Electric Traction Systems:

• Electric traction is meant locomotion in which the driving (or tractive) force is obtained from electric motors. It is used in electric trains, tramcars, trolleybuses, and diesel-electric vehicles, etc.
• They involve the use of electric energy at some stage or the other.
• Examples: battery-electric drive, diesel-electric drive, railway electric locomotive fed from overhead AC supply, tramways, and trolly buses supplied with DC supply.

Railway Track Electrification System:

Direct Current Traction System:​

• In all cases, contact systems are fed from substations which are spaced 3 to 5 km for suburban lines and 40-50 km for main lines Service.
• Substation receive power from 110/132 kV, 3-phase network (or grid). At these substations, this high-voltage 3-phase supply is converted into a low-voltage single-phase supply with the help of Scott-connected or V-connected 3-phase transformers.
• Next low AC voltage is converted to the suitable DC voltage by using suitable rectifiers or converters (like rotary converter, mercury-arc rectifier, metal or semiconductor rectifiers).
• DC motors are better suited for frequent and rapid speed control than AC motors.
• DC train equipment is lighter, less costly, and more efficient than similar AC equipment.
• When operating under the same conditions, the DC train consumes less energy than a Single-phase AC train.
• The erection cost and maintenance cost of the conductor rail is less than that of a single-phase AC system.
• No electrical interference with overhead communication lines in the DC traction system.
• The only disadvantage of the DC system is the necessity of locating AC/DC conversion sub-stations at relatively short distances apart.
   

Single-Phase Low-frequency AC Traction System:

• In this system, AC voltages from 11 to 15 kV at the frequency (50 Hz), (50/2 Hz), (50/3 Hz) Hz are used.
• Electric supply is taken from the high voltage transmission lines at 50 Hz, then in addition to a step-down transformer, the substation is provided with a frequency converter.
• supply is fed to the electric locomotor via a single over-head wire (running rail providing the return path).
• A step-down transformer carried by the locomotive reduces the 15-kV voltage to 300-400 V for feeding the AC series motors.
• To overcome the low power factor and commutation problem in the AC Series motor, a low-frequency AC supply is used.
• Another advantage of employing low frequency is that it reduces telephonic interference.
• Substations are 50 to 80 km apart.
   

Three-phase Low-frequency AC System:

• It uses 3-phase induction motors which work on a 3 kV to 3.6 kV at (50/3 Hz) supply.
• Sub-stations receive power at a very high voltage from 3-phase transmission lines at the usual industrial frequency of 50 Hz.
• This high voltage is stepped down to (3 kV to 3.6 kV) by transformers and frequency is reduced from 50 Hz to (50/3 Hz) by frequency converters.
• This system employs two overhead contact wires and the track rail forming the third phase.
• Induction motors used in the system are quite simple and robust and give trouble-free operation.
• The induction motor used in this traction system has high efficiency and the ability of automatic regenerative braking.
   

Kando System (Single-phase AC to Three-phase AC):

• In this system, the single-phase 16-kV, 50 Hz supply from the sub-station is picked up by the locomotive through the single overhead contact wire.
• It is then converted into a 3-phase AC supply at the same frequency by means of phase converter equipment carried on the locomotives.
• This 3-phase supply is then fed to the 3-phase induction motors.
• Kando system is likely to be developed further.
   

Single-phase AC to DC System:

• This system combines the advantages of high-voltage AC distribution at the industrial frequency with the DC series motors traction.
• It employs an overhead 25-kV, 50-Hz supply which is stepped down by the transformer installed in the locomotive itself.
• The low-voltage AC supply is then converted into a DC supply by the rectifier which is also carried on the locomotive.
• This DC supply is finally fed to the DC series traction motor fitted between the wheels.
• The system of traction employing 25-kV, 50-Hz, 1-phase AC supply has adopted by Indian Railway.