ELECTRIFICATION

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ELECTRIFICATION
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ELECTRIFICATION

• Introduction
• Basics of electric motors
• Brushed motors, Brushless Out Runner motors
• Motor Constants
• Choosing a Motor with makers data
• Interpreting makers data
• Rules of thumb are just that and not theoretical
• Summary
• Useful Web sites

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The Coil
Magnetic field of a wire
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The Coil

• One loop's magnetic field

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The Coil

• A simple electromagnet

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An Electric Motor

• Electromagnet in a horseshoe magnet

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An Electric Motor

• Parts of an electric motor

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An Electric Motor

• The commutator and coils rotates between two
  magnets. This is a three pole motor

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Brushless Outrunner Motor
By Torcman

• The coils are stationary the casing for the
  magnets around the periphery rotates.

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Brushless Outrunner Motor

• Torcman Motor 14 magnets 14 Poles
• Magnetic field goes around 7 times for 1
  rev of Outrunner
• Outrunner type motors have more torque than
  same size brushed motor

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Electromechanical motor constants

• The geometric arrangement of the magnetic circuit
  and winding defines in detail how the motor
  converts the electrical input power (current,
  voltage) into mechanical output power (speed,
  torque). Two important characteristic values of
  this energy conversion are the speed constant kn
  and the torque constant kM.
• The speed constant combines the speed n (rpm)
  with the voltage induced in the winding Uind
  (Volt). Uind is proportional to the speed the
  following applies
• n kn . Uind
• Similarly, the torque constant links the
  mechanical torque M (Newton) with the
• electrical current I (Amp).
• M kM . I
• The speed and torque constant are not
  independent. The following applies
• kn . kM 60 / 2p
• Efficiency Output power / Input power hence
• ? 2p/60 . n . M / U . I

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Electromechanical motor constants
Power balance Pin Ploss Pmech U.I R.I2
M.n.2p/60 RResistance U.M/km
R.(M/km)2 M.n.2p/60 N U. 60/ 2p.km.
R.M.60/2p.km2 But kn . km 60 / 2p

• In the speed-torque diagram, the output power is
  equivalent to the area of the rectangle below the
  speed-torque line. This rectangle and, thus, the
  output power, is greatest at half stall torque
  and half no-load speed.

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Choosing A Motor

• RULE OF THUMB

! Max Rivers who distributes Torcman suggests
twice as much power !
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Choosing A Motor

• If the model weighs 300 g and you want it to be
  3D capable we need gt 52 Watt.
• That determines the size of motor a little bigger
  is a good idea say 60 Watt.
• How many cells will determine the voltage.
• Choose a propeller to absorb the power.
• Remember Watts Volts X Amps
• Choose the speed controller to suit with the
  current
• Big propellers turn slower. Look for a lower kv
  in the same motor size.
• Lower kv means more turns or windings.
• Ducted fans want high RPM so less turns are
  wanted
• AXI give very useful data

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Choosing A Motor

• This data is for the 2208/34

Notice how the voltage is not consistent, the
volt drop in the battery increases with current
drain.
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Useful Information or not?

• Thrust data is for the static condition, because
  of drag and wind speed, model shape, it is
  possibly only of interest for prop hanging
  indoors

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Choosing A Motor
If model weighs 300 g this data tells us the
motor should work, but we cannot tell much more
without using maths! kn is given presumably at
no load. Using the formula we can predict the
speed at any torque and the power (M.n P
watts). Of course we still need to know what
power is needed for a given propeller.
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Choosing A Motor

• So we know the voltage, the power, the rpm a
  motor will deliver now we need some propeller
  data. If everything matches that is the motor to
  buy!
• Another rule of thumb for 69 inch propellers-
• U volt constant then 1 inch less diameter reduces
  power by 25, RPM goes up 5, thrust is down 40
  . The converse is true for 1 inch bigger
  diameter.
• 1 inch less pitch reduce power by 10 at same
  RPM.

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Choosing A Motor

• Warning about - Rule of thumb
• Do not blame me! They are inaccurate!
• The idea is to get a handle on how things relate
  to one another.

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Choosing A Motor
This data was derived theoretically using the
relationship that power and rpm are connected by
a 3v
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Electrification Summary

• Brushless motors are more efficient and more
  powerful than the same size brushed motor.
• Brushless Motors are different not necessarily
  better, it depends on what you want.
• More turns means less speed and more torque.
• If you copy someone else make sure they tell you
  exactly what they have, look at the specs and
  confirm.

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Electrification Summary

• If you go your own way
• Estimate the model weight.
• Decide how you would like it to fly.
• Match the motor to the power needed.
• Decide on battery type
• Pick a propeller to absorb the power.

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Web Sites

• www.megamotor.cz/en/novinky.phtml
• www.kb-modell.de
• www.modelmotors.cz/index.php?idenncdomu
• http//www.torcman.de/index.htm