I started a long post last night only to have to retype it,
You can breadboard that whole thing and verify every thing using a scope if you just want to understand how it works.
Except the FET as it needs to be on a heatsink and the proper gauge wires used to feed the motor for the amount of current that you are designing it for.
Or you can use any IRF XXX (510,z44,z42,640,740,840,540) and a smaller motor or lightbulb to verify its operation.
The regulator circuit is only for the opamp so you can use a LM7812 to save on parts count.
You can also get better performance and precision if you use a dedicated comparator chip for IC1-2 this is some thing that I would consider if you plan on controling such high currents.
It is your basic PWM circuit and I have built one before and I had alot of fun with it as it has endless uses.
I used mine for a basis for a Class D amplifier.
The other thing I did was to use the TL082 dual opamp to generate a precision triangle wave as I couldn't find a single stage circuit that was linear enough for my liking.
I used a 1/4 of LM339 but there are dual and single precision versions available as well.
Using a TTL buffer on the output to clean things up abit it worked flawlessly and even gave me an output exactly duplicating the input voltage.
My output voltage went from 0V to 5V
A sine wave was used as the input signal and the ouput running into a large inductor as a lowpass filter and there was no noticeable distrotion within its range even with a sqaure wave at various voltage levels as the input waveform.
Make sure that you use the diodes across the motor as the inductive kick of the motor could kill the FET if its voltage is not rated high enough.
My circuit was running somewhere in the 100khz to 200khz range.
I hope that helps you !
jer