The problem is that high current transistors really don’t like being half on. They are much happier being all the way on or all the way off (happier being defined in this case as cooler).
So… …what is an engineer with a high current applciation to do? Well, as it turns out, transistors can be very fast at changing from on to off (and off to on). So… what they do is turn the full voltage on for only a percentage of the time.
Basically, if they want half the voltage, they turn the motor on for half the time. If they want 75% of the voltage, they turn the motor on for three fourths of the time. Assuming that the on/off frequency is fast compared to the reaction time of the motor, the current gets sort of averaged and the motor more or less behaves like it is driven continuously by the reduced voltage.
One way of thinking about it would be to have a hammer that could hit somthing with a force of 10 lbs. If you wanted to accelerate a 10 lbs block at .5g’s I could hit the mass with 10 lbs for half the time. The mass would begin to acclerate with each hit. The mass of the block would smooth out the velocity (sort of, if I didn’t look at it TOO closely) and assuming that my hit frequency was fast enough, I could pretend that I had a 5 lbs. hammer hitting the block continuously.
The mass is playing the role of the motor. The hammer is the voltage. The velocity is the current.
I found the concept easy to understand. Basically, the motor gets full power for amount of time per cycle, and the inertia and momentum of the motor will make the motor keep spinning until the next chunk of full power is applied (like, nano-seconds after). That’s how I visualized it, although I’m not sure how much help that is to anyone else, seeing as Joe defined it pretty well too. I guess my description is the “Super-Layman’s-Terms”
