I like the idea of the v belts over timing as one of the things I hope to get out of doing this is to silence the beast. My step pulley Bridgeport runs smooth and quite and I am hoping for the same on this when I’m done.

What I am having trouble wrapping my head around is the motor is 1725 rpm’s, and max rpm of vari speed is 4000 rpm, what diameter pulleys will I need to get that same ratio? I know there must be some equation to figure it all out but I don’t know what it is.

Thanks

Speed ratio is determined by pulley diameters. If you have a 4" motor pulley driving a 2" spindle pulley, the spindle will be running 2X as fast as the motor (4/2). If you turn that around, and put the 2" pulley on the motor, and the 4" pulley on the spindle, then the spindle will be turning 1/2 the motor speed (2/4).

You need to decide what you want your max spindle RPM to be, and what maximum VFD frequency you will run, and select your pulleys from there. You will no doubt be using a larger pulley on the motor than on the spindle, so the spindle is spinning faster than the motor. If using the stock motor (not VFD/inverter-rated), it seems usually safe to run 2X the base frequency. If using an inverter-rated motor, you can often go even higher. USe the largest pulleys you can fit into the available space, to get the greatest belt contact area.

So, if your motor is rated 1725 RPM, and you run the VFD up to 120Hz, that will give you 3450 RPM at the motor. If you want 5000RPM for your max spindle speed, then you need a pulley ratio of 5000/3450 or 1.45:1. So if your motor pulley is 5" diameter, the spindle pulley must be 5/1.45 or roughly 3.5" diameter.

Finally, make sure your low-end RPM is reasonable. A good sensorless vector VFD should provide usable torque down to perhaps 10Hz. For the above example, this would give you 5000 * 10 / 120 = 416 RPM. Note that, depending on the motor, running extended periods at low RPMs may cause the motor to run hot.

Regards,

Ray L.