Running motor at rated electrical power, and measuring its torque
Testing the motor for its max torque gave the following figures (note
some of these are out of spec as the motor max power is about 10.2w).
Motor type: "Step-Syn" 23 frame, 72oz/in, each coil rated unipolar 5.1v, 1.0A, (5.1w)
0.71A 7.62v 10.82w 5.95kg/cm (83.9oz/in) two full windings bipolar
0.56A 5.96w 6.73w 5.15kg/cm (72.6oz/in) two full windings bipolar
So running this motor in bipolar drive with the same rated total
power gave a holding torque of about 83oz/in. The second row of figures
was to match the unipolar torque of about 72oz/in. You can see the
bipolar setup was more power efficient, needing less total watts to
match the (rated) unipolar holding torque.
Further testing for speed and shaft power showed this old motor did
not perform well with a bipolar drive due to it's high inductance,
and I changed the drivers to unipolar to get the most from these
motors. This is a common scenario when using older motors as these motors
were designed and manufactured to give peak performance with
Measuring microstep phase angle
This type of testing is normally the realm of laboratories, but again can
be done with cheap junk-box equipment. If you are designing a microstepping
setup this can be useful to "dial in" the motor and driver to give
more accurate microstep sizing.
My first go at measuring the phase angle was the above horzontal setup. I glued a
thin wooden meat skewer to the end of the ruler and set up a box with a
piece of cardboard under the point of the skewer. Total radius was about 45cm,
but really was not enough to give the accuracy needed. I also decided it
would be handy to be able to measure holding torque at each microstep
position, so the setup was modified to be vertical and with a MUCH
longer pointer radius.
Using a full stick of hot melt glue this time I glued on a 2 metre (6 foot)
length of alloy bar 25mm x 3mm (1" x 1/8"). The huge length of alloy
bar looked delicate but did not give any problems after I glued the motor
down securely. The alloy bar was also balanced carefully before gluing.
The tip of the pointer was exactly 1 metre from the motor radius. Showing
how small microsteps are, the distance from a fullstep to a halfstep
was only about 15mm (5/8")! It is easy to understand why stepper laboratories
use very expensive precision optical encoders!
During measurement I tapped the motor with a screwdriver between microstep
adjustments so it would "settle out" the shaft bearing friction. This seemed
to work pretty well.
The electrical equipment was very simple and consisted of 2 adjustable
bench power supplies and 4 multimeters, to measure the voltage and current
through both motor coils.
Again a small cup hung from the lever was used to measure torque, this
was placed at 50cm radius, half way along the lever and still above the scales.
The results of the testing are shown below in the standard form of a
phase angle quadrant. First the torque of the motor at the halfstep
position was measured at rated coil current. This was at coilA = 100% current
and coilB = 0% current. This is the black cross at the top of the quadrant.
Then mechanical angle was found by pointer position for each microstep,
and the 2 coil currents were adjusted to give the correct mechanical phase
angle AND also the same holding torque as the halfstep position.
Note that the actual current values for the mid-step positions were drastically
lower than the theoretical current values shown as grey circles. This
demonstrates the problem with using theoretical mathematics to try and predict
real world situations. In the real world the motor poles are constructed
in a way that optimises the magnetic field at the fullstep position,
giving a disproportionally high holding torque around this position and maximising
torque per motor frame size. This is also the torque value that the manufacturers
The current values for all step positons are shown as percentages. The top cross
is the halfstep with current 100:0. The middle cross (at 45 degrees) is the fullstep
with current at 57:57. Between them are the quarter step and 8th step positions.
After testing this motor and a number of others afterward I did discover
2 points common to all the motors that may be of use;