[Lee Hart]

Not a dumb question. Sure, you could do it. The potentiometer would need to be about ten times the size of your motor. And it would waste half of the battery energy needed for every acceleration. And at any speed less than full speed, it would waste considerable amounts of power.

That's a bit drastic. Let's not scare the poor guy to death!

For many years, really simple crude low-speed EVs used exactly this type of motor controller. Examples include streetcars, golf carts, fork lifts, and many little personal vehicles. These were serious commercial products made by major companies; not hobby or experimental EVs.

The adjustable resistor in series with the motor is called a "rheostat" (not a potentiometer). At this power level, it is generally built with a number of fixed resistors, selected in by a big rotary switch.

This type of controller was only popular because it was CHEAP! It has no other attributes. It worked best when the vehicle was so under-powered that it was run at full power most of the time. At full power, the resistors are all bypassed and all the power goes to the motor. You ramp the rheostat up from off to full rather quickly (10-20 seconds) to accelerate from a stop to full speed; then cruise at full speed; then ramp the rheostat back down when it's time to stop. Since it's not in use most of the time, efficiency isn't too awfully bad.

Yes, there are still applications for such a thing. For example, I teach kids to build EVs out of old bicycles and other junk they can scrounge (see www.bestoutreach.com). A typical setup is a car fan motor, with a skateboard wheel pressed onto its shaft, arranged so it rubs against a bike tire to turn it. If you simply switch the motor straight to a 12v battery from a dead stop, the tire slips and quickly burns a hole in the bike tire. But if you put a rheostat in series with the motor, you can slowly ramp up the speed to get it going without slipping. The power of the motor is such that full speed is only 10-15 mph, so you just run it at full speed or off. You can't leave the rheostat at anything but full speed or off for more than 30 seconds or so, or the rheostat overheats.

At this power level, you can buy a proper motor controller for $10 or $20. But a rheostat costs $1 or $2. Guess which one the kids pick?

 

[Lee Hart]

Just having a bit of fun and trying to make a point But the big pot is about what he'd need for an EV car.

Point taken. Though, the pot you showed is bigger than the ones used in the streetcars, subway trains, and other huge vehicles that used this method.

most buy a potentiometer and just use two terminals for rheostat applications.

For small stuff, yes. But you really don't see high wattage potentiometers; they are all rheostats. Often they are specifically built for motor speed controls, and have a nonlinear taper so position is roughly proportional to speed.

And sure, you can build EVs with resistor speed controls, or battery switching, but it is unlikely the outcome will be suitable for highway capable car performance.

Of course it can provide highway-capable performance. You and I might not do it because it's not the "best high-tech" solution; but it is certainly still a viable approach. It's no worse than, say, driving a old car with a carburetor because it's what you can afford, and know how to fix.

 

[Lee Hart]

I don't see why you couldn't drive your PWM from a circuit?

Generating the PWM signal is comparatively easy. You can do it mechanically, with vacuum tubes, transistors, ICs, or even microcomputers.

The hard part is building some kind of switch that can switch the inductive load of a motor on and off millions of times and survive. This is one of those problems like riding a unicycle; an expert makes it looks easy -- then you try it, and "ow"!

It's possible to build your own PWM controller; but it's not easy. Don't take this route because you think you'll save time or money. Only try it if you're already experienced at designing and building high power electronics, or want to learn how it's done no matter how long it takes or what it costs.

For the purposes of this thread: The best way to make your own low cost DIY controller is a contactor controller. It's basically a few used or surplus forklift contactors, wired to switch the motor and/or batteries in various series/parallel arrangements to control the motor speed.

It also helps if you can find a motor with a shunt field (also called sepex, or separately excited). The shunt field makes them much easier to speed control -- all it takes is a moderately sized rheostat.