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Thread: HP power draw from alternators

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    HP power draw from alternators


    Taken from a popular KR website:

    "This "dynamo" is from a John Deere model 670 lawn tractor (any X70, actually), and weighs 4 pounds with the regulator. Output is 20 amps minimum when driven by the mower's 6" pulley running at 2500 rpm. Part numbers are AM877557 for the alternator ($160), and AM101406 for the regulator ($67). The wonderful thing about a dynamo is that it uses permanent magnets rather than field excited electromagnets, so it doesn't "need" electricity for the windings to produce electricity. "

    -Mark Langford

    The JD dynamo alternator is basically the standard for corvair conversions. Probably 80% of the flying corvairs use this alternator. The reasons are mostly because they are very lightweight(4lbs with regulator vs +/- 6lbs for a Denso 40-60 amp internal regulator field excited that is very commonplace on homebuilts) and really compact. The denso is by far the most common second choice, used mostly if either the owner feels they need more than the 20amp the JD puts out, or they just feel uncomfortable with the fact that the dynamo charges full draw all the time. FWIW, both alternators have performed very well on the corvairs, and both have proven to be quite reliable.

    One thing I am curious about is the constant mechanical power draw from the JD dynamo. The dynamo, being a permanent magnet type, will not reduce the armature resistance with reduced charging needs like a field excited (Denso)alternator will. I have one of these JD Dynamo alternators, and spinning it by hand for just a few revs takes a suprising amount of effort. Granted, these units were designed to be flat bulletproof for small diesel engines tractors/mowers, so there was certainly little concern over a bit of potential extra engine power draw from a permanent magnet unit.

    Here's a retyping of a snipet from Kent Paser's "Speed with Economy" book that got me thinking on the subject.

    ALTERNATOR CUT OFF SWITCH

    `A normal aircraft or automobile alternator, when operating at it's rated output, can draw as much as 3-4 engine horsepower to operate. This is not caused so much by the mass of the armature because the armature rotates in ball or needle bearing; and, once the mass of the armature is spun-up, it doesn't require much energy to keep it spinning. Rather, most of the energy (horsepower) required to spin the armature is due to the armature spinning within the magnetic field created by the field coil windings. In an alternator, the field windings must be energized by an outside source of DC current (the battery) for the alternator to start producing an electrical output. I have installed a seperate switch on the instrument panel so that I can cut off the current to the field windings; thereby, freeing up as much as 3-4 engine horsepower which can then be used to either increase the aircraft's speed or decrease the aircrafts fuel consumption. Of course, when I shut off the alternator, I also minimize other current draw from the battery so that the battery doesn't discharge completely during a race. Actually, since the engine's ignition is provided by the magnetos (which requires no outside electrical power source), no electrical draw from the battery should be required during a race unless the race format requires radio communication during the event."

    -Kent Paser


    Granted, there could be some debate as to whether it is prudent
    to be able to intentionally shut down your charging system in flight. But I'm more intrerested in finding out how much power a small permanent magnet alternator such as the JD dynamo draws from the engine, especially as it is running at full resistance ALL THE TIME!

    Since my application is in the 100-120hp range(+/- 60-80hp at cruise), if the JD absorbs 3-4 engine hp at cruise setting similar to what Kent Paser suggests a regular Denso type -may- be absorbing, CONSTANTLY, regardless of charging load, that may be something to consider.

    My understanding is that a typical internal regulated field wound alternator (such as the popular Denso units) takes up less and less energy to turn as the charging requirement diminishes. So say a 60 amp Denso charging at full rated power draws 3-4 engine HP, that would presumably drop to say 1 or less than 1 HP under the lowest charging demand?

    If it is reasonable to assume(and we all know what happens as a result) that a denso unit will draw 1hp or less during low charging times (probably most of the time) and the JD dymano could consume 3-4hp all the time, a functional 2-3hp usable engine HP output difference on account of the different alternators could result. As the mounted Denso functionally only weights almost exactly 2 lbs more than the mounted JD dynamo, that would seem to be a fair trade for the potentially significant drop in resistance.

    A member of the corvair conversion community has very recently purchased a $40,000 engine dyno for his business for R&D and customer work with corvairs, so perhaps a proper back to back set of tests can be done to put some hard numbers on the issue in the near future. In the meantime, I'd be happy to hear any thoughts/opinions on the subject.

    George

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    Registered User wsimpso1's Avatar
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    Re: HP power draw from alternators

    Time for math.

    14V system at 20 amps is 280 Watts. Even if the rectifier/regulator threw away half of the power (it is probably much more efficient than that), that would require 560 Watts at the shaft. If the belt system were terribly draggy, at maybe 90% efficient (it is probably more efficient too), that would be about 620 Watts. One HP is 746 Watts.

