Sorry, mode 2 still isn't clear to me.... "2. Bipolar voltage on the load with a rate of 0-50 Hz"
Do you mean...

A) That polarity of the voltage across to the load is switched at 0-50Hz (and power is regulated by the dutycycle of the 1kHz PWM signal) or
B) That polarity of the voltage across to the load is switched at 1kHz (the PWM frequency) and the 1kHz carrier is turned on/off at a rate of 0-50Hz or
C) None of the above meaning I still don't get it :-)