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Motor gear ratio selection

There are several things to consider in choosing motor gear ratio:

1) Cost of gear unit goes up at better-than-linear rate as the ratio (input / output) increases.
2) Mechanical wear-and-tear on the internal workings of the gear goes up with either input or output shaft speed.
3) The motor is most efficient at 4-pole speeds ... assuming operation is at traditional line frequency (either 50 or 60 Hz).
4) Gear losses go up as the ratio (input/output) increases.
5) Generally speaking, motor cost increases as the shaft output torque increases ... although the relationship is not always linear.
6) Different kinds of motors (squirrel cage induction, synchronous, permanent magnet synchronous, wound rotor induction, direct current, switched reluctance, and linear) all perform slightly differently and have some different efficiency characteristics. These need to be understood with respect to the actual loading on the system (i.e. whatever is connected to the gearbox output).

Bottom line - if you're looking at a steady load torque requirement (4.5 HP at 25 rpm on the output shaft of the gearbox, for example), then a 4 pole 5 HP squirrel cage induction will be your best bet. If the load is varying (either by varying speed or by power requirement), you may find another type and/or size of machine will be better suited to your needs.

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