Why the converter cannot be used as variable frequency power supply

Variable frequency power supply flow the whole circuits by AC AC filter and other parts, so that its output voltage and current waveforms for pure sine wave, very close to ideal AC power supply. The grid voltage and frequency output of any country in the world. Variable frequency device consists of all the Exchange (modulated wave) circuit, converter standard names for the inverter. The output voltage waveforms for pulse square wave and harmonic components, the voltage and the frequency change proportionally, do not adjust, AC power requirements are not met. Can't do power supply used in principle, only for three-phase asynchronous motor speed.