Changing Driving Behavior Or How People Use Cars

There are dozens of good commercial and government applications for electrical vehicles, and some of them even make economic sense:-) The truth about battery powered electric cars for private ownership is that without a major and unexpected breakthrough in battery technology they will never be more than urban, short-haul vehicles. For many observers, this means that battery powered cars have no place in the American setting, but I believe they could prove a valuable lever for changing driving behavior. The problem, from the standpoint of a small government advocate, is that it would take a massive government intervention, in the form of electric car subsidies and gasoline taxes to bring it about.

Americans are rich. You can argue that we’re poorer than our parents generation, and in some ways that may be true, but we’re all rich compared to the average Joe of 100 or more years ago who spent most of his time worrying about getting enough to eat or wear. The problem with this wealth is that people can afford to indulge in bad behavior, like driving overweight, overpowered, inefficient gasoline vehicles. As the recent price bump in gasoline demonstrated, doubling the price from around $1.50 to $3.00 a gallon had only a modest effect on changing American’s driving behavior – we’re addicted to gas.

But what if the government slapped a $3 per gallon tax on gas, bringing the price up to European levels. I’m not suggesting that the price paid for gas in England or Germany is in any sense a "correct" price, but I’m pretty sure it would be enough to start changing the driving behavior of people at the lower end of the income scale. Unfair? Absolutely, which is why the government should take that money and spend it on subsidizing electric cars, so the people who can no longer afford a gas guzzler will have little choice but to buy one.

So why do I think it’s a good idea to essentially force a segment of the population into a technology that’s more expensive than running gasoline cars on an unsubsidized basis? Because the limited range and charging requirements will actually start changing the way those people use cars. The private car won’t be some sort of get-away toy for them, it will be a way to commute to a fairly close job or school. People will be forced to learn to make one or two intelligent shopping trips a week, rather than jumping in the car for the mall every time something comes up, and the rest of the time they’ll be stuck with public transportation or walking.

I’ll be one of those people if the day comes, it won’t be out of economic need (hopefully) but because I believe in the limited use model. I rarely drive my own car more than once a week, unless I’m helping friends with a construction project out in the sticks. Somehow, we’ve gotten stuck on the notion that an unlimited cruising range car is a right, in the constitutional sense, when it’s only the last hundred years that the car has been around. There are plenty of cost effective alternatives to owning a gasoline car for the occasional long haul trip most people make, like car clubs or rentals. Technology isn’t just advancing at a rapid pace, it’s pretty good already. It’s our driving habits that need improving.

Electric Motor Horsepower Rating for Automobile Use

In my first blog post I looked at the horsepower rating for the killer app of cars, the Model T, and found that it took America by storm on 20 HP. That’s a lot of horses of the flesh-and-blood variety, but not so many by automobile standards, where most vehicles produced have at least 100 under the hood. When I started running web searches on motors for use in electric cars, I found that they their power ratings tend to be in KW, with the simple conversion factor that . 1 hp = .746 kW. However, the more I read about motors for electric and hybrid vehicles on the web, the more I see references to the rated horsepower – and ignoring it. The logic runs something like this.

Electric motors are rated for continuous output, connected to a power source, they should be able to deliver their rated horsepower until the cows come home. In order to compare them with internal combustion engines, which are rated for peak power, you have to know what maximum horsepower an electric motor can put forth for a short period of time without melting something or catching fire. I know that sounds like a pretty inexact description, but I haven’t come across any sites giving a rationalized rating system, like (the following is made up)

Rated 10 HP
Delivers 20 HP for two minutes
Delivers 30 HP for one minute
Delivers 40 HP for 20 seconds

Not to mention a nice curve showing horsepower vs time through the longest range you’d want to consider. While two minutes is much longer than you’d hope to need for acceleration when entering a highway, it’s nothing when you consider some of the hills you encounter even just in New England. I’m going to keep searching, but I’m getting tired of seeing rule-of-thumb statements that vary from "Electric motors can deliver over twice their rated horsepower for short periods of time" to "… five time their rated horsepower…" That’s a big spread.

As to how the motors are coaxed into delivering the extra horsepower, that’s entirely in the hands of the control circuitry. I’m as innocent about solid state power controls as I am about electric motors, so I have no idea if some of them do this through current regulation or if it’s a simple question of upping the voltage. Maybe in motors that use electromagnets rather than permanent magnets, they overdrive the electromagnet windings rather than the armature windings. I’m not even sure if the failure mechanism is always heat, it could be that the insulators fail and you get arcing at higher voltages, or maybe the air-gap is idealized for a lower voltage. Much more research to do, but I hope I find some primary electric motor vendors who have better specs.

Rectification in Motors and Generators

Ideally, the motors used in an electric vehicle will be able to function as generators when during braking, with the current generated going to recharge the battery. Sounds like a simple enough concept for an electrical engineer to be dealing with, but frankly, I doubt I ever understood the basics of how an electric motor worked when I was at university. It seems to me there was a senior year course elective called "Power Engineering" that wasn’t in my concentration, and maybe that’s where they taught basic electromotive concepts, but I doubt it. I remember friends telling me that it was all math, like most of our courses.

I searched around the library today for a nice basic text on motors, and came across "Electrical Machines" by Kostenko and Piotrovsky. It’s a fun read because it was written as a textbook during the communist era in the former USSR, and therefore, Russian inventors are much lauded and capitalist exploiters are blamed for the early lack of progress in that country. However, it does have an excellent explanation of the basic concepts of a motor/generator right in the first chapter, using a two magnetic poles and a Faraday loop. My ability to produce illustrations while I’m traveling is pretty limited, so I’ll stick to a text commentary.

If you picture two permanent magnets with a nice cylindrical channel cut out of each, oriented across from each other so that the North pole of one faces the South pole of the other, you have set up a nice continuos magnetic field. If you introduce into that field a loop shaped conductor, the armature, you’ve essentially placed two conductors in the magnetic field, which happened to be made from one piece of wire that forms a "U" on one end. As that armature is spun between the two magnets, a current is induced in the conductors (Faraday’s Law). The free ends of the wires are attached to a split cylinder, or commutator, where the current is drawn off each half on the commutator by a brush, forming a circuit with whatever load is put between them. The function of the split cylinder commutator and brushes is to rectify the current so that it always flows in the same direction, though the magnitude will be very sinusoidal. The brushes are fixed in position, one so that it always draws the current off from the conductor near the South Pole, the other from the North. The whole arrangement is rather magical, and proves there is beauty in electrical engineering.

The design formula for electric motors, which are far more complex in practice than our simple loop with two magnets, are highly idealized using rules-of-thumb and cookbook methods. The reason is that the motor components themselves have geometrys that just aren’t describable with continuos functions. I suppose you could do something with numerical methods to describe the flux through the odd shaped windings and magnets, but since motors and generators are built for practical purposes, practical solutions built upon existing data rule the roost. If instead of turning the armature with an external mechanical force (generation) you cause a current to flow through it from an external electrical force, it causes the armature to spin between the polls and you have a motor.

The reading has me itching to build a couple crude DC motors of my own when I get back. I wouldn’t dream (at this point) of actually designing and winding my own motors and generators to build an electric power train for a car. It wouldn’t be practical or cost effective, even if I knew what I was doing, and would add a couple years of lead-up to my first go. It does bring back memories of a few weeks I spent winding transformers some twenty years ago. They were custom transformers with a special ferrite core, a split secondary with multiple taps and a bifilar wound primary, meaning the primary winding consisted of two identical length conductors wound the same number of turns, side by side. I was actually pretty good at it, when the inductance (or was it reluctance:-) was checked by test equipment.