Hi SDX, thanks for some intersting info there.
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Originally Posted by sdx
but DC trains are now equiped with thyristors which "cut" periodically the current in order to deliver the adequate power to the motors. The frequency they cut is around 400 Hz and is perfectly audible on recent trains (jubilee line for instance). Your system will have to avoid these frequencies.
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In the UK, the last couple of generations of 750 V DC mainline EMUs have used AC motors. This started with 465/466 Networkers in the 1980s , and has been continued with the more recent 375/376/377/378 series. The main benefit of this is that the motors can be used for dynamic (regenerative) braking. This not only saves on brake disc usage and is a more 'powerful' brake, but also keeps the Green lobby happy as it enables some of the train's kinetic energy to put back into the Traction Current supply, or even the National Grid theoretically.
Because the motor frequency varies as a function of train speed, some units like 465/2 and 466 (ie, the MetCam units) have a device called an Interference Monitor, which will actually cut off traction power if the train's return current contains a frequency near to 50Hz into the running rails - which I'm told is to prevent damage to the ac track circuit.
And it's not just EMUs; Diesel locos are undergoing similar development. The ubiquitous Class 66 loco was conceived with DC traction motors, but the new Genesis Project hybrid loco (powered byboth Diesel & AC) which GE is currently developing for Freightliner is to have AC traction motors.