I like the explanation of ...

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I like the explanation of Thibaud.
In simple terms, motors are built for a standard frequency, say in America 60 Hz, in Europe 50.
If you plug an american motor in Europe (at correct design voltage) it will turn slower (at approx 50/60 rpm because of the frecuency change).
A variable Frequency drive control intentionally changes the frequency of the feed alternating current, thus becomes a speed control.
Hope it helps the understanding