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frequency of commutaion of bldc diy project

kaunasLT

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Hello.

I was creating a bldc driver using no bldc driver IC but with 3 mosfets half bridges.

The embedded code was switching between the commutation stages, starting slow with an increasing frequency of steps using a timer ISR and moving its start value closer and closer to its max (255 bits).
The high side was given a PWM of low duty (5%) AND' gated with the high sides commutation signal.

the motor started "rough" until the frequency of commutation slowly got faster and faster and in the end somewhat drove smoothly in open loop.
How ever some audible coil whine was heard and measuring the PSU rail for the motor i could see drops of several volts and it couldnt be bypassed with caps. so the inductors inside the bldc must be saturating?

I started with the idea of using Laplace to calculate the time the fet could be on before the internal inductor saturates but found out no actual information are usually given of these hobby bldc's inductance and it would properly also not be scalable for various different bldc motors.

what would be the appropiate frequency of commutation of 3 fet half bridge bldc diy driver? i was very limited in terms of speed since the PIC only had 4mHz clock with no internal PLL so the timer started VERY slow and got to 100 ish Hz
 
Hi,

thanks god we have schematics, timing diagrams and so on, so we don´t need text to describe electronics.
And we have volts and amperes and measurement tools.

Klaus
 

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