E-Power 101

Discussion in 'General Plane Discussion' started by TManiaci, Sep 5, 2008.

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  1. Sep 5, 2008 #1

    TManiaci

    TManiaci

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    So, we have been having discussions on Electric system setup for aircraft. I thought it might be nice to cover the basics of e-power in a thread dedicated to the technology. It's not as simple as it seems, and actually confusing at first. There are many factors to consider in the selection of hardware to power an aircraft.

    All the components have thier own ratings, capacities and performance factors. To properly set up a system the items that make up a "system" need to be selected properly to deliver the most bang for the buck, and provide the least weight for the desired power requirements. The key ingredients in the mix are:

    a. Motor: Wattage/Amperage Capacity, kV (rpm/volt),
    b. Battery: Voltage, Amperage Capacity, C Ratings
    c. ESC: Continuous Amp Rating, Burst Rating, BEC capacity
    d. Propeller: Diameter and Pitch
    e. Power needs of the Control System

    These items all can influence each other. So let's discuss the details of these items and thier attributes that we must consider.
     
  2. Sep 5, 2008 #2

    CA$HEY

    CA$HEY

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    Im in,,im pritty right on everything,EXCEPT,,the 1P,,and 2P thingey
     
  3. Sep 5, 2008 #3

    TManiaci

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    The key consideration is to put together a system that will perform to your desired needs. What is desireable is probably the first question to address.

    Some may disagree, but it the basic rules-of-thumb FOR 3D Aircraft is probably best described as follows in terms of Wattage, where wattage is the best and easiest measure of electrical power.

    Performance of 3D aircraft is one of the most demanding requirements. We all want robust power to pull out of 3D stall speed manuvers. That punch can get you out of trouble, and many 3D acrobatics require monster power to execute. This is probably best described in terms of power to wieght ratio. For e-power, Watts per Ounce or Watts per Pound are the units of measure we can all mathematically derive. The ratings I find best describes our world is as follows:

    Under 10 Watts per Ounce = weak, will fly okay but not enough.
    10-13 W/oz = Good, where most people fly.
    13-15 W/oz = Very good, great pull-out and ample power to do anything.
    15-20 W/oz = Strong, more than you need but a helloffa lot of fun.
    over 20 W/oz = Stupid power... you really over-did it!
     
  4. Sep 5, 2008 #4

    sandman

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    Once you learn the very basics, learn to use a motor power calculator, and then invest in a data logger like the ones from Eagle Tree Systems. With the motor calculation program you can determine battery need, prop selections, etc. and with the logger, you can see if in flight performace matches the predictions. The data logger makes comparing props, batteries, ESCs, motors, much more objective. I have found this invaluable for finding the lightest, most powerful setup for electric aerobatic planes. If you'd more information about using a logger, I'll be happy to contribute here.

    I have also developed a simple test to evaluate motor quality.
     
  5. Sep 5, 2008 #5

    TManiaci

    TManiaci

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    In the simplest terms, the power comes from the battery, is controlled and delivered by the ESC (Electronic Speed Controller) to the Motor and the motor converts the electrical energy into radial motion to turn the propeller.

    So, let's start with the power source, the battery. For high current delivery and light wieght, the best option today that is affordable is the Lithium Polymer battery pack, also referred to as Lipo or Lipoly. These little guys really pack a high energy density. So, let's focus on this popular battery type.

    Battery ratings are a little cryptic, so let's get that lingo dialed in here.

    mAH, or milliAmp-Hours, is the measure of "capacity". 1000 mAH is equal to 1 Amp-Hour (AH). The bigger the number, the more energy that is stored, and the bigger and heavier the battery will be. The units mean the number of Amps the battery will deliver for a given time peiod. So, a 2100 mAH battery will deliver 2100 mA (2.1 A) for 1 hour.

    The "C" rating... a measure of Amperage Delivery Capacity. Most Lipo's provide a "Continuous" and a "Burst" rating. The meanings are obvious, and we generally don't engineer our systems to pull more than the Continuous rating at full throttle, often called WOT (Wide-Open Throttle). The "C" is a multiplier of the AH rating. So, a battery that has a mAH rating of 2100 and a 25C rating will deliver the product of the AH x C, or 2.1AH x 25C = 52.5 Amps.
     
