HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old 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?
jayanthyk192 is offline Find More Posts by jayanthyk192
Reply With Quote
Sign up now
to remove ads between posts
Old May 22, 2012, 09:17 AM
AndyKunz's Avatar
Illinois
Joined Sep 2001
23,620 Posts
There are excellent tutorials available from TI and Microchip. Search their sites for BLDC or brushless DC.

Andy
AndyKunz is online now Find More Posts by AndyKunz
Site Sponsor
Reply With Quote
Old 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.
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old May 28, 2012, 12:29 PM
Registered User
Joined Dec 2011
7 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.



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
amaora is offline Find More Posts by amaora
Last edited by amaora; May 28, 2012 at 12:52 PM.
Reply With Quote
Old May 28, 2012, 07:03 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,185 Posts
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.
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote
Old May 28, 2012, 08:07 PM
Registered User
Ohio
Joined Nov 2003
1,655 Posts
Bingo. A trapezoid is an approximate sine. The current waveform must be trapezoid
due to the L causing current to ramp up.
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old Nov 30, 2012, 08:30 AM
Registered User
Joined Nov 2012
1 Posts
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.
xlib is offline Find More Posts by xlib
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Battery explanation stevolution Batteries and Chargers 5 Dec 11, 2011 09:09 PM
Suggestion Regarding J Bourkes explanation of prohibited language NoFlyZone Site Suggestions / Complaints 2 Nov 04, 2011 11:00 PM
Discussion Steorn's E-Orbo Electric Motor - No back EMF and "Overunity" performance?! dcmotordude Electric Motor Design and Construction 24 Feb 06, 2010 01:06 AM
Old wives tails and Back EMF - 9th wonder of the world? Larry3215 Power Systems 19 Feb 29, 2008 04:28 AM