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Motor

In the multirotor world we exclusively use “brushless” motors. For the difference between brushed and brushless check this link.

 

Basic parameters:brushless outrunner motor

  • max. current (I) acceptance / max. power (W) acceptance
  • kv number
  • tatic thrust per given propeller
  • number of magnetic poles
  • max. unloaded current (Io)
  • internal resistency (Rm)

 

The most important for us to know is the kv number of the motor. It is defined as RPM per 1V voltage, related to a perfect (100% efficient) motor. So for example, a “perfect” 900kv motor would run on 9900RPM on a 3s battery (11,1V). The lower is the kv number the lower is the RPM and the higher is the torque. Motors with lower kv values usually consume less current.

The real RPM can be calculated by this formula: RPM = Kv * (V – Vloss) = Kv * (V – I * Rm)

 

The intake power (W) equals to the multiplication of the voltage (V) and current (A), or the multiplication of the torque and RPM. The output power depends on the motor’s efficiency, but it is always lower then the intake power. This can change depending on the size of props, battery, voltage. To find the best final efficiency of the drive one have to find the optimal motor+prop+battery combination.

The real output power can be calculated by this formula: W out = (V – I * Rm) * (I – Io)

The motor efficiency is given by the ration of output power/intake power. The better the efficiency, the longer the flight time, so for some applications it is worth to obtain high efficiency motors.

 

By the time of planning a multirotor usually we check the following:brushless outrunner motor

  • will there be enough lift power with the specified propellers by the given AUW?
  • is the moter overpowered enough to use bigger props later on?
  • can we use the motor with different (higher) cell count batteries?
  • is the motor construction good enough (quality shaft, magnets, bearings), especially if used for an AP platform?
  • what quality are the motor wires of, would not they break or tear?
  • is the shaft size compatible with the specified propeller hole?
  • is not the shaft turned upside down, regarding the motor mounting?
  • is there an existing “accessory pack” for the motor?
  • is the propeller fixing type compatible with the motor shaft solution?

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