    Last edited: Sep 5, 2008
  6. Sep 5, 2008 #6

    TManiaci

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    Thanks Sandman. You're comments are really appreciated. You are referring to some fairly advanced aspects of this topic. After we get the fundamentals all laid out, let's discuss your experience with loggers. That will tie all the pieces together for everyone.
     
  7. Sep 5, 2008 #7

    snap

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    Looking good t-man, I hope we go to lin-bec vs switching bec,then 2.4 with high voltage electric systems vs 72mhz. If you need any info I love to contribute ,I do a class once a month to certify electric flyers at are field.
     
  8. Sep 5, 2008 #8

    TManiaci

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    I hope we can cover that and a lot more Snap. I'll need some help with ultra-high power system issues, and the 2.4GHz issues. Let's save that for later after we get basics down.
     
  9. Sep 5, 2008 #9

    snap

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    This is very cool, like an online class, looking foward to E-flight 105
    lol
     
  10. Sep 5, 2008 #10

    crash

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    I'm glad you are doing this TMan, now maybe I can figure out which kv motor I need for a plane. I have always been lost on this.
     
  11. Sep 5, 2008 #11

    TManiaci

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    A little more about batteries...

    So, Lipoly batteries are made from a grouping of cells. These cells are each 3.7 Volts. The size of the cell, again, correlates to the Amp-Hours of capacity.

    The terminolgy used to describe a battery system is a shorthand method to describe the series and parallel battery configuration.

    Series is where the batteries are wired plus to minus, plus to minus, etc., such that the Voltage is added, and the net capacity is the capacity of one cell.

    Parallel is where the plus sides are all wired together, and the negatives are all wired together. So, the Voltage is the same as each individual cell or pack, and the capacity adds together. This is the opposite of Series.

    We always refer to a simple single pack as a 3-cell, or 4-cell pack. Those packs are actually 3S, or 3 of the 3.7V cells wired in Series. Therefore, a 3S pack is 3 Cells x 3.7V/cell = 11.1 Volts. They are properly noted as a 3S1P, where there is only one pack in parallel.

    So, the Nomenclature is X(Series)Y(Parallel). Hence, a 3S2P 2100 mAH is two three-cell 2100 mAH packs wired parallel to each other. The voltage is 11.1V, but the capacity is doubled by the mAH rating of one pack, or 4200 mAH in this case. The packs are ALWAYS mAH matched, so you will not see mixed mAH packs in series or parallel.
     
    Last edited: Sep 5, 2008
  12. Sep 5, 2008 #12

    bdavison

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    Series and Parellel are pretty easy.

    Think of Power and Time.

    Series packs give you more power by increasing voltage.
    Parellel packs give you more flying time by increasing capacity.

    Another aspect of lipos is C ratings. C ratings ONLY tell you how many amps you can draw from the pack before it translates it into heat.
    It works like this....

    Each C equals 1 times the batteries capacity. So a 2100mAh pack rated at 10C means you can draw 21A.
    2100mAh * 10 = 21A
    2100mAh * 20 = 42A
    2100mAh * 25 = 52.5A

    Most lipos come with a rating that says something similar to
    "20C Cont. 30C Burst"

    What that means is that the battery is capable of providing a 20C continuous discharge until it reaches minimum voltage. And it is capable of providing 30C currents in short bursts. Most manufacturers rate the burst as a 15sec time limit. Check the label on your pack, or with the manufacturer regarding the time of burst the pack is rated for.


    Burst current is important, because it lets us know how much amperage we can draw from the battery during high performance maneuvers like vertical climbs or rapid throttle changes. As long as you dont exceed the Burst C rating for longer than the manufacturer recommends....the battery will perform.

    Now that Ive talked about C ratings....here is why.

    If you run a two packs in parellel....it raises the amount of amperage you can pull from the pack.

    For example say you take two 3s1p 2100mah packs, and connect them in parellel. And the each lipo is rated for 20c cont/30c burst.
    You have to add the packs capacity together.

    2100mAh + 2100mAh = 4200mAh * 20c = 84A cont discharge rate
    2100mAh + 2100mAh = 4200mAh * 30c = 126A burst discharge rate

    Its important to note....that if you connect the two packs in series, you DO NOT add the capacity of the packs together. You still have a 2100mAh pack, and the amount of amperage you can draw remains the same as if you only had one of the packs. But....you have twice the voltage.
     
