Algorithm For Producing Sinusoidal Output Voltages Using SV-PWM
Algorithm for Producing Sinusoidal Output Voltages Using SV-PWM:
The SV-PWM is concerned with the control of inverter output voltages in a unified manner. It does not control the individual phase voltages separately. The instantaneous magnitude and direction of the desired resultant voltage vector is decided as per the frequency and magnitude of inverter’s fundamental output voltage. The SV-PWM is best realized with the help of a digital computing device, like microprocessor or Digital Signal Processor. The algorithm to be executed is outlined below:
- Get the input data like; input dc link voltage (Edc), desired output frequency ‘fOP’ (this will determine the speed of the resultant voltage vector), desired phase sequence of output voltage (will determine which way, clockwise or anticlockwise, the resultant voltage vector is moving), desired magnitude of output voltage and the desired switching frequency. It will be shown later that the switching frequency (fSW) and sampling time period (TS) are related. During each sampling time period three switching take place, where one turn-on and one turn-off is taken as one switching.
- Calculate magnitude factor ‘α’ from the knowledge of input dc link voltage and the desired output voltage (α Edc = 3/2 times peak of phase voltage). Also, calculate the sampling time period TS = 1/(3 fSW).
- Initialize sector position = I, and angle ‘θ’ = 0. Assume the rotating space voltage vector to remain stalled at this position for the sampling time period ‘TS’. Calculate the time duration for active and null state. Output the inverter switching pulses as per the calculated time durations so as to realize the space vectors in the following sequence: V8(111), V1 (101), V2 (100), V7 (000).
- Calculate the next position angle
for clockwise rotation in the vector space-plane. The reader should be able to work out the changes when the rotation is anti-clock wise. Recalculate the time durations as in step (3) above but this time the switching sequence will be V7(000), V2 (100), V1 (101), V8 (111).
- Step (4) is to be repeated but every time the switching sequence alternates between the sequences given in steps 4 and 5. This helps in reducing the switching losses. The reader may note that this way there are only 3 switching per sampling period. The switching to next space vector involves change of only one bit of the switching word (i.e., only one turn-on and one turn-off). When the space vector enters sector-II (for θ ≥ π/3 ), the vector V 1 is replace by V2 and V2 is replaced by V3. At the same time, angular position is reset to a value within π/3 ≥ θ ≥ 0 by subtracting 60 degrees from the old value. Every time the voltage vector enters a new sector the angle θ is readjusted so that it varies between 0 and 60 degrees. The active state vectors are also reassigned as described above. The process continues to produce a continuously rotating voltage space vector of fixed magnitude and fixed speed.