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Servo jitter at full motor throttle?
  • I bought a 1.7 gram digital servo and a couple of micro linear servos from Hobby King. The battery is a one cell LiPo. I tested them with an AP03 micro motor. When the motor throttle was near 100%, one of the servos started jumping around. The battery, motor, and speed control all started getting warm. What do you think is wrong?

    (via email from Rafael)
  • It sounds to me like the propeller is too large for the motor. At near full throttle, the voltage is dropping and it is causing the servo to malfunction.

    Carlos
  • The user should ALWAYS test the maximum power of their system using a Watt Meter.  They are not expensive, and ensure that the propeller is not too large nor too small.  It is not sufficient to use the recommended propeller, as variations in the motor winding and battery capability will cause a large variation in the final result.  I recently tested a system that was supposed to achieve 2kW on a 14x10 propeller, but the result was only 1kW.  This was caused by the motor resistance and Kv being only slightly different from specification - Kv was 295 instead of 320, and still well within the manufacturing tolerance.  Since Power is related to rpm^3, the slightly lower Kv caused a much lower ultimate power.  All I needed to do was use a 16x10 prop and the system achieved the target 2kW.  I agree with Carlos - it sounds like you are overloading your motor, causing low voltage from the BEC to the receiver + servos.  

    There could be an alternative explanation - electrical noise.  If your system is at correct power and the voltage supply from the BEC is OK (use a volt meter to check), it's probably electrical noise.  I beat this problem a while ago, and the details are in this [relatively short] thread: 
    http://www.rcgroups.com/forums/showthread.php?t=678289
  • That is a really good point. In fact, I just bought myself some ferrite rings just in case I run into a problem just like you describe.

    Carlos

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