How to find out put voltage of a 3 phase pulse width modulated inverter theoritically

[suman.sardar]

Hello,
Recently I have successfully designed a 3 phase pulse width modulated DC-AC power source using PIC18F4431 micro controller and Power Controlled PWM (PCPWM) scheme. Now I want to compare its output voltage (phase voltage or line voltage) level with expected theoretical value. But I am getting different formulas ( or derivations) for phase voltage of PCPWM scheme at different books. So can anyone tell me how to find the phase voltage theoretically for PCPWM scheme.

 

[KlausST]

Hi,

Pure sine?
Do you have a true star point or not?
How is your hardware setup? Three half bridges?

Klaus

 

[suman.sardar]

Yeah pure sine wave.
Yes I have true star point.
Its a 3 phase bridge inverter ( Full connection with PIC and driver is not shown here)

 

[FvM]

There's no "true star point" in the schematic.

 

[suman.sardar]

A rough schematic after connection of 3 phase load looks like this:

 

[KlausST]

Hi,

Any small DC on your star point will shift it to any voltage.
A true star point should be a fixed reference point for the phase voltages.

For me it's not clear what answer you expect.

For any sine voltage: RMS_voltage = peak_voltage / sqrt(2).
For a symmetric three phase system: phase_to_phase_voltage = phase_to_star_voltage x sqrt(3)

For your system ... if the star voltage is half of the DC_rail_voltage.
Then: actual_sine_voltage = (duty_cycle - 0.5) × DC_rail_voltage.

Klaus