This circuit gives a 3 phase square-wave output for a variable speed motor drive. Operation is straightforward, the 4017 counter is synchronously reset after six clock inputs. The she outputs are combined to give the required waveforms. It is interesting to note that although NOR gates are shown, OR gates will give effectively the same result.
The circuit can be extended as shown in Fig. 2 to give pseudo-sine waves if that is required, but that will diminish the simplicity of the circuit.