[PRCo] Re: PCC__Speeds

[boris6 at volny.cz]

Jim,

Some error occured, so I'm sending this message again.
I don't feel this description correct. Please understand, 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 in "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 when the brake pedal is depressed.

Boris

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