Re: 8 car C set

[matthew@mail.usyd.edu.au (Matthew Geier)]

In article <8E6D9FBB3telstraNews@vic.news.telstra.net>,
Michael  <mk@netstra.com.au> wrote:
>I believe, but I could be wrong that a chopper control is based on a 
>thyristor, a semi-conductor used to vary voltage.

 Sort of. A Thyristor cant switch off. They are not particularly usefull
on DC supplies. (AC has an 'off' point twice a cycle).
 The devices are call 'GTO's or 'Gate Turn Off' Thyrisitors, a device similar
to the traditional thyristor but it can be forced off with out waiting for
a 'zero crossing'.

 They don't really vary the voltage either, but 'turn on and off' really fast,
the frequency of which determines the average output voltage. Hence the term
'chopper'.

>The characteristics of the chopper control are such, that power changes can 
>be delivered a lot smoother than that of a resistance grid.

 Depending on the control software of course, some chopper controllers are 
programmed to match the surges generated by the resistance controllers to
allow compatablity between different types of power cars.

 Melbourne's 4D was 'knobled' to match the comeng sets it was often paired
with.

>I believe that chopper controls are also a lot more efficient when it comes 
>to power consumption.

 Quite an amount more effcient, although I dont have the figures. The main
issue is that the power is no longer lost to heat in the starting resistances,
and that regenerative braking is simpler to implement, allowing return of power
under braking.

>Finally, I have noticed that the chopper controls emit a single, high pitch 
>whine when a train is powering, I am not sure if this comes directly from 
>the chopper circuit, or it's an interferance that's being picked up by the 
>speakers 

 At slow speeds the 'chopping' frequency is just above audable. The coils on
the motors vibrate at the chopping frequency. The PA system may also pickup
the noise, but given my experience on Tanagara's most noise is comming from
motor vibration.

>Maybe someone has a link that explains a lot more about chopper control :-)

 Mitsubishi orginally developed GTOs that could with stand 1500VDC. It may be
there is some thing on their web site under electronics and power systems.
Mitsubishi supplied the chopper control pods for Sydney's 'Chopper K'(C) sets,
the chopper interurbans and the Tangaras.

 I find the most interesting of the Australian 'chopper' control trains are the Adelaide 3000/3100 class DMUs. The engine drives an alternator at constant RPM.
The 3 phase AC is rectified to a 'DC' link and then a chopper controller 
converts that what ever is required to drive the motors. I found it quite odd
the first time I rode one, to hear no change in engine RPM as the car
accelarated, and the varying frequency of chopper whine instead, just like
an electrical train.


 High power solid state switching has now progressed, and the ideal drive now
appears to use high current transistors to generate a true variable voltage
(and variable frequency) supply in 3 phase to feed to a simple induction motor.
 Induction motors are very simple devices, as such simple to maintain, thus
supposedly offset the extra expense of the VVVF drive.

 I belive Sydneys '4GT' train will have 3 phase VVVF drives. The last
Outer Suburban Tangara left Gonnians with AC motors, although I dont know
exactly what type of AC drive. It sounds quite different to the chopper
Tangaras.