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Kv versus synchronous RPM

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Old 12-10-2015, 06:33 AM
  #1  
danever
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Default Kv versus synchronous RPM

Hello,

I am new to Electric flight.
Since brushless motors are essentially 3 phase motors, why is the RPM related to voltage, as in KV?
The ESC generates a 3-phase control that must force the motor to rotate at a synchronous RPM, regardless of the voltage (within limits of course).
If a 3-phase motor can't stay synchronous to the ESC, then it will operate at a very low efficiency as it tries to "catch-up" with the ESC driving frequency.
Therefore, as I read in one forum, "KV is RPM per volt at, no load, installing a prop will load the motor with consequently lower RPM".
What happened then with the synchronous drive from the ESC?
Shouldn't the motor RPM remain constant with different loads as long as the ESC driving frequency does not change?

Dan
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Old 12-11-2015, 08:27 AM
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Jennifer Curtis
 
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The ESC senses the position of the rotor
in relation to the magnets, and "phases"
its output to match, so the motor is not
trying to "catch up." It pulses, at much
higher frequency, to regulate power,
(torque and thus speed).


Jenny
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Old 12-13-2015, 04:16 PM
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Quorneng
 
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It is best to think of a brushless motor as a remote sensed DC motor.
The Electronic Speed Controller sends a 'pulse' of DC down two of the wires, the timing of which is sensed by the back EMF generated in the third. This process performs the same function as the commutator in a brushed motor and within limits has no impact on the ultimate speed the motor can rotate at.

The power delivered to the motor by the speed controller is by a high frequency pulse width modulation which is exactly the same process as is used by an electronic speed control for a brushed motor.
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Old 01-10-2016, 05:12 AM
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ron_van_sommeren
 
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RC brushless motors are brushed motors with electronic commutation (the ESC) instead of mechanical commutation (ring and brushess).
No dedicated measuring pulses, just the power signals, and the voltage generated by the motor (BEMF signal)
The system is iso-synchronous. Zero slippage. RC controllers are not VFD's.
More
www.aerodesign.de/peter/2001/LRK350/index_eng.html
→ DIY brushless controller

How does a brushless motor work? - RCU sticky

Excellent active design discussion
BLDC controller

Vriendelijke groeten Ron


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Last edited by ron_van_sommeren; 01-10-2016 at 04:07 PM.
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