I often see statements from "reputible" sources suggesting that the use of oversized motors causes energy to be wasted.
In many cases, this is not true, infact there can be an advantage in using an oversized motor.

For example, from a manufacturers data sheet (attached) I see the following:

11KW motor.
100% load 89.2% eff

15KW motor
75% load 89.7% eff

22KW motor
50% load 90.7% eff

So, if I have an 11KW load, I would be most efficient to use the 22KW motor at half load, followed by the 15KW motor at three quarter load and least efficient to use the 11KW motor at full load.
The oversized motors would run cooler, and therefore would last longer as well. The disadvantages of using an oversized motor would be that the start current will be higher and the capital cost is also higher.

I find the same if I compare the 30, 45 and 55KW motors, and others as well.

As I see it, the use of an oversized motor does not waste energy, it actually can save energy!!

Best regards,

I would tend to agree with most of that, however expect that you're comment regarding starting current would apply to DOL starting only. The amount of torque required to start the load would be the same irrespective of motor size, therefore it should be possible to start at the same level of current (actual amps) via for example a soft starter that allowed for start current adjustment in 1% FLC increments, in other words upper end soft starter products.

Regards,
GGOSS

Now theres a trap!!

If we have a 11KW load and we need to have 50% start torque, then with the 11kw motor, we would have a start current Is to develp that torque.
If now we double the size of the motor, then the start torque relative to the motor is now 25%. The rated current is double the 11KW, but torque is proportional to current squared. If we had the same start current (Amps) then it would a lower start torque per amp and so we would have less start torque.
With the larger motor (twice the size, we will need a start current that is 41% higher than the 11KW motor (start current = rt2 times the start current)

Best regards,

You're absolutely correct. Don't know what I was thinking.

Maybe it's time to open that Fish 'n' Chip shop.

Regards,
GGOSS

Dear marke,
you wrote some interesting considerations. But these considerations are related to standard motors with standard efficiency.
If you compare a standard efficiency motor with an high-efficiency motors (for example, Class Eff1 or Eff1+), you will see better results. In other words, if the motor is not so stressed by heat due to low quantity of copper used, and induction on magnet steel is normal and not too high, motor efficiency will be better. You can find the same conditions in a bigger motor at 50% of load.
Please note that using a bigger motor the power factor will decrease, and you will be obliged to make a compensation.
Furthermore, for frequently started motors, the higher inertia will be a further cost in energy for acceleration.

Regards
Mario