Re: [CAD_CAM_EDM_DRO] Re: spindle motors

Hugh Prescott wrote:

> I have been researching this.
>
> Acuatully their appears to be two types of VFDs.
>
> One type will work with standard 3 phase motors and with "standard 3 phase
> inverter" motors
>
> The motor distrib that I spoke with told me the "inverter" motors were
> standard 3 phase motors but with better electrical insulation because some /
> all inverters are spikey

Yes, this is a common AC induction motor.

> The other type requires a special motor that sends back a "position" signal
> for the inverter to use in commutating.

This is commonly called a "Brushless DC Motor", which is really a misnomer.
Properly, it is a permanent magnet synchronous motor.

The difference is that the induction motor's rotor field comes from the stator's
moving fields, while the DC Brushless or PM Synch. type has magnetic poles
in a permanent location relative to the motor's shaft. The spindle drive needs
to know the position to provide the correct drive waveform at the right time.
The standard Induction Motor VFD is not designed for positioning use.
Some PM Synch. drives can be used for fine positioning control. There are also
'AC Flux-Vector' drives that can do both spindle speed and positioning using
motors quite similar to AC Induction motors (generally external forced air cooling
is added). These need an encoder for fine position sensing, but use current
sensing to figure out there the rotor field is at any instant.

Note that any spindle drive designed for fine position control will cost a LOT
more than n open-loop speed-only control!

Jon