In a DC generator, what is the function of the commutator?

In a separately excited DC motor, if the field winding is open-circuited while the armature winding is connected to a DC supply, what will happen to the motor speed?

What is the primary purpose of using transformers in the transmission and distribution of electrical power?

In a buck-boost converter circuit, if the duty cycle of the switch is increased, what effect does it have on the output voltage?