Re: ATM - Fractional HP Motors for Grinding machines

[The Silent Observer <silent1@ix.netcom.com>]

Adam Perkins wrote:
> 
> I have no idea how a "2 speed" AC motor achieves its
> lower speed, but assume this one to have a "high"
> speed of 825 rpm (developing 1/4 HP at this speed).

Normally it's done by having two sets of windings in either the rotor or
the stator; some are set up as 3 speed, with unequal numbers of turns,
and operate with the smaller, larger, or both; others have equal numbers
and operate with one or both.

> Is there a standard ratio of speeds for a 2-speed
> motor? Is there a loss of power if switched to the
> lower rpm setting?

The speed ratio is determined by the speed at which the back EMF from
the windings acting as a generator equals the input voltage.  In the
case of an equal wound two speed motor, I'd expect the low speed to be
half the high speed.  And there is considerable loss of power; with only
half the windings operating on the same current, there's half as much
torque; given half the torque at half the speed, you're making 1/4 the
power.

> My reservations about this scheme are:
> 
>      -will a too-long drive spindle (floor mounted
> machine and a +/- 50" chuck height) twist into a
> pretzel if I use a size convenient to chuck in the
> press and fit in a 1/2" or so ball-bearing at the
> bottom?

If the shaft is supported along its length, a half inch shaft shouldn't
be a problem.  Have you ever seen the axle half shafts in some of the
small SUVs now on the road?  The Honda version has the half shafts
exposed for a couple feet between the rear diff and the wheel; it's no
more than 3/4" diameter, and that's to carry 1/4 of the 100+ horsepower
engine output under off road conditions!

>      -Is there a possibility of damaging my drill
> press?  I do realize that side to side loads on a
> drill press quill are not good, and will capture the
> end of the drive spindle between two bearings to
> eliminate this load from the pulleys.

If the shaft should break or bend, you're likely to beat up your press
column pretty badly.  Also, of course, water and grit aren't good for
machine tools.  I'm assuming you can protect against the water and grit,
and a broken shaft shouldn't be a huge problem if you can run it through
a pillow block half way and size it to the largest your drill will chuck
(minimum 1/2").

Biggest problem, though, is that you drill press motor probably isn't
rated for long term 100% duty cycle operation at near it's maximum power
output.  If the motor isn't adequately cooled, you might overheat it
unless your combination of ratios is such that the motor is loafing
rather than really pulling.  Or you might not have a problem; this
depends strongly on how much load your machine will develop, and on what
ratios you're able to run at.

-- 
It is well to approach all wonders and miracles by gradual stages or
degrees . . .
                                                           Fritz Leiber

Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.