Use a stepper motor driver to control a brushed motor

The "problem" is that you probably have not speed CONTROLLED your small motors, you have probably 'speed altered' them.

If you had no feedback (either tacho of some sort or back emf related) then scaling it up to the kW range is liable to produce interesting but not necessariy useful results. There are MANY DC motor control circuits / pages / papers on web. Any serious ones will have some form of speed feedback. Searching for eg : high power dc motor speed controller : Will produce zillions of hits. Including:

• This facebook page appears to have cpied copyright material from SIlicon Chip - if you use it send them the cost of a magazine.

• This commercial one rated at 50A for $60 here (http://www.canakit.com/50a-dc-pwm-motor-speed-controller.html )

• And a vast range of maybe-relevant images, here .

Assuming that the outputs of your brushed drivers are a voltage or current proportional to the commanded speed, you can certainly drive a bipolar stepper with two brush drivers. Let's assume that the output of a driver set for 100 gives exactly the drive current needed by a stepper, and that -100 gives exactly the current needed, but with opposite polarity. Then the two driver channels can be driven like this:

^{simulate this circuit – Schematic created using CircuitLab}

The position of the stepper motor is given by the relative amplitudes of the two phases. In this case, the requirement is that they be 90 degrees apart. In the figure, reading from left to right gives rotation in one direction, while reading from right to left gives rotation in the opposite direction. Always keep track of what step you are at, and only change the drives so as to move one step to the right or left.