    It is that simple. Your little guy is costing you less than 1 horsepower. 'Nuff said?

    Billski

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    TFF
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    Re: HP power draw from alternators

    I dont know the answers to your question, but I think that is pretty cool. You just have to be careful about loads on it. Too bad it has to spin so fast; make it wind driven.

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    Registered User Tom Nalevanko's Avatar
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    Re: HP power draw from alternators

    I did an analysis for a different engine (IO-550) and alternator and concluded that approx. 2HP max could be available in electrical current from the alternator. FWIW

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    Re: HP power draw from alternators

    Thanks for the replies.

    Billski,

    In case it was not terribly obvious, this subject is not my strong suit. Your math is based on a field wound alternator correct? Would the resistance not be completely different on a permanent magnet alternator where there is significant mechanical resistance (I just gave it another few turns on the dynamo). I would presume the engine power absorbed based on electrical load alone for a 20amp alternator would not be much. It's the high resistance drag of the permanent magnets that concern me most.


    TFF,

    Funny you should mention a wind powered alternator. Some of you might have heard of Smokey Yunick. He was a self taught Daytona native that became one of the most notorious "rule benders" in early nascar/indy car/trans am racing history. For one of his nascar racers, he mounted the alternator behind the grill in front of the radiator, mounted a wooden propeller to it, and found that at speedway speeds the alternator made plenty of current to keep the battery charged. So he painted it black to hide it from the tech guys, and mounted a real ready to go alternator in the appropriate location on the engine according to the rule book. On race day, all he did was mount an alternator belt on the engine that was cut most of the way through. The tech inspector, of course, wouldn't be looking for a "damaged" brand new belt. As soon as he started the engine and revved the engine enough, the compromised belt would simply break free and fall off (oops) and away he went without any engine hp robbing alternator hooked up. I believe he made it a good bit through the season before they caught wind of what he was doing. He did mention that it did make a noticable difference though. I'll have to go look up that section in his books and remind myself of his genius!


    Tom,

    I'm curious as to how you came up with your 2hp max. By the same though process as Billski's?



    Something interesing:

    Fact Or Fiction: Does Electrical Systems Decrease Power? - Import Tuner Magazine
    Last edited by gschuld; July 9th, 2010 at 11:38 PM.

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    Registered User Tom Nalevanko's Avatar
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    Re: HP power draw from alternators

    60 amps x 24 volts = 1.93 HP (when units are adjusted); actually I could use a 100 amp alternator but with losses to where I wanted the power, it still comes down to around 2 - 3 HP. Of course, this might rob the engine of a bit more HP due to mechanical losses.

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    Registered User wsimpso1's Avatar
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    Re: HP power draw from alternators

    In most electrical stuff, the mechanical efficiency is pretty high - usually way over 90%. And in general, so is the mechanical to electrical conversion. Just to sort of show the lower end of efficiencies and thus the higher end of mechanical power at the input, I used 50% electric and 90% mechanical. That includes everything., and really is very conservative.

    Here is the details a watt of power is one ampere at one volt DC. So, amps times volts is watts. Divide once by the mechanical to electrical efficiency and again by the mechanical efficiency, and you have the shaft watts to turn the alternator. Divide by 746 watts to get the shaft horsepower.

    Now, if you wanted to compute just the drag in the alternator to separate it from the other losses (because you just don't trust my efficiencies), you can do it this way... Measure the torque to turn the alternator - get an average as you go around as many permanent magnet devices will "cog" with the torque going up and down or even positive and then negative. Without an electrical load (nothing connected), your electrical power will be zero, so this will only be mechanical load.

    Your average torque times your rotation speed is the drag power, but you gotta convert units. If your drag is in ft-lbs and your rotation speed is rpm, multiply torque by rpm and divide by 5252 to get HP. Yea, the HP and torque (in ft-lb) are the same at 5252 rpm. Using 24 inch-pounds, divide by 12 to get 2 foot-pounds and spinning the alterator at 2626 rpm gives 1 HP in drag.

    If you have the torque in Nm and speed in radians/sec, just mutiply them together to get energy in watts and divide by 746 to get HP.

    Now think on this a minute. This rig comes off of a lawn tractor. It has what, 14 hp? If that alternator was using several hp, it would have to run at significant power just to do nothing but run... I suspect that the gadget is not drawing a whole lot of power.

    Billski

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    Registered User jumpinjan's Avatar
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    Re: HP power draw from alternators

    You would trust this lawn tractor junk on your airplane? How many million cycles (if that much) will it take before failure? The pulley is stamped sheetmetal!
    About every street motorcycle has this type of alternator in it. Now, that voltage regulator needs special cooling, they need to dump heat. The Mercruiser 470 marine engine, has one of these built into the front pulley, and the regulator is water cooled.
    Jan
    Jan Servaites (Dayton OH - The Birthplace of Aviation)
    (Where we had the brains and not just the wind to make flight possible!)

