On Mon, 24 Apr 2017 12:44:26 +0100, Mike Clarke wrote:

On 24/04/2017 10:44, Huge wrote:
On 2017-04-24, RobertL wrote:
On Thursday, April 20, 2017 at 8:11:00 AM UTC+1, Tim Watts wrote:

[49 lines snipped]

I have no idea if this method is used by any car motors but I think
some trains do - you can hear the "whirrr... drop pitch whirrr"
which I can only explain as pole changing as the motor is otherwise
directly connected to the axle.

I have also noticed that sound and imagined it to be due to exactly
what you have described. A neat way of 'changing gear' electrically
instead of mechanically.

Having listened to Tube trains "changing up" as they accelerate out of
the station, and assuming that they can't possibly have mechanical
gearboxes, I found this quite interesting!

I think they used to run pairs of motors in parallel on starting and
switch over to series at higher speeds. That was some time ago and I
don't know if they still do.

If true, the sequence would have been series for starting, then
paralleled for 'cruise speed'. Effectively a two speed "Electric Gearbox"
created out of nothing more sophisticated than a cleverly sequenced bunch
of contactor switches.

The same trick can be used in hydraulic drive trains where the motors
*are* fed in parallel from the pump to double the starting torque at half
speed (they get the full pump pressure at half pump flow rate each)
before being 'valved' into series for full/cruise speed (each operating
at full pump flow but only half the pump pressure).

For passenger cars, I doubt such an 'electric gearbox' technique would
be applied (other than as a fixed ratio matching between motor and road
wheel speeds in the fashion of a rear wheel differential reduction box as
part of the optimisation of the design of the whole power train).

Since an electric motor is, like a steam traction engine, capable of
providing high torque from a complete standstill, the necessary voltage
controller to control the vehicle's speed can also effectively function
as a combined accelerator and automatic gearbox in one when fabricated
using modern heavy duty switching converter technology thus neatly saving
on the need for an additional box of contactors to switch between series
and parallel motor arrangements (or effective poles in a single motor).

A box of contactors in addition to the electronic speed controller would
most likely be more at home with rolling stock and heavy duty goods
wagons and specialised military vehicles (and possibly off-road 4WD
vehicles) otherwise why embellish an already effective switching
converter solution to speed control with the additional expense of
another box of contactors?

I'm no expert on how the manufacturers *actually* apply these basic laws
of physics as they relate to electric (and hydraulic and gross
mechanical) power trains[1], just someone who has a comprehensive
understanding of the principles the manufacturers are constrained to work
with. :-)

[1] When all is said and done, the basic function of a power train is
just a specialised application of Archimede's Principle of levers.

--
Johnny B Good