Warco WM16 Mill - Conversion to AC motor with VFD

[Jonathan]

VFD with a 1.5kW motor. IIRC, the Altivar drives have a few functions for "torque boost" and "constant torque" which may help.

Help, but not solve. Power is torque multiplied by angular velocity (i.e. speed), so if the VFD manages to keep the torque constant, then the power you get is the speed multiplied by some number. Therefore if you reduce the speed, the power is a smaller number multiplied by the same constant, hence the power is smaller. So if the motor torque output is constant, the power output will be proportional to the speed. For example if you use a 2-pole motor, you'll get the rated power at (just under) rated speed, i.e. 3000rpm, but if you want 600rpm then the power output (again assuming the torque is constant) is simply 1.5*600/3000=0.3kW.

One way to at least partially get round this is to operate the motor above its rated speed. For example if we take a 4-pole motor, so just under 1500rpm @ 50Hz, and run it at 100Hz, you get twice the speed, and get rated power from (roughly) 1500rpm to 3000rpm. Recalling the formula above, it seems reasonable to expect higher than rated power above rated speed, but unfortunately this isn't the case. The reason you (probably) can't get higher than rated power, when you exceed rated speed, is that the drive would need to supply a higher voltage to the motor, however it's limited by the mains input voltage and wont be able to increase that. Having said this is still a good setup - for example if you want 600rpm now, the power output will be 1.5*600/1500=0.6kW, so twice as much. If this still isn't enough then that's when you include pulleys to reduce the speed further - 1:something and 1:"something bigger" would probably be adequate. You need to decide what range of speeds you need, select the motor speed (poles) from that, then work out the required pulleys.

It looks like the maximum speed your machine currently does is 2250rpm, so if we take that as the limit, then a 2-pole (1500rpm) or 4-pole (3000rpm) motor is closest. From the above it's clearly better to use the slower motor. Generally (i.e. might not be a good idea if you use the cheapest motor you can find) it's fine to run a 4-pole motor at twice the rated speed, so in this case 100Hz, accounting for slip it'll be about 2950rpm, but we want 2250rpm, so make the ratio 2950/2250=0.76... pretty close to 3:4 (0.75) so lets use that. Now pick the other ratio to optimize for low speed...