[PRCo] Re: PCC__Speeds

[boris6 at volny.cz]

Jim,

I don’t feel this description correct. Please remember,
that PCC has series DC motors and that field shunt is connected
across (at parallel) the motor field. At the moment when field
shunting is applied (speaking of acceleration), the armature
current is distributed between the motor field and the field
shunt. Weakened motor field causes, that voltage induced on the
motor armature suddenly decreases and as a result, the armature
current is higher. As a result of that, obviously also the motor
field and field shunt currents are higher. All this results in
that the resulting tractive force of the motor is something
higher. So, the field shunting results in weakened field, higher
armature current and higher tractive force. Field shunting is
applied (all-electric cars) at points 80, 85, 90 and 95
respectively. Those 78, 79, 81 and 82 points mentioned it
„The Book“ are probably a sample of prewar
air-electric car. I remember some air cars where the field
shunting was inserted somewhere at point 58 etc. During coasting
(and braking from high speed) the field shunting causes the same
effect to motor armatures – the armature voltage is reduced
and there is no need to have greater resistance in brake circuit.
Air cars had also „permanent brake resistor“ in brake
circuit and the total accelerator resistance was probably greater
too, therefore the field shunting at higher speed wasn’t
necessary. There is also another reason for field shunting during
coasting, that one the most important. When the car is coasting,
one pair of motors is partly shunted (at lower speed) to reduce
the armature voltage and braking current. When the brake pedal is
depressed, F2 contactor opens and the armature voltage of both
motor pairs (as a result of cross connection) suddenly increases
together with braking current. And there is no delay in this
process of applying brake.

Boris

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