Should I oversize induction motor for the loads?

If your load requires a motor, say 50HP, then you should use a 50HP motor. The motor is designed to provide the HP required by the load. If you use a higher HP motor you may lose efficiency and power factor (PF).

In general, all the trick is to use smallest motor that is capable to drive the load at 100% duty so its temperature doesn't go over the one noted on the motor tag. If in doubts - see also what is the motor service factor (SF). To check the motor functionality at your application, measure its current at full speed (FLA = full load amperage). The measured current shall not exceed the nominal - FLA, again, found on the motor tag. There is one more way to check the functionality. If you have a RPM meter, measure at full load (after the speed get stable) the RPM of the motor. The motor tag has its speed (eg. for 60Hz it is going to be around 1760 rpm, thus to the synchro speed the motor lost 40 rpm or 2.2% due to slippage). If you measure the rpm and it is less than the speed indicated on the tag - let say 1740 or so, you are overloading the motor as the slip depends on torque demand. Everything what I wrote above is applicable if your motor it is an induction motor... and in a quasi good condition... Surely, you can use 60hp motor (forget the 100hp) and it is going to be "better" but you are going to pay for it with bigger contactors, circuit breakers, fuses, wires, cos(phi)-PF, kWh etc. The only thing you are going to gain - the motor is not going to "sweat" at the job, it is simply going to be oversized. There are also other stipulations if you plan to use variable frequency drive (VFD) to run the application but it is beyond the topic.

Another thing to consider is that, if this is a SCIM, a larger motor HP than the required load HP will represent a larger inrush (starting current). So, your system needs to be prepared for a larger starting current than necessary for the load. If this is a compressor application I would recommend a custom designed motor for the application but keeping the HP at 50.

Leave your comment (Registered user only)