DC motor rpm

Discussion in 'The Projects Forum' started by skullforger, Aug 2, 2009.

  1. skullforger

    Thread Starter Member

    Nov 3, 2008

    My nephew has a toy rc helicopter. It has a main rotor motor and a tail rotor motor (both little dc motors). Their RPM is linked (if you speed up the main rotor, the tail rotor also speeds up, because it has to generate the necesarry counter-torque). Yesterday, the tail rotor motor broke down, so the helicopter only spins clockwise and is no longer flyable. I tested the wires to it, and they're still ok (about 9V at maximum throttle). So I guess the motor itself is broken. I have a dc motor of about the same size lying around, and I would like to replace it. The problem is that I don't know if the RPM will be the same as the original motor. If the RPM is too high, the helicopter will spin counterclockwise, if it's too low, it will spin clockwise. Can I just put a resistor between the motor and the supplying wire to create a volage drop to lower RPM? Or am I missing something here?

    PS: The original tailrotor motor has a little PCB with 2 tiny capacitors soldered to it. I think I will be able to desolder it and solder it to the replacement motor. (I guess the capacitors have something to do with clearing out minor peaks and disruptions in current).
  2. mik3

    Senior Member

    Feb 4, 2008
    The capacitors reduce the noise produced by the motor and provide the sudden changes in the motor current.

    You can use a resistor to achieve this but it will waste a lot of heat. Make sure it can handle it and not burn. Also, you can use a diode or two in series to reduce the voltage across the motor.
  3. fanie

    Active Member

    Jan 20, 2007
    You also get a host of different motor types that varies in speed and torque due to the way they were wound, amount of poles etc etc. To replace the motor it would be best to replace it with the original but you probably can if the motors are close to the same thing.

    Why not see if you can fix the motor ? It may be as simple as fixing the brushes maybe.
  4. skullforger

    Thread Starter Member

    Nov 3, 2008
    Ok. Thanks for the information guys! I have never opened/fixed a dc motor so I don't know what to look for and how to fix it. I guess I'll try with the resistor or the diodes then. Thanks.