Loud & clear - DC PWM on 319 train (overhead power)

Thameslink train on overhead power. It accelerates OK on overhead (compared to its crap acceleration on 3rd rail). Also it sounds much better. As well as being louder, it also has a 50hz hum of the overhead AC being diode bridged to DC for the DC PWM. These are now being removed from London and being taken up north. DC PWM is an old 80s technology which was the forerunner of modern AC VF systems. The trains motor is turned off and on 100 times a second. The percent of each time period it is on increases as the train speeds up to reduce inrush current when it first starts. The frequency of the switching does not change, only the amount of the pulse that is true changes. This gives a constant low hum as the train accelerates until it reaches the point (about 15 mph) when the motor is fed directly. Compare the difference in sound to when on 3rd rail...    • DC PWM sound on a 319 Thameslink train