Electrical – 3 phase BLDC sinusoidal drive using PWM controller and/or DSP

When there is a delay shutting off the current from one switch after the other complementary switch was turned on then there is a "shoot-thru" condition where there is a momentary short circuit current surge across the power supply thru the pair of switches. This commutation of "make-before-break" is fixed by timing control of each switch gate drive to allow both to be off for some x microseconds period, usually dependent on the T=L/R ratio. This is also called dead-time control.

Normally the diode//R attempts to turn OFF the gate faster than turn ON to assist in increasing deadtime but depending on Vgs slew rate, gate capacitance, source impedance, load inductance, ESR, RdsOn and L/R ratio, there are many contributors to delayed turn off. A simple way is to increase the value of the parallel Gate R but this has its limits with PWM frequency and gate capacitance.

More basics.

http://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridges-the-basics/

Does changing the throttle change the duration of the trapezoids?

Not directly. The duration of the 'trapezoids' is determined by commutation, which is synchronized to the rotor position. The motor will rotate at the speed it wants to, determined by the applied voltage and torque load. The controller must respond to this by commutating at the same speed and phase.

The throttle controls effective motor voltage by applying high frequency 0-100% PWM, so you could say that it indirectly affects commutation timing because motor speed is proportional to applied voltage. However motor speed is also affected by loading, which may vary independently of throttle level.

How does the ESC maintain unity power factor?

The ESC may adjust commutation timing to compensate for the lagging current caused by winding inductance. Some ESCs do it automatically, others have fixed timing advance settings. With fixed timing unity power factor is rarely achieved, and the best setting is usually a compromise between power and efficiency.

Does changing the throttle setting below a certain point change the kt of a motor?

The controller relies on winding inductance to smooth out the current. However in most ESCs the PWM frequency is barely high enough to maintain continuous current flow. As the throttle is lowered (and PWM ratio reduced) current ripple increases until the current waveform becomes a discontinuous sawtooth. Since torque is proportional to average current this lowers the effective torque constant. At higher loading the current becomes smoother so the effective torque constant increases, causing the torque/rpm curve to become nonlinear.

do the similar sinusoidally-driven BLACs (aka PMSM?) energize all 3 phases at once?

Yes, or no - depending on the controller. There is no fundamental difference between BLDC and PMSM. It is possible to power a BLDC motor with 3 phase AC, but with all 3 phases continuously powered you can't extract back-emf for zero-crossing detection. However the PWM can be modulated to shape the trapezoid waveforms into a 'saddle' profile, which becomes a sine wave when the two driven phases are combined.

How to Sinusoidally Control Three-Phase Brushless DC Motors