A variant of the DC system is the AC series motor, also known as the universal motor, which is essentially the same device but operates on alternating current. Since both the armature and field current reverse at the same time, the behavior of the motor is similar to that when energized with direct current. To achieve better operating conditions, AC railways are often supplied with current at a lower frequency than the commercial supply used for general lighting and power; special traction current power stations are used, or rotary converters used to convert 50 or 60 Hz commercial power to the 25 Hz or  Hz frequency used for AC traction motors. The AC system allows efficient distribution of power down the length of a rail line, and also permits speed control with switchgear on the vehicle.