I read an article today in Wired magazine about a plan to get an electric car /
battery infrastructure going in Israel as a beta
test. Somewhere in the article it mentioned something like "don't need gears
because to go faster you just apply more electricity"
... I've seen this same statement mentioned in other articles about electric
vehicles.
But is this true? Would gearing up / gearing down help to preserve the battery's
power once the vehicle gets rolling - just as
gearing up/down does for internal combustion engines ? These statements seem a
bit ignorant ...
I figure this topic applies just as much to robotics, hence my posting it here.
JCD
Yes.
DC motors draw the power needed/used. If you drop the voltage (take you
foot off the "gas") to the level required to maintain your speed, the DC
motor will be drawing exactly what it needs to overcome air resistance and
all the friction effects and nothing more. No energy is wasted as long as
your voltage regulator is not wasting energy (which they all do to some
extent). Gears are mostly needed in cars because internal combustion
engines have very low torque and power and low speeds. Gears are added to
adjust for this fact. DC motors on the other hand have max torque at the
low speed so they have just what is needed. Their max torque is the stall
torque (the torque provided when the motor is not turning at all). Gears
are typically added to DC motors when the max RPM of the motor is higher
than the application needs - so you add gears to increase the torque and
reduce the max RPM delivered (you shift the power curve down to a lower RPM
point). But seldom is there a need to have more than one gear ratio (aka
multiple gears like a car). The only examples I can think of where DC
motors have multiple gears are where the gears are used as a cheap speed
control device. This is, instead of an expensive power control circuit,
they just used gears with no power control other than an on/off switch.
Couple quick reasons: The typical small hobby DC motor turns in excess
of 9000 RPM; most folks aren't as interested in robots going at freeway
speeds. Something a little slower is usually called for.
From a price standpoing, a set gear reduction is cheaper than complex
PWM driver circuitry as you'd find in an electric car.
-- Gordon
Motors capable of both high speed and controlled slow speed are
quite feasible. The classic example is elevator control. Gearless
traction elevators classically have a direct drive DC motor connected to the
traction sheave, and are able to level the car within 1/8 inch or so.
That's a nice piece of control, especially since it's been done with
relays, resistors, and motor-generator sets.
Adept's SCARA robots are gearless direct drive, with "pancake" brushless
servomotors directly driving the axes. Positioning to 0.001 inch is possible.
Motors capable of fine positioning like that usually have a large
number of poles, and are designed to avoid "cogging" problems.
While the control electronics becomes more complex, gearless
servomotors have some substantial advantages. For one thing, you
can't strip the (nonexistent) gears. At the large end of the scale,
they're preferred for shock loads like rolling mills for just that reason.
At the small end, there's no backlash, which is useful when
doing precise positioning.
John Nagle
As I already wrote in the article you quoted but apparently didn't read....
> Gears are typically added to DC motors when the max RPM
> of the motor is higher than the application needs - so you add gears to
> increase the torque and reduce the max RPM delivered (you shift the
> power curve down to a lower RPM point).
There's two issues here. (As John pointed out). There's the question of why
you need gears at all, and there's the question which was actually asked in
the first message, which is why you need multiple gears (aka adjustable
gear ratios - which are called "gears" generically in automotive
applications). Even though both questions could be asked as "why you need
gears" they are very different questions referring to very different
engineering issues. The "gear motors" you see for sell in all the robotics
stores address the second question and have nothing to do with the first
question. They only have one gear ratio in their design (at least I've
never noticed a multiple gear ratio "gear motor").
The torque and power curves of electric motors better fit the application
of powering a car than the power curve of a typical gas engine and as such,
most electric car designs end up with a fixed gear ratio drive train - as
do most robots. Even though I wasn't aware of it, it seems (according to
John) that some electric car designs have used a design with at least 2
"gears" (aka gear ratios) in order to maximize both low end acceleration
and high end speed.
On 20 Ago, 02:35, snipped-for-privacy@kcwc.com (Curt Welch) wrote:
But I know that the PWM control, varying the medium voltage on motor,
modifies the speed but also the max torque. And every motor has a
limit in the maximum voltage.
For this reason it's impossible to obtain directly an high torque
from, for example, a little 6v motor, also using a PWM control.
So with the gears the motor outputs a greater torque at all possible
voltages, at the expense of a reduction in speed. A typical example
are the servo motors.
Is it correct?
G. De Sanctis
Yes. Gears don't change the energy/power (except they reduce it slightly
due to friction loss), but they do trade speed for torque - just like
transformers trade voltage for current. You can increase one, by reducing
the other in any ratio you like, but the power remains the same. Torque x
RPM is power, and voltage x current is power. Electric motors of all types
tend to operate at higher RPMs than most typical applications require so
instead of using a motor with the low end torque required for an
application, it's common to use a much smaller motor with far too little
torque, and then using gearing (or belts, or other types of friction
drives) to translate the high RPM low torque of the small motor into higher
torque with lower RPM needed for the application. The cost and size of the
smaller motor plus gears is usually less then the cost and size of a larger
motor without gears.
