How the circuit completes in electric traction?

Answer 1

If you are asking what control components are used to connect a traction motor to its power source here is what I would suggest as a minimum (based on projects I have been involved with).

You need a disconnect device (like a switch or switch plus relay that can carry the full power). If that switch is not within sight of the motor you need a local disconnect switch to permit shut off by anyone working on the motor. You need an over-current device that will open and disconnect if too much power is being drawn.You need a status indicator for the presence of live power (could be a pilot lamp or just the switch handle position). You need wiring of appropriate size to carry the maximum (locked rotor) current of the traction motor.. You might need a variable frequency drive if the motor is to be variable speed. You might need a meter (could be part of the VFD) to show the motor's commanded speed. You should have a HAND-OFF-AUTO switch for control of the motor. In some applications you might need over-speed and over-current sensors. You might need a motor starter relay with both a failure and a run indication.

Answer 2

By magic.