excitation winding

The excitation winding is connected in series with a small current-limiting resistor.

The second diagram shows a machine with no excitation winding, but instead, a constant voltage is connected to the quadrature brushes.

Fortunately there is a very effective simple solution in the form of two parallel cores with opposing excitation windings and a single overwound pickup coil (Fig. 6).

Because this winding is smaller than that of an equivalent induction motor and can overheat on long operation, and because large slip-frequency voltages are induced in the rotor excitation winding, synchronous motor protection devices sense this condition and interrupt the power supply (out of step protection).