My question is about the difference between fixed switching frequency and variable switching frequency in DC/AC inverters. Specifically, Consider a PWM inverter in which, the carrier frequency is a fixed value. If the inverter is used in a closed loop control scheme, then I see that the switching frequency of the inverter is not fixed. So from a fixed-switching-frequency carrier, I get variable-switching-frequency behavior. Does this mean inverters have basically variable switching frequency?
In the area of electrical machine control mostly fixed PWM frequency is used for simplicity. However this results in harmonics at well defined points in the frequency spectra. Using a randomized frequency for example spreads out the power over a wider frequency range lowering the power of specific harmonics. If acoustic noise is an issue in a particular application this can be one way of lowering it.
You said "from a fixed-switching-frequency carrier, I get variable-switching-frequency behavior." I guess you mean that when you add close-loop control, the inverter no more switches regularly . I think there must be too much saturation or fast oscillation inside the close loop control. You must check out the signals carefully at various stages of the control loop.
If you do see saturation or fast oscillation, the only solution is to fine-tune your close loop control. Personally, I'd start by reducing the control loop gain, or by adding a low-pass filter.
Could you tell us more about the topology you use? What kind of signal do you like to approximate? E.g. if you have a half H-Bridge inverter you try to follow a reference signal. This signal needs to be compared with e.g. sawtooth wave with constant frequency. The reference signal you follow doesn't need to have a constant frequency.
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