Why does BLDC motor has got a trapezoidal back emf?

 

The shape of the magnetic pole pieces/regions will affect the waveform.

 

Because a BLDC motor uses a trapezoidal wave form to drive it.

 

The BLDC motor can also be driven by PWM square wave, the BEMF generated is a 3 phase sinusoidal voltage.
With the BLDC two windings are energised at any one time, the AC sinusoidal motor is constructed exactly the same way but uses a 3 phase sinusoidal for power.
http://www.youtube.com/watch?v=oFI7VW6WGR4
Max.