I will try to explain this as non-technically as possible, so please bear with me (May be two or three posts).

Firstly, there are no real '1-phase' locos in IR. We have DC-DC locos, AC-DC locos and AC-AC locos. The AC-AC locos are what we call '3-phase locos'.

(You...
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can refer to Wikipedia or Quora for understanding the differences between DC and AC).

Basically, an electric loco takes electricity from the OHE wires and uses it to power motors connected to the wheels. Now the OHE can be carrying either DC or AC current. Similarly, the motors can run on DC or AC current. So there are many combinations possible.

DC - DC : The OHE carries DC current, while the traction motors also work on DC current. All the DC locos in Bombay like WCP1, WCG1/2, WCM1/2/3/4/5/6 and WCAM1/2/3 locos were of this type.

AC - DC: The OHE carries AC current, while the motors run on DC current. The WAM1/2/3/4, WAG1/2/3/4/5/7 and WAP1/2/3/4/6 locos are of this type. In these locos, the AC current from the OHE is converted into DC current and fed to the motors.

Diesel locos like the WDP1/2/3A, WDM1/2/3A/4 and WDG2/3A are also AC-DC locomotives. In these locos, the diesel engine runs a generator (technically called an alternator) to generate AC current. This is converted to DC and used for the motors.


AC-AC: The OHE carries AC current and the motors also run on AC current. The WAP5/7 and WAG9 locos fall under this category.

Among the diesel locos, the WDP4/4B/4D, WDG4/4D/4G, WDG5 and WDG6 locos are also AC-AC locos. In these locos, the diesel engine runs an AC generator (technically called an alternator). The AC voltage generated by this is run through a control system and runs the motors, which also run on AC current.

Now, AC current can have many 'phases' (2-phase, 3 phase, 4-phase etc). The common format used for power supply is 3-phase AC power. ( I can explain this if required). Of these 3 phases, only 1 phase is sufficient for operating most common things. You can see this in most household connections. Some houses will have all 3 phases, but some will have only 1 or 2 phases (out of 3).
There are several types of AC motors, most notably 'synchronous motors' and 'induction motors'. Of these, Induction motors are very suitable for railway purpose, as the speed of rotation of the motor can be changed by changing the frequency of the AC voltage.
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all the AC-AC locomotives in India use 3-phase induction motors. But the OHE carries only 1 phase. The electronics on the locomotive convert the 1 phase from the OHE into 3 phases and feed it to the motor. Hence these locomotives are called '3-phase' locomotives.