Re: [CAD_CAM_EDM_DRO] AC VFD question

Joe Vicars wrote:

> Ok, I have this widget
>
> Boston Gear Inverter Vaiable Frequency Drive
> 220Vac 3Ph or 1 Ph
> Programmable speed/dir control for 1 to 5 hp
> 1-400 Hz.
>
> Does this mean I can run an AC motor at 400 hz? Will it go 6 times
> it's normal speed? I don't understand the capabilities of this thing.

Maybe. To properly excite an induction motor, the applied voltage
needs to be roughly proportional to frequency. So, to properly
excite a 220 V 60 Hz motor at 400 Hz, you would need 1467 Volts,
line to line. these inverter/VFDs can only supply RMS output voltage
about the same as the input voltage. So, from 240 V mains, you will
only be able to get about 220 V out. You can definitely run an
induction motor up to 2 X frequency on 1 X voltage, with a reduction
of tourque. (They call this constant power mode.) Somewhere
above that 2x, the inductance of the stator windings will reduce
rotor excitation, and the motor will either coast along at roughly
constant speed with increasing frequency, or it will actually stop.
Anyway, much above 2 x frequency, there will be no power
available from the motor shaft, and any load will stall it.

Now, of course, there is the safety issue. I certainly wouldn't
want to be anywhere near a motor rated for 3450 RPM being spun
at 23000 RPM! This sounds HIGHLY dangerous, and if the rotor
didn't burst directly from centrifugal force, 23K RPM is likely
above the first critical speed, and likely would crash the rotor
into the stator with enough force to burst it for sure!
Needless to say, bearing life would be very short if the rest of
the motor could handle this abuse.

I have run an air-bearing drilling spindle motor at 400 Hz
(wish I could go higher), but it is designed for that speed and
more. I have a Magnetek VFD that also goes to 400 Hz.

Jon