Back EMF calculation for BLDC motor

[cw2611]

Hey!

I need to calculate the back EMF for my BLDC motor to input to the MCE wizard. I have calculated the value based on your document found here: https://www.infineon.com/dgdl/Infineon-How-to-Measure-Motor-Parameters-ApplicationPresentation-v01_0...

The value I got was 3.71. Should I enter this value in the wizard?

I have attached the waveform also. Please verify if this is correct.

PS: I am working on IMC302AF064XUMA1 microcontroller.

 

[psb]

Hello @cw2611 , 

  The waveform seen doesn't ensure an accurate and also a precise measurement. Request you to follow the same procedure with a complete rotation of the motor giving three electrical cycles, trying to maintain it at a uniform speed and measure the value of the back emf constant cycle by cycle and show the result here. 

[cw2611]

How about this waveform? This is what we are getting with one mechanical rotation by hand.

[psb · Accepted solution]

Hello @cw2611 ,

From rough calculations, it seems that the back emf constant is around 2.065 V (ln-rms)/krpm.   
The speed in the beginning is seen to be uniform and that portion can be considered for calculations. 
Kindly calculate in the 2 below cases and verify. 
1. Calculate the time t1 (s) between the first negative peak and the subsequent positive peak. The frequency will be (0.5/t1) Hz. Calculate the Vpp between these two peaks, and apply them in the formula. 
2. The same procedure as above, by considering the first two negative peaks, and the frequency will be 1/t2, t2 time between the two peaks. 

Verify and confirm if all the values are almost equal. 

[cw2611]

As per our calculations, the constant seems to be 4.007 (ln-rms)/krpm.