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#1 jayanthyk192 May 22, 2012 04:52 AM

back emf waveform explanation

I wanted to know how the back emf waveform is trapezoidal in nature. I understood the sinusoidal waveform of AC motors but this i cannot. Can anyone please tell me how this happens,how the trapezoidal voltage is induced?

#2 AndyKunz May 22, 2012 09:17 AM

There are excellent tutorials available from TI and Microchip. Search their sites for BLDC or brushless DC.


#3 jglenn May 26, 2012 08:03 PM


Trapazoid results from square wave excitation. Something to do with inductance, probably.

#4 amaora May 28, 2012 12:29 PM

Try to integrate BEMF wave shape to find out the magnetic flux wave shape. This is because of the following equation that links flux and emf.

http://latex.codecogs.com/png.latex?E=L{d\Phi \over dt}

Flux wave shape is dependant of mechanical desing (winding laying, bevel, etc.) of the motor.

However, I have never seen anything like a trapezoidal BEMF of real motor. There was a topic about this question here (but no answers).

That topic - http://www.rcgroups.com/forums/showthread.php?t=1645446

Also, paper almost on topic - web.mit.edu/scolton/www/SCThG.pdf

Sorry for my English

#5 Bruce Abbott May 28, 2012 07:03 PM

4 Attachment(s)
If you look at the voltage across a whole phase instead of between one terminal and ground, you can see that it's not very far away from a sine wave. At part throttle the true back-emf waveform becomes visible, and this varies depending on motor construction.

#6 jglenn May 28, 2012 08:07 PM

Bingo. A trapezoid is an approximate sine. The current waveform must be trapezoid
due to the L causing current to ramp up.

#7 xlib Nov 30, 2012 08:30 AM

I'd like to specify that that equation is dimentionally incorrect, the derivative of the flux times an inductance doesn't result in a voltage.
btw, bemf shape depends on motor geometri and not on the drive.

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