This may be known, but I haven't read it anywhere, so I thought I would put it out here to see what you say.
I've seen that controllers have a limit on drive frequency to the motor, the so called Electrical RPM(ERPM) limit. Since microcontrollers are fast, it seems odd to me that the microcontroller would cause the limit. After looking at some numbers it seems that it could actually be the PWM frequency limiting the ERPM.
Since a BLDC is commutated 6 times per cycle, and there is 60 ERPM per cycle, the ERPM to commutation frequency has a simple relationship of 10:1. (I hope I'm correct?)
http://www.kellycontroller.com writes:
100.000 ERPM is a commutation frequency of 10kHz. Since the PWM frequency is 33kHz, it seems Kelly limits the ERPM to a about 3 PWM periods per commutation period.
Hobbyking writes:
210 kERPM is a commutation frequency of 21kHz. The PWM frequency is 24kHz, which makes it limit to about 1 PWM-period per commutation period.
So, to me it seems the physical limit of a controller using PWM at 20kHz is 200,000 kERPM, and the only way to increase the ERPM is to increase the PWM frequency (which increases losses in transistors), or changing the strategy of driving the motor away from the PWM:ing 3-phase bridge.
I've seen that controllers have a limit on drive frequency to the motor, the so called Electrical RPM(ERPM) limit. Since microcontrollers are fast, it seems odd to me that the microcontroller would cause the limit. After looking at some numbers it seems that it could actually be the PWM frequency limiting the ERPM.
Since a BLDC is commutated 6 times per cycle, and there is 60 ERPM per cycle, the ERPM to commutation frequency has a simple relationship of 10:1. (I hope I'm correct?)
http://www.kellycontroller.com writes:
Up to 40,000 electric RPM standard. Optional high speed 70,000 ERPM, and ultra high speed 100,000 ERPM. (Electric RPM = mechanical RPM * motor pole pairs).
Optional Ultra High Speed to 100,000 ERPM
Price: $99.00
1. Dual 32 bit micros, with 33.3kHz PWM
2. Up to 100,000 Electric RPM
100.000 ERPM is a commutation frequency of 10kHz. Since the PWM frequency is 33kHz, it seems Kelly limits the ERPM to a about 3 PWM periods per commutation period.
Hobbyking writes:
http://www.hobbyking.com/hobbyking/store/__8921__TURNIGY_K_Force_120A_HV_OPTO_5_12S_Brushless_ESC.htmlMax Speed;
2 Pole: 210,000rpm
6 Pole: 70,000rpm
12 pole: 35,000rpm
210 kERPM is a commutation frequency of 21kHz. The PWM frequency is 24kHz, which makes it limit to about 1 PWM-period per commutation period.
So, to me it seems the physical limit of a controller using PWM at 20kHz is 200,000 kERPM, and the only way to increase the ERPM is to increase the PWM frequency (which increases losses in transistors), or changing the strategy of driving the motor away from the PWM:ing 3-phase bridge.