4. Variable speed operation

At very high power levels, synchronous machines are still used for power generation, and therefore for fixed-speed operation. For small and medium-power applications, they are used in variable-speed motor mode, which is of course perfectly capable of reversibility. In both cases, the physical principles involved and their modeling are identical, but the controls are different.

As we have just seen, a mains-coupled machine operates at fixed voltage and frequency, and we can simplistically say that we only manage power transfers within the imposed stability limits. In motor mode, on the other hand, speed and torque profiles are variable, and dynamics and precision are high. Last but not least, digital control techniques enable better energy optimization by adjusting control parameters.