HT Motor Capacity Reduction

The motor will consume what it needs to drive the load, in your case, 0.6 MW. It will of course draw more current than this due to efficiency and power factor. If it is delta-connected now, you cannot connect in star since the power will drop by a factor of 3, ie, it can deliver only 0.5 MW.

It may run at slightly poorer power factor and efficiency at 0.6MW than at 1.5 MW, but short of replacing it, it is not worth bothering about. One plus point is that being only partially loaded, it is likely to last longer.

Please check out this link for more on motors : cowern

Nicely put.

Many people believe, incorrectly, that a larger motor automatically consumes the power rating of the motor. But a motor only consumes as much power as it needs to accomplish the work load it is attached to, plus losses. Losses can be further divided into several categories, most of which are fixed regardless of motor size, i.e. friction, wind, I/R etc. etc. The only one that varies with motor size is the core (magnetic hysteresis) losses in the iron. More iron, more losses. But of the total losses, this never represents more than about 25%. but that is 25% of the LOSSES, not of the motor power. So if your motor efficiency is rated at >90% (as many newer designs are) then the magnetic losses represent <25% of <10%. If your motor power draw is further reduced by a light load, as in this case, then that is <25% of <10% of 40%. So you are experiencing at worst an additional 1% losses with relation to the full load power draw of that motor, probably a lot less. Take that into consideration before chasing it with expensive solutions.

Power factor issues can be easily corrected with capacitors.