back emf waveform explanation - RC Groups
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May 22, 2012, 05:52 AM
Registered User

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?
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May 22, 2012, 10:17 AM
AndyKunz's Avatar
There are excellent tutorials available from TI and Microchip. Search their sites for BLDC or brushless DC.

May 26, 2012, 09:03 PM
Registered User

Trapazoid results from square wave excitation. Something to do with inductance, probably.
May 28, 2012, 01:29 PM
Registered User
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.

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 -

Also, paper almost on topic -

Sorry for my English
Last edited by amaora; May 28, 2012 at 01:52 PM.
May 28, 2012, 08:03 PM
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Bruce Abbott's Avatar
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.
May 28, 2012, 09:07 PM
Registered User
Bingo. A trapezoid is an approximate sine. The current waveform must be trapezoid
due to the L causing current to ramp up.
Nov 30, 2012, 09:30 AM
Registered User
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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