Re: ATM - Fractional HP Motors for Grinding machines

[Del Stanton <sd20@earthlink.net>]

IN both an AC Induction Motor and an AC Synchronous Motor
the motor's field windings and the associated steel
laminations produce a rotating magnetic field in the space
that will be occupied by the rotor.  In a two pole motor
this magnetic field rotates at 3600 revolutions per minute,
in a four pole motor it rotates at 1800 revolutions per
second. (Which I shall refer to as the "synchronous speed".) 

An induction motor develops its torque because the rotor
rotates more slowly than the rotating magnetic field (that
rotates at 3600 or 1800 revolutions per minute).  Because
the conducting bars imbedded in the rotor are rotating more
slowly the magnetic field (that  is rotating at the
synchronous speed of 1800 or 3600 revolutions per minute)
cuts the conductors, induces voltage in them and thus
creates the current in the conducting bars that magnetizes
the steel laminations of the rotor.  This magnetic field
makes the rotor want to follow the rotating magnetic field. 
The rotor does not rotate slower than the synchronous speed
because of losses, but because rotating more slowly is the
only way any currents are induced in the rotor's
conductors.  If the rotor was rotating at the synchronous
speed the rotating magnetic field and the conductors in the
rotor would be rotating at the same speed and NO current
would be induced in the rotor, it would not have a magnetic
field and the motor could not provide any torque.  As
increasing load is put on the motor the speed drops, that
generates more current in the rotor and increases the torque
available to drive the load.

A true synchronous motor has a DC excited rotor that becomes
a magnet that rotates at synchronous speed (1800 or 3600
revolutions per minute).  A synchronous motor develops its
torque by lagging some degrees behind the rotating magnetic
field, the more load applied to the motor the farther it
lags behind, but still rotating at the synchronous speed. 
If excessive load is applied the motor falls out of
synchronicity and stops.  Most synchronous motors (I
believe) come up to speed as induction motors and then lock
into synchronous mode when near to synchronous speed.  If
the rotor was EXACTLY in step with the rotating magnetic
field - that is the rotor's magnetic field were perfectly
aligned with the rotating field - then the synchronous motor
would not be able to generate any torque.  The torque comes
from "stretching the spring" of the magnetic field - the
further the magnetic field is stretched the greater the
torque.  Thus the synchronous motor generates torque that
increases as the rotor drops back from the rotating field
and is "pulled along" by it.  Note that is is a phase angle
change - the rotor is still rotating at the synchronous
speed.

When a synchronous motor is used to drive a movie camera the
considerable variation in load on the motor due to the
intermittent load of the film advance mechanism. This
variation makes the camera drive motor's rotor lag varying
amounts during the film pull down cycle (24 frames per
second). When cameras were used to record TV shows for later
broadcast (long before videotape was developed), this
variation made a "shutter bar" (a flickering horizontal
line) appear in the middle of the television image.  (The
camera had to resolve the different frame rates, 60
interlaced fields per second (two fields interlace to make a
television frame) for TV with the 24 frames per second for
the camera - but I will not go into how that was done.)  The
shutter in the camera was driven by an independent
synchronous motor that was directly coupled to the shutter
disk. There was a "loose coupling" between the shutter disk
and the normal shutter driving shaft shaft that was driven
by the main motor.  This loose coupling  consisted of a hole
in the shutter disk, spaced some distant away from the axis
of the shutter disk.  The usual shutter drive shaft had a
rubber bumper that was somewhat smaller than the hole
mounted at the same distance away from the axis of the shaft
- to act as a "crank" to drive the shutter disk.  This
assured the the shutter was at the correct angular position,
the shutter motor being rotated on its mount until the
shutter would run with the bumper centered in the large hole
in the shutter once the camera came up to speed. The
difference between the diameter of the hole in the shutter
disk and the diameter of the rubber bumper was sufficient
for the camera's maim motor to vary its angular position, as
required by it intermittent load, without the rubber bumper
contacting the edge of the hole.  Thus the shutter was in
the correct angular position, but isolated from the cyclic
angular errors in the position of the usual shutter driving
shaft.

Del Stanton    sd20@earthlink.net

lowther@att.net wrote:
> 
> 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).
> >
> 
> Ac motors 'natural' speeds come from the number of poles.  The 'regular'
> 1725 is actually a 2 pole motor with an ideal speed of 1800.  Losses
> account for the 1725 rating.  I just gave a washing machine motor to
> someone for a lathe.  After examining the schematic, it looks like they
> use the low speed winding to degrade motor perfomance to get a lower
> speed.  I could see no place where the hi speed winding was ever
> switched out of the circuit, just where the low was switched.  I
> remember years ago at a training seession everyone was shocked to learn
> that washers use more power on low than high.  These motors even had
> completely seperate  low windings.  I guess it must be an effect of
> being off the 'natural' speed.
> 
> --
> Ken Lowther
> Youngstown, Ohio
> http://www.atmsite.org