During normal operation, the exciter rotor and the diodes D1-D6, which are affixed to the motor shaft, generate a rectified direct current voltage to provide field current to the motor according to the exciter field current supplied separately by an alternating current exciter drive. https://www.mrosupply.com/v-belts/v-belts/b-section/178315_b94_gates-rubber/
The phase sequence of the alternating current exciter causes the slip to increase from 1 to normally 3 at maximum rotation. This is why the rotating field applied to the exciter stator must rotate in the direction opposite to the direction of rotation of the motor. If the rotary field has the same direction of rotation of the motor and they have the same rotation, then the voltage and current induced in the rotor of the exciter will be zero.
This type of rectifier configuration is only applied for starting using frequency inverter. During motor operation, the rotator field of the exciter stator induces an alternating voltage in the exciter rotor, even when the motor is completely stopped. The excitation of the field is controlled by the control of the exciter through the amplitude of the voltage.