I intend to control a motor with 3 120° phase shifted sinusoidal signals. I already implemented the SPWM to output a sinus shaped voltage, but i am a little bit confused at how should i implement it with my custom design inverter. The inverter has 3 legs/6 Mosfets which can be controlled indivudally.
I will quickly describe what i have in mind and would appriciate any comment/clarification on if it is a good idea or not.
With a 3 phase sinusoidal signal, the outputs are always a permutation of + + – or – – + (momentary sinus values). If you add them up you should get 0. And the inverter legs are HHL or LLH accordingly(Of course LLH or LHL. etc is possible). Lets take the HIGH-HIGH-LOW case :
Here i thought i can identify the different phase (LOW in this case) and control the other 2 with PWM (according to a look up table). What i am confused at is, is it really necessary to seperately control the 3. leg(which should be low ) ? Cant i just turn on the low side switch fully on, while the other 2 are positive and being controlled by a varying PWM duty cycle. This will function as a current return path and since the sum has to be 0, it will automatically take the value it is supposed to take. And in the LLH case, H-leg will be on as long as the other 2 are L and controlled by PWM. This will be alternated with a state machine.
Please let me know if the way i am thinking is correct. I am aware of more advanced methods like SVPWM (Space vector) but would like to stick to SPWM.
Best Answer
Yes you can and this is a common tactic. It significantly reduces switching losses as for 1/3 of the time a leg will not be switching.
It comes with a couple a considerations.
Increase ripple current for the same switching frequency
The starpoint of the load will have a large common mode swing.
If these two points can be tolerated, this discontinuous PWM scheme is viable