Re: [NSW] Saturday's TRain Sightings

[Eddie Oliver <eoliver@efs.mq.edu.au>]

David Johnson wrote:

> A chopper is (how do I put this simply?) a Gate Turn Off Thyristor.
> 
> Normal Thyristors are turned on, but can't be turned off until the supply
> voltage is cut.  A Gate Turn Off Thyristor allows the Thyristor to be turned
> off under load.  The way it is set up, is the Thyristor is turned on and off
> many times a second outputting a square waveform.  The pulse width is adjusted
> to increase/decrease the average voltage.  The narrower the pulse width, the
> lower the voltage (and current).  The wider the pulse width, the higher the
> voltage (and current).  When the train is at full speed, the pulse width is
> 100% (permanently on).  It is very similar to Pule Width Modulation (PWM) model
> train controllers.

And the term "chopper" arises because the original DC is "chopped" into
pieces, i.e. the bits where the power is on versus those where it is
not. 

Eddie