  13. Sep 5, 2008 #13

    bdavison

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    The above is a good example of why it is sometimes better to run two smaller packs with high C ratings vs a single larger pack with a lower C rating.

    For example.

    Say you have two 2100mAh packs in parellel with 20C cont/30C burst
    Thats 84A cont/126A burst.

    Or you have a single 4200mah pack with a 12C cont/25C burst
    Thats 50.4A cont/105A burst.

    So its important to not only look at the voltage and capacity of the lipo, but also the C ratings.
     
  14. Sep 5, 2008 #14

    Showtime

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    Great info Guys Keep it coming.
     
  15. Sep 5, 2008 #15

    TManiaci

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    Great input there BD, thanks!

    So, What Brian is alluding to in simple terms is that battery configuration and selection can be "tuned" to provide a variable array of possible performance types.

    There is no "optimum" setup. Your preference is the issue. You can design a very light system with tons of power, but very short flying time, or a long flight capacity that is very heavy, and everything in between.

    Generally, we all seem to target a flying time in the 8-12 minute range. That brackets the capacity as a starting point for deciding on your battery selection.
     
  16. Sep 5, 2008 #16

    TManiaci

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    Summurizing the battery nomenclature, the X(Series)Y(Parallel) provides a means to decribe battery wiring senarios. From this and the capacity, you can derive mathematically the voltage and capacity.

    XSYP

    Voltage = 3.7 Volts x X

    Capacity = (Cell AH Rating) x Y

    Remember that the "C" factor multiplies the capacity in units of Amp-Hours (not milli-Amp Hours), so divide the mAH by 1000.

    So, a few examples:

    6S2P 3200 mAH 25C

    X = 6, so 6 cells x 3.7 V/cell = 22.2 V
    Y = 2, so 3.2 AH x 2 = 6.4 AH (6400 mAH)
    Amp Load Capacity = 6.4 AH x 25C = 160 Amps
    Wattage (Power) Capacity = Voltage x Amperage (P = V x A)
    ...so, P = 22.2V x 160A = 3552 Watts

    4S4P 4800 mAH 20C

    4 cells x 3.7 V/cell = 14.8 V
    4.8 AH x 4 = 19.2 AH (19,200 mAH)
    Amp Load Capacity = 19.2 AH x 20C = 384 Amps
    P = 14.8V x 384A = 5683 Watts
     
    Last edited: Sep 5, 2008
  17. Sep 5, 2008 #17

    bdavison

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    Im patiently waiting for E-Power 105....I got some ESC/Motor stuff thats gonna blow your mind.
     
  18. Sep 5, 2008 #18

    TManiaci

    TManiaci

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    LOL, you are funny.... you think we have buried everyone with too much tech-talk?
     
  19. Sep 5, 2008 #19

    Showtime

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    I dont think so...
     
  20. Sep 5, 2008 #20

    TManiaci

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    Okay... how about we talk about the next component in the system, the Electronic Speed Controller, or ESC as it is usually called.

    This device is quite a jewel of technology. There are two basic types of Motors: Brushed and Brushless. We'll talk more about that later, but the point here is that there are ESC's for each type. The ones we will focus on are the Brushless type, which now dominate our e-power world.

    The Brushless motor is really a little 3-phase AC electric motor. It's not DC in the conventional sense. That's why there are 3 leads on the brushless motors, and the field windings in the motor are always in multiples of three.

    The Brushless motors are far more efficient as they convert electrical energy into kinetic energy (rotary motion). There are no brushes to wear out, and they don't require a starter winding to initiate rotation in the proper direction.

    The Brushless ESC has very sophisocated 3-phase inverter circuits to generate the wave-form that approximates power from a conventional 3-phase AC power source. That output is also controlled with timing and other features like braking to make the motor behavior more elegant and efficient.

    So, the DC battery (Lipoly) connects to the input side of the ESC, and three output leads connect to the motor. The other connection is a 2-way deal, where we Supply power to the Reciever (and Servos) AND we Recieve Throttle Control channel commands to control the motor output. The Power Supply and Signal are in the same connection that connects to the Reciever (Rx). When we have this arrangement, and are using the power from the battery thru the ESC to supply the Rx with power, it is known as a Battery Eliminator Circuit, or BEC.

    The BEC Power Circuits on the ESC come in two styles.... Linear and Switching. For small systems (say under 50 amps) the linear BEC is pretty common and works well.
     

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