Yea, here I was thinking that a 3450 motor had the same torque rating as a 1725. So much for that pie-in-the-sky concept.

The motor can deliver 5 HP regardless of the pulley ratios- the MAXIMUM AVAILABLE torque will multiply or divide by the speed ratio:

Torque = HP * 5252/rpm

1) a 5HP motor spinning at 1725 RPM CAN make 15.22 Lb-ft of torque.

2) a 5HP motor spinning at 3450 RPM CAN make 7.6 Lbs-ft of torque.

if you reduce the RPM of motor #2 by half with pulleys, gears, or whatever, it will be capable of making 5HP, 15.22 ft-lbs of torque, and the output pulley shaft will be spinning 1725 RPM. The load will see the same amount of power delivered either way.

Now look at this from a cause/effect standpoint:

HP = Torque * RPM/5252

A motor produces torque, and horsepower is a consequence of any rotation that occurs. A stalled motor is rotating 0 RPM, producing 0 HP, and full torque. Some people think the HP creates the torque- How can 0 HP create anything?

5HP is the rated power of the motor. Induction motors will try to develop the power required to move the load at rated speed. If the load is too much, the motor pulls too much amperage and you will need to reduce the load RPM with a smaller motor pulley (motor sees less opposing torque or resistance). This is just what happens when selecting a higher gear ratio for climbing a hill in a car.

If the load does not require full torque, you can increase the motor pulley size. If the compressor only pulls 15 amps instead of 22, you can assume that the load is less than 5 HP- the ratio is approximately the load/full current:

hp= 5*15/22= about 3.4 HP

HP = Torque * RPM/5252

If you can up the pulley ratio by 10%, delivered horsepower will increase by 10%.

If you don't want a 7.5 HP motor, you may have to gear-down to avoid overworking the 5HP motor. You will be getting 5 HP and pumping 2/3 the air volume as with a 7.5 HP motor.

Bulldog aka Ronnie

Thanks for the clairification! Finally, somebody who could explain what was happening! It appears that I'll be sticking with the 5hp as the 7.5hp motors are very, very expensive.