the ride at full charge, on standby ,until the next time it is called on for stand alone power.
Also , charger drive power is some what proportional to charger output wattage, so tht at low charging currents, your "5 HP" generator is consuming much less power.<p>[ September 11, 2005: Message edited by: ROBERT REED ]</p>

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Re: where does the power go?

Post by terri » Sun Sep 11, 2005 10:39 pm

You sure you didn't misread the decimal point on the "5 HP?"<p>
That's about 3,000 watts, just doing it in my head. I thought most alternators and generators put out, ohhh, say 50-110 watts in passenger vehicles.<p>
Even considering belt and other friction losses, that sure seems high to me. Even if it were really 0.5 HP, that still seems high, since 0.5HP is still about 300 watts. I wonder if they're including the power required for the cooling fan, which runs off the same belt in these bugs....? <p>
The generator turns on and off, as needed, by the relay-operated voltage regulator relays, as Chris noted, and puts out DC directly because the commutator on it switches the rotor (it's callled an "armature" on generators) coils as it spins, acting as a mechanical rectifier. This is a major source of problems in the generator, and the graphite brushes need replacement every once in a while, as well as the copper segments on the commutator itself needing service.<p>
The Alternator does indeed put out 3-phase AC, but through the field windings. The rotor is switched on an off by the electronic regulator as demand varies. The rotor is not "commuted," like with the brushes on the generator, but is powered through its own brushes and slip rings (rather than as with the segmented commutator on the generator.)<p>
The "commuting," or "switching" to produce DC from the AC output on the alternator is by a set of diodes pressed into the housing of the alternator, rather than a segmented commutator and brushes as on the generator.<p>
The generator can also be used as a motor, and on a 12HP lawn tractor I had once, the generator was switched to being a starter motor for the gas engine. When the engine kicked on, the generator switched back to being a generator, as on a car. I imagine they still do this, but I haven't mown a lawn in twenty years, so I don't know for sure what they do nowadays.<p>[ September 11, 2005: Message edited by: terri ]</p>
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Chris Smith
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Re: where does the power go?

Post by Chris Smith » Sun Sep 11, 2005 11:09 pm

He missed the decimalpoint...<p>25 amp generators are quite common [300watts or point .5 HP]<p>50 to 55 amp is the standard small alternator [600 to 660 watts, almost a HP]<p>100 amp alternators are common today [1200 watts]<p>Even 150 and 200 amp alternators for preformance trucks and racing gear.

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Re: where does the power go?

Post by k7elp60 » Mon Sep 12, 2005 9:12 am

I can guarrantee that it takes power to turn a generator. I once had a surplus 12 volt generator that was capable of about 40 amps @ 12 volts. I had a machinist friend fabricate a knurled shaft that I attached to the generator. The same friend was a welder so he fabricated me a stand for a adult bicycle. The bicycle sat on the stand and the rear tire rode on the knurled shaft. I could turn the generator while I was peddling fast, without a load on the generator. With a 50 watt 12V lamp as a load I could hardly turn the generator with bicycle power.
I thought I could get my exercise and at the same time charge a 12V car battery. I gave up the project for lack of energy.

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Re: where does the power go?

Post by terri » Tue Sep 13, 2005 12:02 am

Yes, it takes power to turn a loaded generator. Even a little 7 watt bicycle generator can be felt when you turn on the bike lamps. And, before computer-controlled idling, turning the headlights on in a car would result in a 60-70 RPM drop in the idle --certainly noticeable.<p>And for many decades hams have tried to hook generators to bicycle frames for emergency communications power --and have been disappointed by the effort required to keep the finals lit. (An old expression, the "finals" being the final amplifier tubes in a ham rig.) Even with modern xsistorized equipment, it takes a ham in superb condition to run an 80 watt rig from a bicycle generator.<p>During WWII, there were hand-cranked generators to run military radios, but, again, some poor dogface got tired running even a small xmitter and receiver setup.<p>The point I was making was that it seemed like 5 HP to run an automotive generator was a lot, and I surmised that this 5 HP either included running the rather large cooling fan on these air-cooled VW engines, or the poster had misread a decimal point.<p>In the (thankfully) dead and gone years when the slide rule was the premiere calculating device, we had to be very careful with decimal points, and we were mercilessly trained in the art of calculating approximate results in our heads before sticking a decimal point on a slide-rule result.<p>There were a lot of techniques used to place this decimal point, and so when I saw the "5 HP," I did a quick calculation in my head: "half of 700 watts is about 300, times ten is 3000" --I got the "half" because of the "5" in the stated HP requirement. One got to do this rough, rough, rough kind of shortcut calculation so instinctively when we were trained on the slide rule, that it was automatic when I saw the "5 HP."<p>"Reality check," we called it, even back then, in the late forties and early fifties of the last century.<p>This quick and dirty mental computation was a skill which was very valuable then, and is still valuable, in my opinion. Some of us in high school (bths.edu) could even track the decimal point through five or six multiplication and division operations, and with maybe a log stuck in there, to boot. <p>I wasn't smart enough to go that far with this skill, but even with my own limit of two or three multiplication/division operations, I still find the technique useful.<p>[ September 13, 2005: Message edited by: terri ]</p>
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Re: where does the power go?

