At no load condition the generated voltage at transformer secondary output is 140 V ac. When a 60W resistive load is connected the voltage drops to 27 V ac. The pmg generator rating is 1 KW 415 V and transformer rating is 415/230 Vac 1 Kva. Why does the voltage drops so much and what would be the reason behind this scenario?
Torque applied is maintain at constant for both scenario. But during the load condition there is a significant drop in rotation speed.
If input torque is held constant then you are fashioning a constant current generator and not a constant voltage generator. Input mechanical speed would need to be held constant to produce a "near-enough" constant output voltage.
Mechanical power in = \$2\pi n T\$ where n is speed (rev/sec) and T is torque. You have kept T constant but allowed n to fall therefore the mechanical power into the generator has fallen just when you wanted power to increase!