Doubts about generating three phase PWM for AC Induction Motor
Hi everyone ;)
I am developing a driver (inverter) for a three phase AC induction motor, which would be controlled from a dsPIC33EP for motor control (dsPIC33EPxxxMCxxx). I want to deveope the program via Simulink.
I have read the guides and AN related to the dsPIC33E family and also studied the examples given in the Lubin's website but I still have some doubts about generating PWM.
Here are the doubts:
- Is it right that, for generate a three phase PWM, I have to supply three equal sinusoidal duty cycles to the PWM high speed block, delayed one from the other in order to obtain 120 degree displacement? (See picture) Doing so it works but, I have placed two delay blocks of 7 and 14 samples (7ms and 14ms) and for the first 7ms and 14ms the second and third phase are zero, this could be a problem for the motor. Do you have suggestions?
- Also, I have seen that inside the block can be set the "Initial Phase (s)". So can be put a single duty cycle equal for the three phases and then obtain the displacement setting the Initial Phase (s) inside the block? (See picture) Doing so and measuring with the oscilloscope the phases are not displaced in time, why? How it works?
- I would like to control the equivalent output frequency to the motor with a potentiometer (30 Hz, 50 Hz, ...). Do you have suggestions on how can I implement it in Simulink?
Thank you very much ;)