Post by ecerfoglio » Tue Sep 13, 2005 5:39 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> "Reality check," we called it, .....<p>This quick and dirty mental computation was a skill which was very valuable then, and is still valuable, in my opinion.
<hr></blockquote><p>Even in this times of calculators, computers, spreadsheets, etc it is a * VERY * valuable skill. :) <p>It´s common to hear somebody give a result of
58.3456976 instead of .6 or "about half" :( <p>It´s so easy to "misspell" a number on the keyboard, or to missread a display, that if you don´t do the "Reality check" you may give a completely wrong result. But, of course, with 10 or 12 decimal places :mad:
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terri
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Re: where does the power go?

Post by terri » Tue Sep 13, 2005 6:09 am

That's the difference between "precision" and "accuracy."<p>I have a bathroom scale which gives a very precise result to 1 decimal place, but it's inaccurate by about 10 lbs. on some days.<p>Whoa! This morning it's off by 11 lbs. Surely it wasn't that cake someone left in the break room yesterday!
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Re: where does the power go?

Post by Robert Reed » Tue Sep 13, 2005 5:05 pm

Terri, Ecer
Your comments are so right. Approximation fisrt and then precision. Now you have an idea of what you are looking for.
In the beginngs of many a circuit design, I always use approximations before zeroing in with higher math. This saves a lot of time and points out errors up front before diving in head first.Like you two, I also was taught to have enough understanding of the problem to be able to see a ballpark result with just quick computation in my head.
I think that half the kids today using "new math" and calculators have missed a lot of this teaching. I beleieve some of them could multiply two times two and come up with an answer of fifty six on a misfunctioning calculator and never know the difference.IT MUST BE RIGHT BECAUSE THE CALCULATOR SAYS SO!!<p>Matts "5 HP" generator putting out 30 amps At 14.5 volts produces 435 watts. Given 85% gen. efficiency would require 0.69 HP of drive. By the way, I quickly roughed this out in my mind first to obtain 0.65 HP

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Re: where does the power go?

Post by terri » Tue Sep 13, 2005 9:48 pm

So what's the vote? Was the "5 HP" a mis-read decimal, or did it include the fan, which runs off the same belt?<p>Seems to me about 5HP would be about right for running one of those big VW cooling fans with the cooling flaps full open at 50-60 mph.<p>My vote is that the quoted specs included the fan driving power.
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Re: where does the power go?

Post by rshayes » Wed Sep 14, 2005 4:35 am

If I remember correctly, that VW cooling fan was driven by a single fan belt. Transferring one horsepower through one of those belts is rather marginal.<p>I might believe about 1/2 horsepower for the generator and another 1/4 to 1/2 horsepower for the cooling fan.<p>The generator output is 30 amps at 12 volts or a total of 360 watts. This is slightly less than 1/2 horsepower, so I could believe a figure in the .6 or .7 horsepower range, but I would be very dubious of the 5 horsepower figure. The generator efficiency should be far more than 10 percent.

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Re: where does the power go?

Post by terri » Wed Sep 14, 2005 6:01 am

OK, that's one vote for the "misread decimal," with a comment that 5HP to run the fan through a single belt is too much --and is more like 1/4 to 1/2 HP.<p>I recall that the monstrous VW cooling fan was about 15-18 inches in diameter and ran about 3/4 of the engine RPM, judging from a poor visual recollection of the relative sizes of the sheaves on the engine and the fan itself. <p>I would guess that 5HP through a single belt and also that 5HP to run the fan is not out of the ball park.<p>Somebody correct me? <p>This is all just from memory, sitting on my couch this AM, with no reference works, wating for the coffee to get done, hoping I'm not going to be too late to catch the 7:07 AM bus.<p>[ September 14, 2005: Message edited by: terri ]</p>
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Newz2000
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Re: where does the power go?

Post by Newz2000 » Wed Sep 14, 2005 6:58 am

The "5 HP" figure is second hand information. There is only one belt to the front of the generator then the back of the generator connects up to a big fan housing. These cars are air cooled and have big fans.<p>However, the engine is rated for less than 60 HP so loosing nearly 10% to the generator/fan is a sad story.<p>It definately makes "Herbie Reloaded" a little more humurous knowing that these little cars have so little juice. If you've seen that movie, did you notice that when herbie was originally decked out for racing that he had a killer stereo system in the back? When was the last time you saw a race car with sub-woofers? (I have a 4 year old boy, so we've seen it in the movie theatre twice now and will likely buy it when it comes on video)

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Re: where does the power go?

Post by terri » Wed Sep 14, 2005 8:48 am

OK, I'm going to admit error and say 5HP for the fan is too much. I took a quick look at the Machinist's Handbook on the way out the door to catch the bus and a single V-belt in this kind of situation can only transmit about 2.73 HP.<p>I missed the bus.
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