What is the work of a controller in electric cars? & how does this controller works?

there are 2 types of controllers dc to ac and dc to dc in electric cars.What is the work of a controller in electric cars? %26amp; how does this controller works?
DC to DC in old electric trolleys works as follows:





A coarse control is gained by switching the traction motors from parallel connection, where each motor gets a full 600 volts, to series connection where each motor gets 300 volts. Finer control is achieved by switching large power resistors in series with the motors.What is the work of a controller in electric cars? %26amp; how does this controller works?
This works because of the waves that goes to the antennae.