SOLUTION

Single-phase to the 3-φ system or kando system

In this system, 1-φ AC system is preferred for the distribution network. Since single-phase overhead distribution system is cheap and 3-φ induction motors are employed as traction motors because of their simple, robust construction, and the provision of automatic regenerative braking.

The voltage used for the distribution network is about 15-25 kV at 50 Hz. This 1-φ supply is converted to 3-φ supply through the help of the phase converters and the high voltage is stepped down transformers to feed the 3-φ induction motors Frequency converters are also employed to get high-starting torque and to achieve better speed control with the variable supply frequency.

By using silicon controlled rectifier as an inverter, it is possible to get variable-frequency 3-phase supply at 1/2 to 9 Hz frequency. At this low frequency, 3-phase motors develop high starting torque without taking excessive current.

The advantages of this system include higher starting efficiency, less number of substations, simple substation design, and lower cost of fixed installations.