The larger motor wouldn't use more current or power to produce the torque,
it would just physically have to be larger (and more expensive) - more
turns of wire in the coils and heaver wires to prevent loss due to
resistance (and maybe more poles?), and larger magnets if it were a PM
motor.
That's an interesting question. There are two issues; first, is
a gear reduction needed between the motor and wheels, and second, is
a shiftable transmission needed.
Although it's possible to direct drive the wheels from the motor,
and there have been some motor-in-wheel vehicles (mostly mining
trucks) usually there's a gear reduction between the motor and
wheels.
Some designs have a shiftable transmission; some don't. Tesla
Motors started out with a no-shift design, but couldn't hit their
desired top speed. So they went to a 2-speed gearbox. Then they
went to a liquid-cooled motor and back to a single-speed gearbox.
With this, they can get enough low-end torque to get their desired
initial acceleration while running at a constant high ratio
which maxes out the vehicle speed around 125MPH. If they were
willing to accept a 100 MPH top speed or 6-second 0-60MPH
acceleration, the drivetrain would be simpler and cheaper.
The Toyota Prius has a single-speed transmission, but an
unusual planetary gear arrangement which connects the gasoline
engine, electric motor, and driveshaft.
Most modern electric cars use AC motors and semiconductor drives.
Brush DC motors are a headache at the current ratings required
for good highway performance.
John Nagle
Thanks to everyone that has replied to this one!
I am still stuck with the mental picture that once a motor gets spinning (ie -
after the car is moving from a dead stop) you would
then need less power to keep it spinning, so would perhaps 2 gears be able to in
ANY way maximize the life of a single charge for
a battery in an electric car for a given amount of mileage ?
This then leads me to another question: are there any types of motors that have
"electrical gearing" --- meaning a motor in which
the *number* of coils that are energized is varied to achieve different levels
of mechanical power (torque ?)
Thanks again !
JCD
Sure. Larger motors often have "starting" and "running" windings,
with a centrifugal switch to change modes. Traction motors for
older transit vehicles and locomotives switch various
combinations of windings from series to parallel as a primitive method
of speed control. The jerk at transitions is very noticeable in older
subway cars. This approach was invented by Frank Sprague in the 1880s.
This is all classic stuff. Today, any motor big enough to need
a full transition controller probably has a solid-state PWM drive
instead. It took a long time, but at last power semiconductors come in
locomotive size.
John Nagle
No, gears will _always_ cause the battery to drain faster because they add
additional energy loss in the power train due to friction. AKA, they cause
the battery energy to be wasted heating up the gears. In the designs that
John mentioned which used 2 gears, it seems they were added to create a
"sports car" design to allow for max 0-60 acceleration while maintaining
a maximum top-end speed. But they did it at the cost of lower range (faster
battery drain).
Electric motors translate electrical energy into mechanical energy. You
can completely control the drain on the battery by controlling the voltage
(PWM duty cycle) you put into the motor. When the car gets up to speed and
you want to stop accelerating, the controller reduces the duty cycle of the
PWM which reduces the drain on the battery to the level needed.
To increase battery life, you have to reduce wasted energy - energy that is
is used to heat things up instead of move the car. Gears heat up due to
friction loss. Every bearing in the drive train wastes energy, and every
gear you add adds enough bearing or two. Belts heat up and waste energy as
they flex and as they rub against the pulleys. The wires in the motor have
resistance which cause energy to be wasted as heat.
Electronic controllers are not 100% efficient either. They waste energy as
heat as well (out the heat sinks). It's always a design trade off to pick
what is most important for any given application. In general, because of
advancements in power electronics, it's more optimal to control energy flow
and torque with the electronics than to use mechanical systems like gears
these days.
Also, keep in mind, that when we are talking about cars driving at highway
speeds, there is a huge amount of friction due to air resistance at those
speeds so to keep a car moving at those speeds actually burns up a lot of
energy (battery life). You don't actually get to "back off" all that much
anyway when you get "up to speed". They energy loss due to air resistance
goes up with the square of the speed (if I remember correctly) which means
for each 5 mph faster you want to go, you have to add increasing more
amounts of more power.
If you want to extend battery life, drive at 15 mph instead of 70 mph.
That will do more for you by orders of magnitude than everything else
combined. I don't know the actually numbers, but you could probably go 10
times as far at 15 mph than you could travel at 70 mph (however, it will
take very long to get there). The faster you drive, the more the energy of
your battery is wasted heating up all the air and the less it's used to
make you move.
Actually, no. Optimal speeds for IC engine cars are around 45MPH, but
that's because an IC engine uses enough fuel at idle and loses enough
heat that there's a penalty for going too slowly.
Electrics don't have the idle losses that IC engines do, so the optimal
speeds for electrics are slower. The solar-powered competition cars run
around 25-45 MPH; most can go faster, but it's inefficient to do so.
John Nagle
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