Why do electric powered automobiles need to be so complicated?

I understand why gears are required with an internal combustion engine, to keep the RPMs at an optimal (or required) range. Also it is difficult to adjust the engine RPMs by a finite amount. For example, it would take a very well tuned engine to hold an exact RPM of 1200, and then increase smoothly to exactly 1210.

I didn't think that was an issue with an electric motor because there isn't such a limited optimal range (they can run well at 1 RPM, or

10,000 RPM). It also seems like they are able to hold exact RPMs, and adjust smoothly from 1200 RPMs to 1210 RPMs (for example) when coupled with a computerized speed control to allow just the right amount of current to flow.

If the above is correct, about electric motors, then you have plenty of power cruis> Simple. If you gear the motor so that it can start the car moving, you'll

There most definitely is an efficiency optimal "sweet spot" on most motors that is exceptionally limited.

Torque conversion of one sort or another is a must.

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Many thanks,

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I'm afraid this is wrong. You can make a motor spin at just about any speed you want. But plot the power output, and heat generated, and you will see why gears are needed.

HP, is not the same at any RPM. Basically a motor draws less current, and produces less torque as it spins faster. When you remove the load, it's speed will increase to the point where the output torque match the frictional losses in the system. The motor at this point is producing no useable power. The get useable power out of the motor, you increase the load, and spin it slower, as you do so, the current flow increases, the heat generated rises, torque rises, and the delivered power increases. As the load increases further, and the rotational speed decreases more, the power reduces, with further rotational speed decreases. The output HP, is the product of the torque, and the rotational speed. At 0RPM, a motor again actually produces no power at all!. At low revs, you cannot increase the supplied voltage (to give more torque), without burning out the motor. In theory, the maximum power should be at half the 'free running' unloaded RPM. It is possible to design traction motors with wide useable rev ranges, but they are effectively larger motors 'turned down', to flatten the output power curve. As such, larger, heavier, and less efficient, than a smaller motor with multiple gears. Even a wide range motor, would not cope well with the range of revs needed for a car application.

Best Wishes

Notwithstanding it'd be flagrant socialism, maybe even communism, which in case you haven't checked lately isn't exactly in the United States charter.

Thanks, Rich

Well, it _is_ doable, albeit admittedly probably kinda wasteful. I once saw a 25 HP 3-phase motor on one of those controller thinngies that produced full torque from zero all the way up to rated RPM (which wasn't very fast - whatever the standard 3-phase 60 Hz induction motor runs at).

It sounded kinda weird while running up the RPM - it was like the PWM was at some harmonic of the drive frequency or something.

Cheers! Rich

charter.

Basic medical care is communist?

Close enough so far.

And then you infer a completely wrong relationship.

Motor power is a function of torque (which is more or less constant at a given _current_), and rotational speed. Maximum torque produces zero mechanical power at stall (when the rotor is motionless). When it is moving slowly, it produces little power. When it is moving fast, it's producing lots of power.

_That's_ why you need gears. At low speed, you need considerable mechanical power to accelerate the vehicle. You only have a certain torque available. You can do as the railroads do, and size your motors for good cruising power. You then accept slow acceleration.

Or, you can do as no one I know of does: size the motor for full acceleration. You then spend a lot of money for a motor that is grossly overpowered at cruise speed.

Or, finally, you do as the vehicle people do (see, they really do know what they're doing): you size the motor for good cruise power (or maybe a bit more than really needed), and use a transmission to multiply the motor's (essentially fixed) torque to a useful level for acceleration.

Then you could smoothly accelerate up to 100

As I pointed out above (and in my previous post), the power is not constant with rpm. Only the torque is. This is a serious difference.

John Perry

[snip]

The torque is constant ??

In a DC motor the torque is maximum at zero RPM and zero at maximum RPM's, due to back EMF.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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I love to cook with wine.      Sometimes I even put it in the food.

If the government forces one person to pay for another's, yes. If the government takes over the health care industry and decides who gets what care, certainly. How do you see nationalizing any industry?

--
  Keith
> 
> 
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Funding a system of free, local clinics nationalizes nothing.

And, hate to tell you, the US government already levies all sorts of taxes onto one set of people for the benefit of others. Did you ever attend a public school, or drive on an Interstate highway? Ever been vaccinated?

