Re: Techie Question

On May 31, 7:18�pm, Cod Roe <c...@xxxxxxx> wrote:
Thanks for the brilliantly comprehensive answer Ray!  A few questions if
I may:


So the speed of rotation is set by the frequency of the AC supplied

Yes indeed. Asynchronous motors have "slip" which means that they
rotate at a slightly lower speed than that set by the frequency of the
stator windings. Synchronous motors used on early TGV models have a DC
winding on the rotor fed by slip rings which lock the speed of the
rotor to the stator exactly, but require more maintenance and are more
expensive and hence they have fallen out of favour.

which in turn governs the rate of change of the magnetic field,

The frequency of the field changes, but not the magnetic flux which is
determined by the curent supplied which in turn is proportional to the
applied voltage to the stator windings. Three phase drives have both
variable voltage and variable frequency output to the motor

the event of wheelslip the speed of the motor doesn't run away with you,
right?  

That's correct for asynchronous motors employed in all three phase
powered trains. If the motor loses adhesion it will attempt to speed
up due to the reduced torque, creating an opposing flux in the rotor
to that in the stator windings which will brake the motor back to the
synchronous frequency minus slip very quickly.

However, is that enough on its own, or do you want to slow that
wheel down until such a point as it starts to grip again, in which case
I can't see how self correcting wheel slip occurs?

See above which I hope explains all!

Lower working voltage

For 750V DC power supply, the DC series wound motor will receive the
full line voltage once the motors are in notch 3 or 4 out of 4

Three phase motors used in nearly all present models have the stator
operating at a maximum voltage of approx 400V. The insulation quality
is much better than older DC motors and hence the voltage stresses are
lower. This also allows higher operating termperatures for the
insulation before it degrades and thus provides a much higher short
term rating. With no insulation in the rotor (DC motors have a lot of
highly stressed insulation in the armature), it makes them much more
bombproof and it is extremely hard to overcook a three phase machine,
but easy with a DC motor

Hope that helps and provides a bit more insight for you.

I still can't understand why 66's soldier on with DC series wound
traction motors when their manufacturer (EMD) almost exclusively uses
three phase AC induction motors for the North American market with
their inherent advantages and improved perfromance. Oh, well yankee
engineering is either cutting edge (space programme, military kit etc)
or plain backwards in building Ford Mustangs with leaf springs and
live rear axles. I drove one- nice V8 noise, terrible fuel economy and
appalling handling - roundabouts at more than 20 MPH and the back end
breaks away. My Mum's Corsa 1.4 SRI outhandles it by a mile, the
Mustang would stand no chance against my Porsche Boxster whatsoever.
Ooops, not boasting, honest!

Ray

.

Relevant Pages

  • Re: Techie Question
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