The correct answer is D. induction motors.
Cogging is a phenomenon that occurs in induction motors when the rotor teeth are aligned with the stator slots. This can cause the motor to vibrate and produce noise. Cogging can be reduced by using a different rotor design or by using a variable frequency drive (VFD).
Three phase transformers are used to transfer electrical power from one circuit to another. They do not have a rotor, so cogging cannot occur in them.
Compound generators are a type of DC generator that has both a series field and a shunt field. The series field is connected in series with the armature, and the shunt field is connected in parallel with the armature. Compound generators are used in applications where a high starting torque is required, such as in locomotives.
Series motors are a type of DC motor that has the field winding connected in series with the armature winding. This means that the current through the field winding is the same as the current through the armature winding. Series motors have a high starting torque, but their speed is not constant.
In conclusion, the term ‘cogging’ is associated with induction motors.