seyyah
Advanced Member level 2
rms value of pwm signals
I'm trying to find the paramaters of an induction motor by applying some tests using 3ph inverter. But i'm confused about some basic terms and calculations about spwm, rms values etc. In one of the tests i apply sinusoidal signal to one phase (by this method motor doesn't revolve) and measure voltage and current. Then i calculate phase angle between current and voltage. I am calculating the angle via reactances and resistances etc. Because i know inductance, resistance, frequency etc. I made a simulation. I applied sinosoidal signal and calcuations were right. But in real i don't have a pure sine signal and i have to apply this signal via spwm. But when i apply spwm to one phase everything changes in simulation.
1st; as pwm is unipolar voltage doesn't go to negative.
2nd; due to this fact, according to the motor's inductance , phase current has a dc offset.
My questions are: Assume that i have 10khz pwm signal and producing 50Hz sinusoidal signal. My DCBus is 310V. And assume that there is no dead time.
1- With the above quantities, what should my rms phase voltage be?
2- I apply a low pass filter to pwm signal to measure the voltage with a cut off frequency of 1/10 of the modulating frequency. With this method, can i measure all the applied voltage nearly or it eliminates some of the harmonics and i measure less than real voltage?
3- While i make calculation should i use the fundamental signal or real signal?
4- In simulation, i apply 50Hz pure signal and calculate XL=2•Π•50•L. In SPWM situation how must i calculate this? Must i use modulation frequency of 10KHz or output frequency of 50Hz?
5- According to you, should i measure φ (angle between voltage and current) or can i calculate correctly as in the simulation?
6- In simulation i measure higher rms current in pwm applied method than pure sinus applied method.(~%50 higher) Is this due to dc offset of the output current? How can i correct this?
A bit long but i'll appreciate if you answer, beacuse i don't know what to do, and i stuck.
I'm trying to find the paramaters of an induction motor by applying some tests using 3ph inverter. But i'm confused about some basic terms and calculations about spwm, rms values etc. In one of the tests i apply sinusoidal signal to one phase (by this method motor doesn't revolve) and measure voltage and current. Then i calculate phase angle between current and voltage. I am calculating the angle via reactances and resistances etc. Because i know inductance, resistance, frequency etc. I made a simulation. I applied sinosoidal signal and calcuations were right. But in real i don't have a pure sine signal and i have to apply this signal via spwm. But when i apply spwm to one phase everything changes in simulation.
1st; as pwm is unipolar voltage doesn't go to negative.
2nd; due to this fact, according to the motor's inductance , phase current has a dc offset.
My questions are: Assume that i have 10khz pwm signal and producing 50Hz sinusoidal signal. My DCBus is 310V. And assume that there is no dead time.
1- With the above quantities, what should my rms phase voltage be?
2- I apply a low pass filter to pwm signal to measure the voltage with a cut off frequency of 1/10 of the modulating frequency. With this method, can i measure all the applied voltage nearly or it eliminates some of the harmonics and i measure less than real voltage?
3- While i make calculation should i use the fundamental signal or real signal?
4- In simulation, i apply 50Hz pure signal and calculate XL=2•Π•50•L. In SPWM situation how must i calculate this? Must i use modulation frequency of 10KHz or output frequency of 50Hz?
5- According to you, should i measure φ (angle between voltage and current) or can i calculate correctly as in the simulation?
6- In simulation i measure higher rms current in pwm applied method than pure sinus applied method.(~%50 higher) Is this due to dc offset of the output current? How can i correct this?
A bit long but i'll appreciate if you answer, beacuse i don't know what to do, and i stuck.