ron, as I read it he means a PWM generator that can do 30 amps, input/output voltage are the same, between 12 and 24 volts DC. At 30 amps that's why I suggested looking into Mosfets, (no diode drop just a very low ESR) but there are many depending factors on what neutrino actually wants to do that will effect what the best choice is.
Ron. We're on post 2 or 3 at best. Lets keep the original poster in mind here please. EVERY original post must be treated as the first one. You're already derailing understanding by foretelling of a perceived intent of which none has been given.
We're here to do one thing and one thing only, to help to increase understanding, not to judge.
Will the control be provided manually (e.g. using a rotary or slider control, or switches) or by a microcontroller/PC ?
Is the engine/motor one having field coils (i.e. is it an inductive load) ?
And what kind of control? Speed control to both directions? Is the 30A current just an estimate (stall current)?
In general, you need an H-bridge that can handle the 24V and 30A. Do you wan't buy one or build it yourself?
Then you need the PWM signal. You can do this with analog circuit:
1) DC reference, triangle wave and a comparator.
2) 555 timer
You can also use a microcontroller to create the PWM signal.
What is the application? Is it a simple human operated machine or a more complex and automatic? Do you need accurate speed control?