James Wilkinson Sword wrote:

On Sat, 10 Mar 2018 21:01:10 -0000, Roger Hayter wrote:

James Wilkinson Sword wrote:

On Sat, 10 Mar 2018 09:36:36 -0000, Chris J Dixon
wrote:

The Natural Philosopher wrote:

I thought electric trains connect(ed) their motors in parallel mode to
start off, which is why they always (used to) start with a jerk.

Series, more like, to limit current.

That's right, start in series, then later switch to parallel.

Jerk is just the inability to reduce it ENOUGH.

Quite so. Until the advent of power electronics in rail traction,
then control was generally either by tapchanger, for ac supplies,
or series resistors for dc schemes. In both cases the steps (or
notches as they are known) are finite, not continuously variable,
and there will always be jerks. The art of the design engineer is
making them as smooth as practicable.

Somewhere in the loft I have the notching curves I drew for the
Class 313 rolling stock.

I wish someone would put soft start onto conveyor belts in supermarkets.
Actually it is kinda funny to put milk or bottles of vodka upright then
laugh when the supermarket has to replace them for free.


I expect they sometimes wish the mentally subnormal had minders to tell
them to put glass bottles on their sides.

So they roll into the next person's food. Why not just make the motor
work properly? This is the 21st century.

You keep saying that. It would not be simple or cheap. At the moment
you just need a crude set of rollers and bearings, and a sufficiently
powerful motor to overcome limiting friction at maximum torque. You
would need some combination of better bearings for the belt rollers,
more precision and rigidity of the whole conveyor structure, a much
larger motor with either a variable frequency inverter or a continuously
variable gearbox. And a control system to replace the current on/off
switch in the mains supply. If you don't believe me, try and make one
yourself.

--

Roger Hayter