Q. How do the AC-DC locos (WCAM series) switch from one power source to another on the run?
Ans. At DC/AC changeover points as on the Virar-Vaitarna section, WCAM locos can switch from one power source to another without stopping.
The WCAM-1 has a selector on the rightmost side of horizontal control panel for selecting the pantograph. It has four positions, DC, AC, DC-ALT and AC-ALT. In DC and AC-ALT mode, pantograph with two collector shoes is raised; in AC and DC-ALT, pantograph with one collector shoe is raised. The DC pantograph has...
more... two shoes and is thicker in its contact area than the AC pantograph because it has to carry a larger current corresponding to the lower voltage.
The ALT positions allow the DC pantograph to be used for AC traction or the AC pantograph to be used for DC traction, in case of damage to one or the other pantograph. I.e., the pantograph itself does not control whether the DC or AC circuitry is in use; the selector switch controls this. In real life, the driver rarely gets a chance to see which pantograph is up; all he knows is the position of selector switch. Sometimes when a damaged pantograph is replaced, a pantograph of a different kind (one shoe instead of two) may be installed; the loco still works, although perhaps suboptimally.
About the only time the driver must raise or lower the pantographs when the locomotive is in motion is at the AC-DC changeover point a little north of Virar on the Virar-Vaitarna section -- at a 'dead zone' or neutral section where there is a length of overhead catenary with no electricity supplied to it, between the AC and DC catenaries. This usually extends for a length of about two or three catenary sections. About a kilometer before this dead zone, a sign alerts driver with a '1000 meters' warning followed by another for '500 meters' and then a sign saying 'Dead Zone'.
Going from Mumbai towards Dahanu, the driver shuts most of the equipment in the loco off (air compressor charged, traction motors cut off, motor generator switched off, etc.), then lowers the DC pantograph and just waits while the loco coasts without power through the dead zone. until the AC section of the catenary is reached. At this point, he raises the AC pantograph. After about 30 seconds, the voltmeter shows 25 kV and he restarts the traction and other equipment.
Note that this arrangement of the catenary is different from that at Igatpuri (see above for DC/AC loco switchover). There, all locos have to stop and wait for the line voltage to be switched on in the intermediate neutral section.
Just before the dead zone, there is also a sign, 'Open DS for speeds below 40km/h'. The 'DS' is the main Disconnecting Switch, a manually operated circuit breaker in the DC supply path from the pantograph, that isolates and grounds the 1.5kV DC downstream circuits from the 25kV supply. If the speed is below 40km/h, the driver needs to keep on accelerating until the very last moment and then throw this switch to isolate the DC circuits on the fly.
This tricky manoeuvre is necessary when the speed is that low, because of the danger of losing momentum and stopping in the dead zone without power in case of any adverse conditions like emergency brake application, or brake pipe parting, etc. (The dead zone is one length of catenary and considering the cross-over structures on the DC and AC sides it is nearly two lengths, hence the loco and train have to have enough momentum for the loco to get across this distance).
