[Azalin]

Are all variable frequency motors high speed? Are there any low speed (about 6k-8k RPM) but high torque motor that can be driven with a VFD?

[handlewanker]

Hi Az.......I don't think that so far a "compact" motor has been "invented" or designed that can go past the voltage amps input and still be running when it's just turning over slowly.

To get a motor to run slow and have lots of grunt means you either raise the voltage to high levels or stay with the mains voltage and cram on the amps.....this is where it gets tricky because the heat that occurs is inside the windings and as it cannot get out the winding insulation cooks and motor kaput.......you can have the low rev torque with normal current consumption but the motor is huge by comparison.

It comes down to frame size, so in my opinion and I have looked at this problem and decided that if you compare electrical force to hydraulic force....the thing missing is the heat in the hydro solution.

A hydraulic motor can be quite compact and still go from zero to 5,000 rpm range with huge torque from zero to top revs due to the off machine power package not being attached directly to the machine except for the hydro hose lines......the hydro motor is also infinitely variable and has the high torque at all rpm.

If you run the hydro motor at low rpm and cram on the work load the governor just puts out more hydraulic force to maintain the rpm......electric motors burn up when this happens.

The other side of the coin is torque multiplication by multiple gear ratios or belt drive.......and variable belt drive is infinite.

A hydro motor on EBAY in our vicinity is approx. 100mm diam x 150mm long and cost A$220......a hydro pump cost the same and a 3 phase motor new is about A$150......add the oil tank and controls etc for a power package.

You could drive a spindle at 10,000 rpm with a 2:1 belt drive from a hydro motor and still go as low as 100 rpm or so with torque to spare.
Ian.