Re: ATM Help sizing a stepper motor

["Bill T." <twentiethwave@hotmail.com>]

Matt,

Take a look at

http://www.servosystems.com

Their web page lists several new stepping motors, but they also carry an
extensive stock of used, good quality steppers not listed on the web.  They
will gladly mail their interesting catalog of new and used stuff.  You can
spec a "new" motor, and ask if they have a used equivalent.

Almost all manufacturers make physically the same stepping motors -- the
frame sizes, current ratings, shaft sizes, etc. have been standardized for a
long time.  Only quality varies.  The main issues with stepping motors are
the number of phases and the current rating.  Check Mel Bartels' doc files
for recommended motors, if that's what your building.  For slow shaft speed,
high torque applications you usually want a fairly low current rating, say
about 1.0 to 2.0 amps, in a four phase motor.  3 volt, 2 amp, 34 frame size,
four phase, 5 and/or 6 wire motors are common on the surplus market.  The
volts don't matter that much, it's the current that counts.  Low amp motors
are hopeless at high speeds, but work well in slow, saturated PWM
applications.

If you can find a good, used 34 frame size, single stack motor I'd recommend
that over a 23 frame size.   34 frame size means the mounting flange is 3.4"
square.   The wide, heavy, flat rotor inside the 34 frame adds a
considerable amount of inertial "fly wheel" effect, as compared to the
narrow, lower mass rotor in a 23 frame.  In my experience this extra inertia
is very desirable in slow speed, high inertial load applications.  Also, try
underdriving the motor with less than the specified amps for extra
smoothness.  And usually a single dropping resistor setup is smoother
running than a double dropping resistor setup.

But either a 23 or 34 frame is adequate for a 100 lb. balanced scope.  Your
10
RPM figure implies the load is driven through a large gear ratio well within
the capabilities of these motor sizes.

Bill Tondreau