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    Registered User Dan Thomas's Avatar
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    Re: HP power draw from alternators

    The hp draw will diminish as the battery reaches full charge, even though the field consists of permanent magnets. If there is no place for power to go, there is no current flow and the back-EMF on the alternator drops off and the power required to run it will fall as well. It's not as ideal as a field-controlled alternator but better than most other options.

    You can see this effect at work on cordless drills, which have permanent-magnet fields in their motors. The older drills used to spool down gradually when the trigger was released, and then someone got the bright idea of a "dynamic brake" as a selling feature; the switch just shorts the motor's leads together when the trigger is released, and the motor becomes a generator and the huge load imposed by the short stops it rather quickly. Must have cost at least 30 cents more to produce such an advanced tool.

    The alternator's regulator will be a transistorized doodad that varies its resistance to keep the system voltage constant. Since it's in series with the output and not shunting power to ground, it should get hottest when the RPM is highest and the electrical needs are moderate. When the battery needs lots of current to recharge it, the regulator's resistance will be minimal to allow maximum current flow, and when the battery is fully charged and near the maximum voltage produced by the alternator, the need for resistance will again be minimal since the current flow will automatically be little. Resistance is what make heat, and the heat generated by the regulator will be the biggest power loss in the whole system. And as others have said, the HP requirements will be relatively small. Three or four HP would burn out the average V-belt in short order. I had a regulator fail on my inboard boat once and the output voltage rose to 18 and the battery got seriously overcharged; the V-belt didn't last too long, either.

    I used to have a tiny wind-driven generator on my airplane to keep the radio battery up. The generator was a ball-bearing 36VDC motor from some sort of office machine, and the regulator was an LM314 chip. It gave me 1.5 amps at 14 volts, enough to keep the small battery up (most of the time the radio is receiving, when current demands are tiny) but the electrical noise its brushes made was a pain to filter out.

    Dan

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    Re: HP power draw from alternators

    Dan, I'm pretty sure that regulator is grounded so it's not simply in series with the PMA. That's why it needs all the cooling fins as it's dumping all the power you don't need to maintain system voltage. The stator coils would get quite hot as voltages went up, which they would, as rpm increased and power demands went down.

    PMAs are on many motorcycles but it's primarily because they are simple, cheap, and reduce parts count not because they are a great idea electrically. Voltage regulators on bikes fail constantly due to the wide rpm, hence voltage, swings they must go through. Probably wouldn't be so bad on an airplane, or a lawnmower, as they run at fairly constant speed so you could size it accordingly.

    Maybe you could use the waste heat in the cabin.

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    Registered User Dan Thomas's Avatar
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    Re: HP power draw from alternators

    Quote Originally Posted by roverjohn View Post
    Dan, I'm pretty sure that regulator is grounded so it's not simply in series with the PMA. That's why it needs all the cooling fins as it's dumping all the power you don't need to maintain system voltage. The stator coils would get quite hot as voltages went up, which they would, as rpm increased and power demands went down.

    PMAs are on many motorcycles but it's primarily because they are simple, cheap, and reduce parts count not because they are a great idea electrically. Voltage regulators on bikes fail constantly due to the wide rpm, hence voltage, swings they must go through. Probably wouldn't be so bad on an airplane, or a lawnmower, as they run at fairly constant speed so you could size it accordingly.

    Maybe you could use the waste heat in the cabin.
    Grounding of the regulator is necessary to get the voltage reference. I would be very surprised if it's shunting 20 amps to ground. That would generate enormous heat: almost 300 watts of it.

    Dan

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    Registered User wsimpso1's Avatar
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    Re: HP power draw from alternators

    Thanks guys. Somebody else thinks so too...

    Bill

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    Re: HP power draw from alternators

    As usual, any time I ask a question here I am greeted with a wealth of knowledge. Many thanks. The example of how much engine HP could possibly be absorbed by an alternator on a 14hp engine is a great reality check. I will keep the calculations in mind for future reference. Dan Thomas, Billksi, and others, thanks

    George

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    Registered User Hot Wings's Avatar
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    Re: HP power draw from alternators

    I know this thread is a bit dated but I have been researching using this type of device on my project and ran across this article that I think explains the voltage regulation pretty well for those of us that keep hooking diodes up backwards, or have no chance of remembering resistor color codes.

    continuousWave: Whaler: Reference: Permanent Magnet Alternators


    It would be nice to know what method the little Kubota uses since it makes enough 'tricty for me and fits where there is space.
    Can one adapt a PWM regulator from one permanent magnet alternator to work on another?
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

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