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.
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.
Well written software on an auto should make it change rather like a well
But it's a pretty poor auto that 'lurches' forward. And where it changes
down where there is no need. The now ancient pure mechanical GM auto in my
old Rover is pretty well always in the gear I'd choose if it were a
manual, and certainly never lurches forward. Of course if you floor it at
moderate speeds it will drop to 1st gear and take off like a scalded cat.
But not something that happens by accident.
*The man who fell into an upholstery machine is fully recovered*
Dave Plowman email@example.com London SW
The severe upper limit is ion fact the driving voltage, When the motor
back EMF hit's that, it wont go faster.
Torque is directly proportionment to current. If you monitor that and
pedal position and use the difference top adjust a PWM chopper, then the
behaviour will be as you describe. It is not 'given' though. It has to
be designed in.
"When a true genius appears in the world, you may know him by this sign,
that the dunces are all in confederacy against him."
Not only - but motors can be geared virtually - by changing the number
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.
However, I'm pretty sure most traction has at least one set of fixed
gears as motor rpms tend to be higher than desired wheel rpms.
People would simply say 'too long, didn't bother to read it' and go back
to believing simple fairy tales as per usual.
Lets just deal with one aspect. Number of poles. There is a given amount
of iron you can cram into a motor, that, plus the magnets you use
(unless they are electromagnets) determines the amount of energy *per
magnetic reversal* you can get in and out of the system
This leads to a simple rule: the more revs (reversals) the more power.
Until other factors limit you, as high frequencies start to impose their
own extra losses, as does friction.
Adding more poles allow for more reversals per shaft rotation. It is
equivalent to a gearbox. However only using *some* of those poles nets
you nothing, because you are simply using less ironwork. Its pointless.
No one does it. It is simply like having a smaller motor that weighs more.
Electric motors are UTTERLY different from other motors.They dont really
have a 'power rating' re se. Only when fed from a given voltage.
I've run motors designed for 3V, generally used at 6-7v, at 11-12 v and
got more than three times the power out of them. The limitation was
brush bounce and wear in that case, and the availability of gearboxes to
get the insane RPM down to something usable..
In practical terms a multipole motor is bigger and more expensive than a
two or three pole motor* BUT if the gearbox you need to use is more
expensive and heavier...then the multi-pole wins slightly.
Some of the model motors used to come with kevlar wound rotors, to stop
them exploding. More revs, more power.
That's part of the simple story. The more complex story involves
understanding copper losses (resistance) iron losses (hysteresis and
frictional motor losses) plus issues with the controlling electronics
(peak currents, inductance, switching losses and so on). And it it uses
permanent magnets, the field strength, weight, and performance versus
temperature of those as well.
Each one of which is a book chapters worth.
*two magnetic, three wound poles is the most basic uni-directional motor.
"Women actually are capable of being far more than the feminists will
OK - fine. But you haven't explained why it's "bollocks". I've found
references to train traction using, at some points in history, pole
Also the type of motor described in the Wiki was an induction motor so
no permanent magnets.
Most EMUs these days use induction motors, even on DC lines so the power
electronics is there either way.
I'm not sure how having a larger number of lighter stator windings is
worse than 2-3 large windings either. Would any large traction motor
really use a 3 pole configuration - it seems unlikely - the starting
torque would suffer.
sigh. You see. I explain, and it makes no difference.
A 3 pole motor and a 3:1 gear box is EXACTLY equivalent to a 9 pole
motor of the same size.
If you found references to pole changing it was almost certainly using
all the poles but putting them in series or parallel pairs.
If you only have contactor type switches that's a very crude way to
limit starting current. i.e a 6 pole motor can be connected as a 3 pole
motor with either twice or half the RPM per volt and twice or half the
starting torque or current. Depending whether you series or parallel the
But that isn't 'switching poles off' .
And all it really does is ensure you can start without burning the motor
out. These days an electronic controller does that better.
"When one man dies it's a tragedy. When thousands die it's statistics."
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