Speed control for high power DC 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 .