[PRCo] Re: PCC__Speeds

[Pepa Kalousek]

From: Jim Holland <PghPCC at pacbell.net>
> 1.>-- You say that changing from 50% to 67% field-shunting yields
> faster  *acceleration.*    This would just be the
> *Rate--of--Change*  in speed, not the ultimate speed  --  is this
> correct??

No, this also causes the balancing (ultimate) speed somewhat higher. But
when the overspeed relay cuts the power somewhere between 42 and 50 mph,
there is no way to increase speed. The result then is only that higher
rate-of-change.

> 2.>-- Would this change in field-shunting affect spotting
> circuitry when the power pedal is released?    If yes, would it
> explain why the PRCo  1775--1799  cars had quited a strong
> dynamic drag when the power was released?

During coasting, one pair of motors is shunted in relation to car speed (at
high speed - F1 and F3 field contactors are closed to yield partial-cca 35%
or FULL-67% shunting of this pair) while the second pair of motors is
shunted permanently, without any reference to speed, by means of F2
contactor. But this shunting is only PARTIAL (cca 35%, haven't my
calculation here). You can easily imagine, that when we change the F1 and F3
shunting from 67 % to 50 % and the F2 from 35 % to cca 25 %, it MUST cause
some change in the motor exciting and the spotting current increases to
higher value, primarily at high speed, when the accelerator's resistance
cannot be added and is not sufficient to consume the electric energy
supplied by both pairs of motors.
We did the same experience here with newer series of T3. All previous cars
have FULL shunting by F2 during coasting and also additional shunting by F1
and F3 at higher speed, so that the spotting current do not exceed 100 Amps
at about 60 km/h (37 mph), which is in sharp contrast with 200 Amps of newer
T3 with only PARTIAL F2 shunting. Also remember, that the armature current
of FULLY shunted motor causes much lower braking effort than the same
armature current of only PARTIALLY shunted motor. That's possibly why the GE
car has strong dynamic drag when power is released at higher speed. But I
haven't seen any GE wiring diagram so there might be also another reason of
that drag. You may consider this my interpretation a little jerky, but might
be understood.
This is an appeal to the Pennsylvania Trolley Museum PCC specialists - does
anybody know, if those GE cars are suitable to change from 50 to 67% field
shunting by means of short-circuiting part of field shunt resistance?

Boris