I run a loaded BLDC motor. Current meter shows 30A. How much current at average will flow through each phase? In BLDC 6-step commutation current flows to a phase and comes out of another, then it changes to next phases. Do it means that each phase pases 2/3 of the full load on average, as it passes full current 2/3 of the time?
Electrical – BLDC motor phase max current
brushless-dc-motorcurrentmotorphase
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So, if I want the speed to remain at, say 3000rpm, and increase the thrust from, say 400g to 500g, I increase the current through the motor. I do this by increasing the PWM duty cycle. But, this means the speed will also increase from 3000rpm to a higher value, right? How can this be maintained then?
You would only want to increase the torque (thrust) if the mechanical load were threatening to slow the motor down - by increasing the PWM to counter that increase in load torque you are largely keeping the motor at constant speed. Remember a simple DC motor (even a brushless type) has a very clear torque-speed characteristic that means, for a given speed at a given torque, an increase in torque of \$x\$ will reduce the speed largely by \$x\$ also.
It's a not a perfectly linear relationship and changes entirely when field windings are implemented but it's fairly reliable: -
The absolute best way to measure the current in a line is to use a current probe such as the TCP0030A. They are a bit pricey, but totally worth it if you are doing motor control.
Of course, if you are doing this in an application, you can use three ACS711 (pdf warning) chips (or similar). These are good enough to use for current control loops or similar, but not necessarily good enough for lab-grade measurements.
I have used both successfully.
What happens in a stall condition depends entirely on the drive architecture. Most drives have some sort of stall protection built-in so that they don't burn up the motor.
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If assumption can be made that your motor controller (ESC) will switch full battery voltage in PWM pulses then your measured battery current will be equal to RMS (about 0.7 times amplitude for sinusoidal or function of a duty cycle in PWM)) of the current flowing through 2 phases cables (2 out of 3 motor feed cables) plus loss in your ESC (controller). Calculation is better done as follows:
your battery power taken P bat = Vbat * Ibat
adjust for the loss in ESC Padj = Pbat * k where K is your ESC efficiency (0.9 for 90% efficiency of ESC)
Measure amplitude of PWM pulses Vpwm (should be close to Vbat less resistive loss in the wiring), if you want only to get estimate you can assume that PWM voltage amplitude is the same as Vbat
Your motor current through 2 phases (if star wiring) will be Imotor = Padj / Vpwm , this Imotor is RMS value.
Some digital oscilloscopes show this value calculated for an observed waveform for any duty cycle of waveform.