Kv versus synchronous RPM
12-10-2015, 06:33 AM
#1
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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
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
12-11-2015, 08:27 AM
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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
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
12-13-2015, 04:16 PM
#3
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.
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.
01-10-2016, 05:12 AM
#4
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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
Attachments from
www.aerodesign.de/peter/2001/LRK350/index_eng.html
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
RonAttachments from
www.aerodesign.de/peter/2001/LRK350/index_eng.html
Last edited by ron_van_sommeren; 01-10-2016 at 04:07 PM.
