Two things:
BD241/242 are rated for only 3A so you should look for "stronger" transistors and probably darlingtons will be better choice..
I don't know at what frequency are you operating but in PWM motor speed control low frequency (≤500Hz) works better ..
Also, I prefer configuration of the bridge as per this example: **broken link removed**
the drive circuit dose not work properly.
pwma and b must be level at 0 to 30 volt.
the better way use the full bridge mosfet (IRFP150) and isolated trans pulse to drive them.
please ,note that a .7 voltage drop will appear between the base and the emitter in each transistor which will consume power P=VI = 1.4*I w.
all of these power are consumed in the BJTs.
it is better to use mosfet .