Motor size

Hello

I need to find out the size (KW) of an AC motor so I can size up a VSD, the name plate is missing, was wondering could I measure the resistance of the windings and compare with other motors, my motor supplier says its not as simple as that.

Any help would be appreciated

Determine the frame size of the motor. Measure the shaft, base to center of shaft, spacing of holes on base, etc. Hook it up & determine the rpm. Frame size will be common for a range of hp sizes. Take it to a motor shop. If the label is gone it probably needs bearings anyway. Is it already connected to a piece of equipment? Measure the current of the motor while it is running and compare to FLA charts.

If the motor is connected to your machine and you can load the machine to the maximum it will ever see, measure the current under those load conditions and size the drive for those output amps. Or, to say it another way, size the drive by amps and ignore hp or kw.

Once you have those max load amps, you will need to estimate how much short-term overload torque will be needed. If the operation is smooth with no sudden load upsets and no sudden speed changes, buy a drive with only 10% short-term overload capacity. These are usually called "variable torque" or "normal duty" drives.

If, on the other hand, your operation involves a lot of sudden speed changes or heavy shock loading, buy a drive with 50% short-term overload capacity. These are usually called "constant torque" or "heavy duty" drives.

Actually, sizing a drive by current rather than power is generally the better way to do it anyway so don't struggle too much to find the motor's power rating.

If you may run your motor and measure its current you may determined FLA from following rule - FLA = 1/3 idle current. Ofcourse the relation is approximate.

...and if you happen to have individual motor PFC capacitors make sure you check the current before them, motor side. Especially important on a motor with little load. VFDs & PFC caps don't mix so make sure you do not have them on the load side of a drive.