John

PWM VF controllers can use very sophisticated feedback techniques to monitor current in each leg of a bridge circuit to produce maximum torque at virtually any RPM, and they now have sensorless vector controllers that use the back EMF to determine rotor position and velocity. You can program the controller for different modulation frequencies to reduce audible beat frequencies and resonances which is probably what you heard. Induction motors can be designed with tradeoffs for low end or high end torque, efficiency, and variable speed. At zero RPM, the excitation frequency is just about the "slip" speed, usually about 20-50 RPM, which may be 0.5 to several Hertz, depending on the number of poles and other factors.

A two pole motor runs at just under 3600 RPM at 60 Hz. There are 4 pole, 6 pole, 8 pole, and 12 pole motors, which run correspondingly slower but with higher torque, so your motor might have been a 12 pole at about 600 RPM. It can be driven up to 400 Hz, at which it will run at 4000 RPM, but the torque will be much less because you do not have a corresponding increase in voltage above nominal line voltage. You can probably run a 240 VAC motor on 480 VAC at 120 Hz and get twice its rated HP.

Much higher voltages would probably overstress the insulation, and push the limits of available IGBTs and MOSFETs used in the bridge circuits. I am researching the practicality of winding motors for much lower voltages and then pushing the HP by high frequency PWM. From what I have heard from professional motor engineers, magnetic losses will limit this boost to about 2.5 to 3x. Higher frequencies may require thinner laminations of higher grade steel, or other changes. An induction motor is much like a rotating transformer, and most power transformers work quite nicely up to about 1000 Hz. It would be very nice to make a motor almost 20 times smaller than its 60 Hz counterpart of the same HP. Of course, a 2 HP frame could not handle the torque of a 40 HP motor, and unless it was wound with more than the usual maximum of 12 poles, it would spin at a possibly dangerous 10,000 RPM.

Returning to the original question of this post, the complexity is the cost of making something efficient and practical. You could bolt wheels on a couple of big series wound motors and control the speed by selecting 6V,

12V, 18V, etc., but it would be hard to drive safely and would not be practical or efficient. Adding sophisticated electronics, using BLDC or induction motors, adding regeneration, incorporating a multispeed or infinitely variable transmission, and maintaining optimum torque on all wheels under all conditions, is a very complicated task but results in an efficient, practical, and elegant design.

Then there is the most serious limitation of the capabilities of batteries, which limit range, increase cost and weight, and require regular maintenance and replacement. Hybrid vehicles overcome some of these limitations, but greatly increase the complexity, especially for those that combine the output of the ICE and electric motors to drive the wheels.

Whew! 'nuff said.

Paul

Yes, a series motor in particular does this at a given voltage. For a parallel motor it is less true. But that's a practical matter of how commutation is done. In both cases you can get closer to the maximum torque over a wide range of speeds by increasing the voltage as the speed increases (exactly as you can in an AC motor by modulating the drive voltage as speed increases).

Think about the fields between the rotor and stator of any motor. The maximum torque occurs when the ferromagnetic material of the cores starts to saturate. If we can choose the type of motor, we certainly won't choose for a vehicle a mechanically commutated motor that has a voltage/current limit determined by considerations other than maximum torque and efficiency over a wide range of speeds.

There are good reasons why no one considers building a vehicle with anything other than AC induction or synchronous PM motors.

jp

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Yes, essentially. It's based on taxation, which is theft. It is wrong to sieze the fruits of one man's labor to pay the bills of another, it's just that simple.

I'm a lot of things, but I'm not a thief and I don't aid and abet them.

And if you don't think it's theft, then when the tax collector comes to your door, say, "Oh, gee, I didn't vote for that tax, so I decline to pay, thank you very much." and see how long it takes to get out the big guns.

Thanks, Rich

On Tue, 06 Jun 2006 02:19:40 -0400, Paul E. Schoen wrote: ...

Do you suppose Big Oil is sitting on fuel cells?

Thanks, Rich

It sounds to me like John's still stuck on the IC explosion engine, with its tiny little power band. I still say that with the "right" motor and controller design, a mechanical transmission shouldn't be necessary.

Just like you say, Jim, a DC motor has its most torque at stall, just like a steam engine. (I have a couple of design ideas for that, too, but that's for another time. ;-) )

Cheers! Rich

OK, that was the root of my misunderstanding. Some guy told me, several years ago, that electric motors had a straight line on one of those power lines/plots. I guess he either was completely wrong, or possibly was talking about motors which also included a lot of fancy equipment... in which case the plot wasn't for the motor only.

I did kind of question that statement, in general, since anything must have some kind of limits of effeciency. I couldn't imagine a motor spining at 1 google RPMs, effeciently moving a 2,000 pound vehicle at

10 MPH =]

Roger Hamlett wrote: