The transitions of a WDM-2
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Amongst the list of the idiosyncracies of our beloved Alco WDM-2 is the 'zhatka' or jerk during acceleration. As the loco is accelerating, all of a sudden the power cuts off for a fraction of a second and then picks up again. This happens at typically 39km/h. The jerk can often be felt over the entire train specially if accelerating up a slope.
I remember I was way in the middle of the 1028 Up Kashi Express and I could feel it in the coach. This was many moons ago and then I really had no explanation for it. Now I do. Here goes:
When the loco starts from stand still all the traction motors are in Series Parallel. As the loco accelerates the control system switches the combination to full parallel in so many steps to allow the motors to accelerate.
- 0 - 29 km/h : Series Parallel
- 29 - 39 km/h : Series Parallel Shunt
- 39 - 80 km/h : Parallel
- 80+ km/h : Parallel Shunt
Thus as the loco changes from Series Parallel Shunt to Full Parallel the Generator output is cut and the fuel to the engine is momentarily stopped to reduce the flashing on the switch gear and strain on the drive train.
Shunt position is also known as 'weak field' position, where a resistor is placed across the windings to reduce their resistance and increase the amount of current that can flow.
Now sometimes the driver thinks it is appropriate to allow this transition to come earlier so as to pick up speed faster. In that case he shouts for his assistant to 'M.T.' (Manual Transition) and the asst. switches the Toggle switch on the Long Hood bulkhead and the loco jerks instantly.
The other transitions cannot be felt at all in the loco.
If the 'M.T.' switch has been operated and the loco has subsequently stopped, then the MT toggle switch has to be brought back to the normal position before starting once again. If the toggle remains in the 'M.T.' position, then the loco is very lethargic with very poor pickup. In fact I have been told that if the switch remains on in the 'M.T.' position then the main generator gets overloaded and has actually burnt out in a couple cases in the Pune shed. There was a period when this MT switch was outlawed and a wire was inserted through the switch dolly to prevent it being used (railway pidgin - it was 'dummied'). But apparently now the drivers have learnt to use the MT effectively and the locking wire has been removed on all the Pune locos.
One more observation: the Gooty (GY) locos the motoring / braking lever used for selecting the degree of dynamic brakes has motoring 1, 2 and 3 positions. The drivers explain that this is akin to the gears in a car and helps to get the transition faster. In the Pune locos the Motoring / braking lever has been dummied with the same locking wire in the so it cannot be moved beyond the Motoring 1 position.