We were called in to replace an old imperial frame 180 kW motor on a circular saw because they needed a faster cutting speed. Shipping a motor of that size to a remote location was expensive, plus all the mounts and pulleys would have to be changed (imperial to metric and smaller frame size.). Instead we changed the VFD, the forced cooling fan and made some changes to the programmed cutting cycle.
With an older motor you often get more copper, and a lower efficiency. But you do get the ability to "soak" up higher cycles in peak load over time because of the thermal inertia the extra copper gives. (Overload a modern higher efficiency motor with a high cyclic load and you will begin to see over heating growing to the point of thermal failure.) Older motors often have a larger frame size which can mean you can force more cooling air through.
With an older motor on a cyclic load you can set up the VFD to run the motor harder in the peak cycle and then back off on the decreased load. Overall the motor handled this quite happily - and we protected it using an electronic shear pin setting, as well as tight thermal modeling, checked against a number of actual runs.
The result - one very happy customer with much of his budget unspent! Happy accountant too!
The next trick was we offered a specialist 1.8 m blade which allowed us to deal with the waste more effectively and increased the cutting rate.
Yes, the motor was not the most efficient we could buy - but at the end of the day the system was more efficient - and at less capital cost. The electricity costs over a year were slightly higher, but still low enough in the overall scheme of reduced capital (and interest/ROI) and improved system through put to more than compensate - again happy operations manager.