Newbie with a project

Discussion in 'The Projects Forum' started by Bruino, Nov 1, 2008.

  1. Bruino

    Thread Starter New Member

    Oct 28, 2008
    4
    0
    Hi guys. I'm new to electronics, please be patient.

    I would like to build a generator for my rc helicopter. I have a brushless motor which produces aprox. 7 to 18 volts , depending on revs of main motor. I would need it to supply 5.4vor 6v to my ignition supply battery, which is a 4.8v 2500mAh nimh.

    The ignition requires 6v max and 1700mA. It would be identical to a car alternator, charging the battery and supplying power to the ignition.

    I'm not sure what amps the motor will supply.

    Thanks in advance.
    Shaun
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Actually, your post is a bit unclear. I saw it as using an electric motor for the rotor at first. On second reading, it seems that you have a spark ignited engine (I left off with glo-plugs), and wish to add in the motor to act as a battery charger.

    If that is the case, you could add some rectifier diodes to isolate the motor, put in a filter capacitor, and run the output through a 7806 regulator. It could get a bit hot, so arranging for a small heat sink and airflow would be critical.

    Can your motor generate anything like enough current to charge the battery and keep up with the ignition? If not, it will keep you flying a bit longer, but may not give much satisfaction due to the performance hit. You will have the extra weight, plus the generator will pull power from the engine.
     
  3. Bruino

    Thread Starter New Member

    Oct 28, 2008
    4
    0
    Hi sorry for being vague.
    It is a petrol 2 stroke engine, fairly big 23cc with aprox 2.5HP. A tank of fuel will run for 30 min. but battery will not. If generator could supply power to spark ignition and battery would only be used at start up and as a back up that would be great.
    Cheers
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Welcome to the Forums, Shaun. We try to be patient :)

    OK.
    Power in Watts = Voltage x Current, normally expressed as P=EI.
    So, P=6V x 1700mA = 6 x 1.7 = 10.2 Watts. That's just your requirement for keeping your ignition going.

    Let's figure out what the equivalent resistance is of your ignition system.
    Since Resistance = Voltage / Current (in Amperes), normally expressed as R=E/I, and E=6V, I=1.7A, so R=6/1.7 = 3.53 Ohms.
    You could roughly simulate that by using three 10 Ohm 10 Watt power resistors all wired in parallel as a load; three perfect 10 Ohm resistors in parallel will measure 3.333... Ohms.

    Connect them up to the output of the motor, and spin it up. While the motor is spinning, measure the voltage across the resistors. This will enable you to calculate the current throught the resistors:
    I=E/R (Current = Voltage / Resistance)
    If you measured 6V, then you would calculate:
    I = 6/3.333 = 1.8 Amperes.

    10+ Watts would be quite a load for a model helicopter engine, and your motor would add weight to the aircraft. Generators/alternators are far from 100% efficient, and neither are couplings such as V-belt drives. It would not be unrealistic to assume that for 10W out, you'll need to put 20W in to overcome the inherent losses.

    1 Horsepower = 746 Watts (approximately). If you know what the horsepower rating is of your engine, then you can figure out what percentage of the power the generator will be reducing that by.

    There is also the "weight and balance" of the aircraft. This is critical for ANY aircraft. If you change the CG (center of gravity) by even a small amount, the aircraft may become unstable. Also, increasing the gross weight of the aircraft will diminish flight time due to increased fuel consumption required to maintain altitude and speed. If the gross weight approaches the lift that can be produced by the engine, the climb rate will be severely impacted, even to the point where the aircraft is not able to begin or sustain flight.
     
    Last edited: Nov 1, 2008
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    Model aircraft generators for in-flight recharging have not been very widely applied. First, there is the weight issue. Second, you would need to recharge while it is being used. That can be tough and may not even be possible with some battery chemistries. It will work with lead acid. I suspect it would be increasingly complex with NiCd, NIMH, and LiPo.

    The real question that struck me is why you cannot get 30 minutes flight time from a 2500 mAH battery. Is your battery shot? Is your ignition shot? Can you use a bigger battery (like a 4000 mAH LiPo)? And finally, have you considered switching to "gas glow" and running without a spark plug or switching to a magneto ignition. A 1200 mAH battery for radio and servos alone will give you more than enough flight time.

    John
     
  6. Bruino

    Thread Starter New Member

    Oct 28, 2008
    4
    0
    Hi
    Thanks for info.

    There are a few of these on the market which supply 5.4v and 3A. The weight is approx 100grams which is negligible. I would like to build one myself as a challange.
    Running the ignition off the bat alone would require me to have quite a few bats for a full day at the club.
    Cheers
    Shaun
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    I thought you wanted 30 minutes flight time, now it seems to be a full day.

    We just use two battery packs and charge one while flying the other.

    John
     
  8. Bruino

    Thread Starter New Member

    Oct 28, 2008
    4
    0
    Thanks For help
     
Loading...