May 22, 2012, 04:52 AM Registered User Joined May 2010 82 Posts Discussion back emf waveform explanation Hi, 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?
 May 22, 2012, 09:17 AM Illinois Joined Sep 2001 36,636 Posts There are excellent tutorials available from TI and Microchip. Search their sites for BLDC or brushless DC. Andy
 May 26, 2012, 08:03 PM Registered User Ohio Joined Nov 2003 1,655 Posts https://www.google.com/search?q=esc+...iw=790&bih=429 Trapazoid results from square wave excitation. Something to do with inductance, probably.
 May 28, 2012, 12:29 PM Registered User Russia Joined Dec 2011 16 Posts 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. $E=L{d\Phi \over dt}$ 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 Roman Last edited by amaora; May 28, 2012 at 12:52 PM.
May 28, 2012, 07:03 PM
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Hastings, New Zealand
Joined Jan 2001
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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.