In article ,
Boltar wrote:
"Angus Bryant" wrote in message ...
frequency) has this effect. The advantage of using pulse-width modulation
(PWM) switching to achieve this is that the switching process is very (90%)
efficient, since the devices only pass high currents at high voltages (hence
burn lots of power) when switching. You could use a linear amplifier (i.e.

How does that efficiency compare with the old DC systems where at full power
the motor was pretty much just connected directly to the powern rail(s) as
opposed to the new systems where you still have the electronicsn driving the
motors?

B2003

I don't have data to hand, but I imagine the efficiency is very high
for both the old DC systems, and the modern induction motor systems.
Unlike mechanical engineers, electrical engineers are quite good at
making their machines efficient. You'll be looking at 95%+.

The real advantages of the modern AC traction systems over the older
DC systems are the following:

1. DC motors have brushes, which cause mechanical noise and which wear
out and have to be replaced; AC induction motors do not.
2. You get more power per kg of motor with AC induction motors than you
do with DC motors.
3. The control of Ac induction motors is done entirely electronically - there
are no mechanical parts (eg, relays, tap-changers) like the old DC systems
have and, again, which wear out quickly.

Furhter to what other people have said, the 'gear change' sound
is found only in the systems manufactured in the mid-90's. Newer systems -
and here I am thinking of Northern Line tube trains and the Heathrow
Express - have no such 'gear change' sound.

The reason for this is that the AC systems made up to the mid-90s use
a power electronic switch called the gate turn-off thyristor (GTO)
which has a maximum switching speed of only a few kHz. Therefore,
as the train speeds up, the frequency of the PWM square wave keeps
on being taken down a notch so as not to exceed this maximum switching
frequency. More modern systems use insulated gate bipolar transistors
(IGBTs) which can switch up to 20kHz and don't have to have their
freuqency notched down as the train speeds up.

Hope that this makes sense!

David.