Class 20E

On the 20th of November 2013, I wrote a blog post titled: ‘My Take on It – New TFR Class 20E’.

In short, I gave my personal view on the locomotive and my optimism about ordering locos from China (Previous locos ordered from China for Namibia were poor performers and even suffered from cracked frames, so the general public – read railway enthusiasts – didn’t view the ordering of these locomotives in a good light).

Below are a few technical details about the locomotive from my Facebook Friends Philmar Du Plessis and Christiaan Bezuidenhout.
The photos supplied to me are by: Christiaan Bezuidenhout, Francois Mattheüs, Japie Terblanch, Noel Welch and Hennie Heymans.

The locomotives have received the nickname China Dolls and have a Bo-Bo wheel configuration. Two (20E – 031 and 20E – 032) have been specially painted to haul the Blue Train between Pretoria and Cape Town, replacing the duel voltage 14Es.
The class 20E is a dual voltage locomotive that is designed to operate on either 3kV DC or 25kV AC. The locomotive is fitted with AC traction motors and controlled via the TCU (traction control unit). A Change-Over switch determines whether the locomotive operates under DC or AC. It has standard pantographs fitted with carbon collector strips.

When operating under DC, the HCB (high speed circuit breaker) connects the pantograph to a DC-link (stabilized DC supply). The DC supply feeds a combination inverter/rectifier that inverts the DC to a 3-phase supply, feeding the 3-phase traction motors. In electrical braking (DC) the 3-phase motors generate power back into the inverter, which converts into the rectifier which in turn feeds back into the DC-link and back to the overhead when in REGEN mode. If the overhead line voltage goes to high, then the control system enables part of the REGEN current to pass to the rheostatic grids. This is called blending, the locomotive will always attempt REGEN back into the overhead 1st and if possible will pass the maximum current. If the overhead line is non-receptive, than it will pass all the current (or most) to the rheostatic grids ensuring as close as possible to a 100% effective electrical braking system.
With AC traction it is possible to “plug the motor” in reverse leading to a very effective electrical braking to nearly 0km/h. This is called “flat topping” giving you a constant high kN braking from 54km/h to around 2km/h. (In DC traction this is not possible because of the physics of a DC motor and you have a knee point where you achieve maximum electrical braking. If you move below or over the specific speed where the knee point occurs, your electrical braking taper off).
When operating under AC, the VCB (vacuum circuit breaker) connects the pantograph to a traction transformer with several tapping’s to reduce the voltage to the required values. The traction transformer windings are then coupled to a rectifier/inverter that feeds the same DC-link (stabilized DC supply as when supplied by DC overhead). The DC supply feeds a combination inverter/rectifier that inverts the DC to a 3-phase supply, feeding the 3-phase traction motors. In electrical braking (AC) the 3-phase motors generate power back into the inverter, which converts into the rectifier which in turn feeds back into the DC-link, The traction transformer rectifier/inverter then inverts the DC into AC which feeds back into the traction transformer (which then acts as a step-up transformer) to get the voltage again at approximately 25kV). This cannot be Reneged straight back into the overhead unless the frequency and phasing matches the overhead 100%. Very clever software measures the phasing and frequency on the overhead, the required shifts to correct the frequency and phasing is then communicated to the rectifier/inverter which in turn adjusts the output that when it passes to the overhead, is 100% in synchronization, (this happens in 5 cycles, a 10th of a second). It is also a blending system and it will attempt to pass maximum current back into overhead. (Just for interest sake, the class 15E REGEN about 26% of the total power consumed back into the overhead that can be used by other trains in the same electrical section or reused by Eskom). If the overhead line voltage goes to high, then part of the current is rerouted back from the DC-Link into the rheostatic grids to ensure as close as possible to 100% effective electrical braking. Many microprocessors are used in the control system to execute what was explained, the TCU (traction control unit) determines how the IGBT’s must be turned off and on according to the driver handle demand. This all happens in micro seconds.

The pantograph will only drop if the maximum permissible current is exceeded, i.e. short circuit conditions. The fire detection system is wired into the pantograph circuit too.  The VCB or HSB will open under other fault conditions, one condition is when it senses a high voltage jump.  Very difficult to simulate, but what is normally done is to operate in a section and force the overhead voltage as low as possible, then change tappings in the next electrical section to get the overhead voltage as high as possible, you then get a voltage jump when you pass the phase break or isolation section.  We also do a dead short circuit to ensure that the HSB clear the fault.
The locomotive also features ECP Brakes (ECP stands for ELECTRIC CONTROLLED PNEUMATIC, in other words the air brakes are controlled electronically, Every 2nd wagon has a “computer” that identifies the wagon with an address block.  Via the 150V cable running along the train, they transmit different frequencies, interpreted by the “computers” .  Each wagon’s brake is therefore individually controlled.  You can isolate some wagons with braking problems if needed.  You can apply brakes to part of the train too. If the train is stopped over a hill, you can release the brakes in part of the train until the train is bundled again etc).

Sadly, the 21E is visually the same as the 20E as is the 22E


2 thoughts on “Class 20E

  1. I find these blogs very interesting. Thanks for taking the trouble to put this together.
    As you said, they are not particularly good-looking locos at all! It seems that two locos to a train is more or less standard; so I suppose saving on a cab each time may be worth it.
    I wonder if these new AC/DC locos will affect the lifespan of the 18’s, 11’s and 7’s: any idea?
    (You put me to shame: I’ve done nothing on my blog for ages!)

    • Thanks for taking time to read!

      I’m not sure about the 18Es but I know they have started replacing 7Es in a few places (Such as the Northern Cape). I suppose after all testing and teething problems are sorted out, they will phase the older locos out.

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