Fractional windings are invariably used in synchronous machines.

Fractional windings are not invariably used in synchronous machines; rather, they are a specific design choice applicable under certain conditions and for achieving certain objectives. Synchronous machines can be designed with both integral and fractional windings. The choice between using fractional or integral windings depends on various factors including the desired electrical performance, manufacturing considerations, and cost implications.

The primary reason for using fractional windings in some synchronous machines is to improve their electrical performance characteristics such as reducing harmonics, improving torque smoothness, and distributing the winding more evenly. Fractional windings help in achieving a more uniform air gap flux distribution, which is especially beneficial in applications requiring high precision and smooth operation.

In summary, while fractional windings are a valuable tool in the design of some synchronous machines, they are not universally used in all synchronous machines. Their application depends on specific design and performance goals.