Another issue, according to a retired commercial electrician friend,
is the
newer high efficiency motors run from slightly hotter to much hotter.
The
weigh from ~10% to ~30% less. I suspect these motors will not last
nearly
as long as the older ones, and when you add a variable speed drive I
would
guess the service live will be much shorter.

We used to say that thermal conditions were almost always the lifetime
factor. Now, winding failure due to voltage stresses can be an early
failure mechanism as well. The designs will improve that.

I know that transformers from the 50's and 60's are heavier and run
much
cooler then most "modern" transformers.

The "paradox" is that less metal can mean less losses, but less metal
mass means temps rise. Another factor is, years ago, class H
insulation (higher heat capability) was more brittle than class F and
was used only in very high ambient heat applications, because it would
fail years sooner in cooler apps than would class F. As improved
class H insulations were developed, the standard designs became class
H. The designers said eureka!, we can now go much hotter.... and one
can fry eggs on some transformers.

Also, in the case of liquid filled transformers, when industry moved
from combustible oil to fire resistant fluids, operating temps for a
given size system had to rise as the new fluids did not have as good
thermal transfer capability as oil and the cooling radiator systems
became less efficient.


Even most EEs don't understand that the common AC motor will make a
very useful genset. You can't draw as much current as the motor would
draw, but the simplicity is hard to beat.
Here is one way to do it.
http://www.qsl.net/ns8o/Induction_Generator.html

Interesting link. Wound rotors with connections brought out to slip
rings are more efficient, and can have amazing speed torque
characteristics as an AC motor with solid state drives and controls in
both circuits.

In old, large motor generator sets used to provide power to reversing
steel mill drives, when the mill is reversing under current limit, the
motor is generating into the generator which is motoring into the AC
motor, which then generates back into the power buss.

The DC motor or generator is essentially the same piece of equipment,
only matters whether power/torque is being applied or used. Today's
hybrid cars are an example. When slowing down the "motor" is
generating into the batteries, and when accelerating the motor is
supplying power from the batteries as a motor.

Bob