>>>READER-TO-READER QUESTIONS AND ANSWERS
batteries? Would the charge controller
be over tasked? Or, would it be better
to just buy a much bigger unit? I want
scalability dependent on me, not what
the company offers.
#06077 via email
[#3072 - March 2007]
I built an ATMEL-based stepper
controller that sent the step commands
through four power MOSFETs that
connected directly to the coils of a
"four volt, 1.2 amp/phase" unipolar
stepper. It was an inexpensive motor
but it worked great — good torque and
speed and started right back if stalled.
I bought more of the "same" motor — it
had a different label and didn't work at
all — stalled, shook, and no torque. I
got several different motors from different companies, all with a rating of 4-5
volts at 1-1.2 amps (I'm using a switching supply rated five volts, 3. 5 amps)
and they all respond differently. None
work as well as the first (and cheapest)
motor. The MOSFETs have protection
diodes and I tried adding diodes to the
power lines — no change. I bought a
commercial controller with a motor
rated 12 volts, . 4 amps, which turned
out to be a 12-volt version of my
original motor, and it worked very well.
The other motors run better with
that controller, which has variable
current, but still vary in performance.
I can't (nor can the motor company)
figure out why there is so much
variation in output with motors with
the same ratings. Is there some way I
can get the rated power out of these
motors without spending a fortune? Is
there another motor rating or spec that
I can use to judge these motors?
Stepper motors are rated by volts/
amps per phase, phase inductance,
steps per revolution, number of
phases, detent, and holding torque. A
quick way to tell motor performance
at given current and voltage per phase
is to compare the holding torque since
it is the maximum torque the motor
can develop at low speeds and
the highest load. Another important
parameter is winding inductance; the
lower, the better for high speed
performance, since a low inductance
allows winding current to build up
faster. From what you are describing,
you may have an incorrect phasing
sequence or system resonance.
Another possibility is a difference
in phase inductance or not enough
current, or demagnetized (
overheated) motors or winding shorts, just to
give you the starting points. Keep in
mind that torque is roughly proportional to current in the linear region.
Excellent references are Stepping
Motor Basics at EAD Motors (www.
eadmotors.com), and the best one I
have seen is Jones on Stepping Motors
at University of Iowa (www.cs.uiowa.
edu/~jones/step). There are also
other resources at Camtronics, Inc.
(dynamometer testing). If you need to
get into the more sophisticated five-phase systems, there is Oriental
Motors at ( www.orientalmotor.com/
[#2075 - February 2007]
I need an alternator circuit that
will allow me to charge an eight-volt
lead-acid battery in a WWII army
vehicle using a standard automobile
alternators that use an external
regulator, the two blades are at right
angles. As I recall, the blade on the
right connects to the field; the other is
for the idiot light. This circuit is similar
to one I sent in earlier. The LM2594 is
a PWM regulator operating at 150
kHz. The diode is a three amp
Schottky to catch the backswing of
the inductance of the field winding.
The TIP41C is a TO-220 transistor and
should not need a heatsink because it
is used as a switch. The eight volt
battery is actually 8. 4 volts and needs
9. 2 volts to charge.
in turn, controls the alternator output.
This URL provides details and a
schematic for building your own
simple adjustable regulator: www.
A modification to the circuit is
needed to operate with an eight volt
battery. The 6. 2 volt zener diode and
the 3.3K resistor in series with it need
to be changed to a 3. 9 volt zener and
a 2.2K resistor. The 3.3K resistors in
series with the 1K pot also need to
be changed to 2.2K — see the
schematic below which includes the
#1 Newer automotive alternators
have built-in regulators, so you will
have to either remove the internal
regulator or find an older one
that requires an external regulator.
On Delco alternators, you can tell if
the regulator is internal or external
by the connector. If the blades are
parallel, the regulator is internal. On
#2 Most alternators have a built-in
regulator, which must
be removed and
replaced with one
designed for eight
volts to charge your
eight volt battery.
The regulator controls the current to
the field winding of
the alternator which,
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