# Calculation of back EMF by ADC measurement in MC9S12ZVM_BLDC_Sensorless

Question asked by qinzhi liang on Sep 18, 2018
Latest reply on Oct 10, 2018 by MATEJ PACHA

Dear friend:

I have a problem of calculating of back EMF in the MC9S12ZVM_BLDC_Sensorless,which is Official routine.

Why is calculate the back EMF by the flowing  routine?

{// LEFT justified result ADC data

DCBusVoltageHalf = MLIB_Mul(ADC0ResultList[1]>>1, 13653 ,F16); // DC Bus Voltage * 5/6 / 2

bemfVoltage = phaseVoltage - DCBusVoltageHalf;

.....................................

...................................

}

According to information of AN4704.

we get a new equation from Eqn.7              ec=(2*uc-uDCB)/3

ADC0ResultList[1] is uDCB by ADC measurement. But this value is normal 1/5 by Internal shunt.

because LEFT justified result ADC data, now ADC0ResultList[1] is 16 times normal value.

ADC0ResultList[0] is uc (Phase voltage），this value is 16 times normal value by LEFT justified result ADC data.because Internal shunt, This value is divided by 6.

Finally, we get a equation ec=(4*uc-2*uDCB)/3 by Front program,

this EMF is two times theoretical value .

Please tell me where is wrong.