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Thread: Electric Power Plants

  1. #1

    Electric Power Plants

    I was reading about the electrical prototypes in EAA SportAviation, July issue. The articles mention various power plants that deliver 27 hp with 20 KW and 54 hp with 40 KW and various voltages---no amps discussed. Is there a basic article or forum that explains the electrical parameters of motors/generators used in this airplanes? Thanks

  2. #2

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    HP and kW are both measures of power. 1HP = 0.746kW so 27HP = 20kW, etc.

    The current (amps) depends on the voltage. Volts X amps = watts so your 20kW motor would draw 833A at 24V, 416A at 48V, 200A at 100V, etc.

  3. #3
    Got that--and helpful---so, how do you calculate how long the battery will operate the prop ? Obviously depends upon work load but is there a typical formula for prop/gross wt of plane calc?
    Thanks

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    Matt Gonitzke's Avatar
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    The battery's capacity will help you determine how long you can operate the electric motor. For example, a 500 A-hr battery would deliver 50 amps for 10 hours, or some other product of current and time that multiples to 500 A-hr. So the 20kW motor being operated at 100V draws 200A and would have a battery life of 2.5 hours with the fictional battery I made up as an example.

    I guess I'm not completely sure what you mean when you refer to 'prop/gross weight of plane' calculations. If I may assume you are trying to size a powerplant and propeller for a particular aircraft, a good aircraft design or aircraft performance book will contain the numerous equations necessary to size the powerplant and propeller.

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    Quote Originally Posted by Matt Gonitzke View Post
    The battery's capacity will help you determine how long you can operate the electric motor. For example, a 500 A-hr battery would deliver 50 amps for 10 hours, or some other product of current and time that multiples to 500 A-hr. So the 20kW motor being operated at 100V draws 200A and would have a battery life of 2.5 hours with the fictional battery I made up as an example...
    ...and before anybody gets excited, that's a 20 kW-hour battery. That's a big battery. With current Li-po battery technology, that's 220 lbs of batteries. For only 27HP.

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    Matt Gonitzke's Avatar
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    Indeed. That was made up to make the numbers easy, and highly unrealistic with today's battery technology.

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    Now don't quote me on this because it is coming from memory, but I believe the HP rating for a combustion engine and electric motor are specified differently. I believe an electric motor is rated on the amount of continuous power it can deliver while a combustion engine is rated by the peak power it can deliver. That difference is very important.

    So if you need a 200HP combustion engine to make your aircraft fly, an electric motor that will give you the same performance will have an HP rating lower than 200HP. Really you have to get into the mathematics of the whole system. You need to figure out the amount of thrust you need that will result in the airspeed you need to produce the amount of lift you need to get the payload(aircraft+people+stuff) in the air.

    Once you have that thrust you can calculate how fast the propeller needs to spin(RPM) to produce that thrust, then check the datasheet of the electric motor to get an estimate of the current draw at the calculated RPM and a given voltage. Expect the actual current draw to be 10-20% more.

    What are you thinking about doing f_lunn? I'm an electrical engineer and pilot, so I like where you seem to be heading.

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    Matt Gonitzke's Avatar
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    As far as aircraft performance is concerned horsepower is horsepower. The equations don't care what is producing the power. It is up to the designer to use the appropriate numbers for each flight condition. An electric motor can produce maximum torque at essentially any RPM, and also would not exhibit performance degradation with altitude as an air-breathing powerplant would.

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    True, 1HP will always be 1HP. I just wanted to point out the difference in how the motors advertised HP rating are derived since I don't believe that is common knowledge.

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    Aaron Novak's Avatar
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    Quote Originally Posted by KJ Hamblin View Post
    True, 1HP will always be 1HP. I just wanted to point out the difference in how the motors advertised HP rating are derived since I don't believe that is common knowledge.
    Incorrect, in the aviation and marine world engines are rated and a specific RPM that is typically where you would "prop" it, or whats called the "rated RPM". What you are speaking of sounds more like automotive salesmanship. HP is HP (rpmxtorque)/5252.

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