PURPOSE: To equalize the temperature distribution of a field coil by providing the ventilating grooves in the insulating plate communicating the space between the field coil supports with the space at the end portion of a field coil.

CONSTITUTION: A rotor is equipped with a yoke 1 provided on the axis, a magnetic pole 2 provided at a specified interval in a circumferential direction of this yoke 1 and axially extended, and a field coil 3 wound around this magnetic pole 2. Between the neighbouring field coil 3 there are multiple field coil supports 4 axially placed at specified intervals. An insulating plate 5a is provided between the field coil 3 and the yoke 1. In this insulating plate 5a ventilating grooves 8 are provided communicating the space between the field coil support 4 with the space at the end portion of a field coil. Thus the cooling blow is sent to the space between the field support 4 and the temperature distribution of the field coil 3 can be equalized.