The field winding is connected in series with the armature, so the current in the armature is also the field current. This makes regulation of the motor simple, as its voltage is proportional to the required current value, allowing the power regulator to control the battery voltage by switching or adjusting the frequency. The series electric motor has good starting torque, but the torque characteristic is very soft, with a rapid drop in torque as the RPM increases. If the load is reduced, the RPM can increase to the point where it may cause damage to the motor. Therefore, it is not recommended to operate without a load on the shaft. These motors are used in electric cars and electric traction vehicles such as trains, subways, and trams due to the high torque at low speeds and ability to self-adjust speeds based on the load.