Re: Notching-up scenarios

On 23 Mar, 08:34, Chris J Dixon <ch...@xxxxxxxxxxxx> wrote:

The available notches on both ac and dc units were pretty
standard, and were largely due to the dc traction motors then in
use.  The most common arrangement was to start with all four
motors in series, and then reconfigure as two parallel strings of
two in series.  The voltage supplied to this system was
controlled either by series resistors for dc supplies, or via a
transformer and tapchanger for ac supplies.

For starting, all 4 motors in are series.

Notch 1 was for shunting, and held the applied voltage at a low
level.

Notch 2 voltage was gradually increased until full volts are
available.

Notch 3 reconfigured the motors into two parallel strings of two
in series, and again increased voltage gradually until full volts
are available.

Notch 4 introduced field weakening.  This changed the motor
characteristic at higher speed.

IIRC, the lower acceleration rate was available in notch 2.

On some earlier systems, acceleration was controlled by actually
measuring it directly, using either a pendulum, or mercury tube.

For DC units using either contactors or camshaft resistance control,
the notches described above by Chris and the way in which they worked
are correct with one exception:

A current limit relay (CLR) controls the progression of the camshaft
position or contactor sequence so that the next position could only be
taken once the current had fallen back to a predetermined value
governed by the back EMF of the motors. The CLR has a current winding
and a control voltage winding acting on the relay's core. If you
selected Notch four from the off position, the control voltage winding
would receive a lower voltage, permitting the CLR to allow the
camshaft or contactors to progress more quckly through their
positions, hence improving accelleration. A 10% improvement rings a
bell.

The "Notch 4 Start" was widely taught and resulted in a 4 EPB to get
off the blocks very smartly indeed. I doubt whether the replacement
Networkers can match this initial accelleration.

Ray
.

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