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
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.
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.
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.
As I already wrote in the article you quoted but apparently didn't read....
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.
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.
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 ?)
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.
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?
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.
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.
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