Re: [CAD_CAM_EDM_DRO] Re: hi-freq motors[perske etc]

In a message dated 22-Nov-01 09:16:16 Central Standard Time,
shymu@... writes:

> What is the reason for using hi-freq motors/spindles ?
>

Frank: Undoubtedly, someone knowing much more about "AC machines" will have
answered this more properly before MY response appears, but here goes,
anyway:

The rotary speed (RPM) of an AC motor depends upon line-FREQUENCY rather than
voltage. The voltage has to be "right", but, for example, raising the
voltage on a 240 VAC motor to 300 VAC will only serve to cause its rapid
destruction and discoloration, though a just-perceptible increase in
"available torque" might have been available for a (very) short time after
so-raising the voltage.

HOWEVER, if you raise the FREQUENCY from 60 Hz to, say, 120 Hz., you will
DOUBLE the RPM of that motor, assuming it "can take it", and there is no
mechanical load which will preclude this "over-speeding". (The HF motors are
DESIGNED for that, and the VOLTAGE is engineered to fit in with the HF, as
well).

Now, if you raise the AC-frequency to 400 Hz., you will have a motor spinning
at either 12,000 or 24,000 RPM (depending on how the AC motor is wound,
inside). But the amount of copper is often reduced, etc., so a "400 Hz
motor" can't be run on some much-lower F, like 60 Hz, without cooking, etc.

But most utility-companies in the U.S. sell us only 60 Hz. power. One needs
a "power converter". Forty years and more ago, this was done "only" with a
motor-generator, a big ol' boat-anchor that looked like a longer-than-usual
electric motor with NO shaft coming out either end. But TWO "terminal boxes"
on the side. One is for 3-phase AC in (240 or 480 V, 60 Hz), and the other
is for 400 Hz. out. The voltage-out depends entirely on the manufacture and
the agreement with the customer purchasing same, hopefully correctly chosen
to match that of the HF motor to be powered therewith. Now, some uses can
be powered with a "static freq. converter" which is a "black box" (often
"machine-green" or grey, etc.!) which "electronically converts" the power for
you, with NO rotary converter ("motor-generator"). The new VFD's can do just
that, Vary the Frequency, and Drive the motors at various speeds, though I am
guessing only within a "practical range", due the the fact that the VOLTAGE
requirement of an inductive device like a motor varies considerably as the
Frequency varies! (Takes some elec.-engr. smarts I ain't got!)

Now, say you have just bought-in a 3½ hp. 400 Hz. 3-phase Perske
router-motor, with a "to suit" motor-generator, and decide to wire it all up,
temporarily, "just to see", before you install these items on that new CNC
machine you are building: MAKE SURE you have BOLTED 'em down, at least with
some VERY-heavy and new Jorgenson clamps, cause, when you switch-on
"shop-three-phase" to that lashup, your ZIP-CODE will change! And, you do
NOT want an unmounted motor-generator spinning across the shop-floor, due to
Newton's laws, when the armatures inside begin to spin!

Lotsa luck! Jan Rowland, Troll


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