I'm somewhat confused in the physical/mechanical train from the motor to the end load. I have a screw/nut arrangement to move a platform up and down (think about an elevator). Induction motor, pulley and then the screw system. At the moment it needs a 3kW motor to do it's job.
For reasons we want do change the screw to a multiple leaded one so that the effective pitch is greater (I mean, keeping the same ridge spacing a two leading screws in one turn moves double of the space). To keep the speed the same the idea is to use a VFD to drive the motor at half speed.
So there's the physics question: to move the load I have to expend some amount of Joules (gravity and mechanical losses) and, since the motion speed is the same, the power needed stays the same (the work time doesn't change). At the end of the day the work done is the same (ignoring differences in losses due to speed). Even if the screw turns slower the power needed is the same (because of the torque conversion changes).
However an induction motor gives constant torque. So if at 1500 rpm can give 3kW if I run it at 750 rpm the torque remain the same but the power is only 1.5kW, every motor curve says so. Even at zero rpm torque is more or less the same but the (mechanical) power is zero (because it's not actually turning).
Does this mean that I need a more powerful motor or have I missed some energy conservation step? (the obvious solution would be to use more motor poles to have a motor with 3kW at 750 rpm, of course)
