Despite the fact that the cost of an oversized motor goes on forever, more than half the respondents to our survey say they over-specify motors. Is the response different between mechanical and electrical engineers? How do you feel about motor oversizing?

1) loading on motors is variable in machines e.g. scrap grinder,pulverizer,mixer
2) layman uses do not no how much feeds to machine
3) for safety side mgf use over size motor
4) temperature variable in 3 season like India 7 to 45 c so to avoid over heating in pick temperature
5) cost and maintenance of vfd is more (small industry) not use it due to dirty atmosphere
6)their is no effect on current if we run on loading conditions only if it run empty it costs more

I saw in some applications oversized motors do more harm. If the motor overload protection set according to motor's name plate it will protect motor but not the part witch motor attached to (gear box or other). Bigger motor, higher price, more room, less efficiency.
Of course there are many applications where motor must be oversized.

In the end, the size of the motor should always be relative to what you are planning to do with the material you are working with. If putting an over size motor will give you head room or allow the drive and components to run cooler, fine. I would even consider an over size motor based on duty cycle/factor. Keep in mind that depending what you do with the motor, it could also potentially damage material/equipment if you do not set things up properly.

Apart from higher purchase and installation cost, an oversized motor will require more energy because of reduced efficiency range. If you have no inmediate plan to increase load you should install a properly sized motor and replace it for a bigger one only when required.

Also power factor of a motor that runs at less than full power gets progressively more lagging. This affect the sites power factor and in some countries energy regulators require minimum power factors coming from sites, otherwise fines are imposed.

In certain applications, over sizing becomes mandatory. Like in Paper machine - one section need to synchronize with other (preceding or succeeding) sections and the based on basic machine speed, the individual section need to correct itself with in a specific time period (very minute scale in case of high speed machines). This effectively means the motors need to be rated for transient demands and the desired lrating may be much above normal running load.

Also the type of load decides the selection predominantly. If the load is High Moment of Inertia type and accelerating/ speed correction times are small, the motors and drives are calculated to ratings equal to acceleration load + friction loads. Applications are Dryer section, MG/ Yankee dryers, large roll press sections etc

With the above in view, the paper machine vertical adopts to two ratings - Normal Running Load (NRL) & Rated Drive capacity (RDC).

Same can be said about Centrifugal Fan applications where Moment of Inertia's are high.

But since most of these applications are driven through VFDs, which reflect near unity power factor to the source, the reactive power are fairly controlled to small values although Bearing, Windage etc. active power losses increase with Higher power ratings of the prime mover.

In centrifugal fan/ pump applications, defining quiescent point of operation and application of Variable speed drives yield optimization of drive ratings in the interest of Power & Energy savings.