Traction motor
As the DC motor starts to turn, the magnetic fields inside start to join together. They create an internal voltage. This electromagnetic force (EMF) works against the voltage sent to the motor. The EMF controls the current flow in the motor. As the motor speeds up, the EMF falls. Less current flows into the motor, and it makes less torque. The motor will stop increasing its speed when the torque matches (is the same as) the drag on the train. To accelerate the train, more voltage must be sent to the motor. One or more resistors are removed to increase the voltage. This will increase the current. The torque will increase, and so will the speed of the train. When no resistors are left in the circuit, full line voltage is applied directly to the motor.