Underloaded Motor

Yes, but you lose torque as well. Torque is reduced by the square of the voltage reduction, so if for instance you reduce the voltage by 15% to reduce magnetic losses to save energy, the torque goes down to .85², which means 72% of FLT. If the load stays low, that usually isn't a problem but if not, you need something that can respond quickly to re-apply full voltage.

This is what the Nola Power Factor Controller does by the way, which is the basis for many of the "energy savers" on the market today. They measure the power factor and if it's low, they use phase angle controlled SCRs to reduce the RMS voltage to the motor, which reduces the magnetic losses. Then by sensing the PF, they are able to automatically determine when the load increases (because the PF goes up) and fully gatethe SCRs on to apply full voltage. The problem with tose "energy savers" is that they don't really save very much energy, and when their installed cost is factored in, they take a LONG time to just pay for themselves, let alone return extra cost benefits. Thy also need to be used in cool dry environments as well, because when he SCRs are phased back, they are rejecting about 4.5W of heat per running load amp (3 phase) and will cook themselves if mounted in a dust / water tight enclosure. So the applications are much more limited than the purveyors make them out to be..