On Thursday, April 20, 2017 at 8:11:00 AM UTC+1, Tim Watts wrote:
On 19/04/17 23:03, NY wrote:
"DerbyBorn" wrote in message
2.236...
Murmansk wrote in
:

I've just been for a ride in someone's Nissan Leaf - it was
impressive, so quiet and amazing acceleration.

Do electric cars have a gearbox (an automatic one I presume)? Or does
the motor just run faster the faster you go? I was told by the owner
the optimum speed for economy of battery usage is about 55mph.

It doesn't sound like it's changing gear.


No gearbox or clutch.

Mainly because an electric motor, when correctly driven, can generate
torque from zero speed and don't have such severe upper limits to the
speed because there are no reciprocating parts which have to rapidly
reverse direction.

Not having a gearbox (automatic or manual) must make it much easier to
drive because torque/acceleration is directly dependent on accelerator
position and not a combination of that and gear ratio - you don't get
gentle acceleration which suddenly becomes kick-in-the-back acceleration
as the automatic transmission reacts to your gradually increasing
accelerator position by suddenly deciding to change down in a situation
where a manual driver would hold onto the present gear. I find this the
hardest thing about automatics: I accelerate out of a roundabout and the
car suddenly lurches forward so I ease off the throttle slightly and it
changes back up - very difficult to hit the happy medium.

It is also easier to build in torque limiting to any value, to prevent
the car lurching forwards if you press the accelerator too hard when
setting off from rest, or for a cruise control to also accelerate you
briskly and yet smoothly from one speed (eg rest) to the another speed
speed. The fact that (AFAIK) no cars implement this doesn't mean it's
not possible to do.

Not only - but motors can be geared virtually - by changing the number
of poles.

Say you have a 24 pole motor. By deciding how to drive the windings in
phase or antiphase, you can pair off adjacent poles for an effective 12
pole, 8 pole, 6 pole etc setup.

More poles = slower and higher torque, less poles = higher rpm.

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 axel.

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.
Robert



Robert