RX62T YROTATE-IT-RX62T Motor Control Kit
UM-YROTATE-IT-RX62T Rev.1.00
Page 33 of 51
Jan 15, 2014
17. PWM modulation technique
Among the various possibilities, a particular form of PWM modulation was chosen. In this modulation technique, the
voltages to be imposed are shifted in order to have in every moment one of the three phases of the motor
connected to the system ground. This allows reducing the commutations of the power bridge of one third, in respect
to other modulation techniques. In fact the phase that is connected to the system ground doesn’t require any
commutation, having the lower arm always on and the upper arm always off.
The method is based on the fact that, having no neutral connection, we are interested only in phase-to-phase
voltages, or in the voltage differences between the phases, not in the voltage level of the single phases. This allows
us to add or subtract an arbitrary quantity to the phase voltages, on condition that this quantity is the same for all
the three phases. So, obtained from the algorithm the three phase voltages requests, the minimum is chosen and it
is subtracted to all the three requests.
With this method, the applied voltage star center is not at a fixed level, but it is moving.
The maximum phase-to-phase voltage that can be obtained (without distortion of the sinusoidal waveform) with this
method is equal to the DC Link voltage, as in other methods (like Space Vector Modulation).
VBUS
