Download How step-motor performance

How step-motor performance
varies with type of control
tepping motor-based systems
offer an economical method of
motion control. Different types of drives
(or
controls)
expand
step-motor
performance for added versatility.
Relative performance of a typical
or standard hybrid step motor under
various drive conditions is shown in the
diagram. (Popular in industry, hybrid
step motors combine characteristics of
permanent magnet and variablereluctance step-motor types.)
For
the
greatest
wiring
flexibility, the motor's eight leads are
connected to the ends if the four coils
that make up the phase windings. The
coils can be connected in series or in
parallel to operate with a bipolar drive,
or in series with a center tap run from a
unipolar drive. Relationships between
motor parameters when using these
connections are summarized in the
Online Extra item.
Use half of all the copper
Unipolar drives (diagram curves 3, 4, 5)
supply current to a given motor phase in
only one direction. These simpler drives
use only half the copper available in any
phase at a given time. The result is less
torque at low speeds than with the same
motor operated from a bipolar drive of
the same input power.
Bipolar drives (curves 1,2,6)
supply either positive or negative current
to a given motor phase.
Drive
electronics are more complex here, but
using all the copper in the phase
windings results in about 40% more
torque at low speeds than from a
unipolar drive with the same motor.
Unipolar or bipolar drives can be
further divided into voltage (L/R) type
and constant current type (which
includes a chopper or PWM).
Voltage (L/R) drives (curves
4,5,6) apply rated voltage to the motor
phases and rely on winding resistance to
limit current. The least costly drives in
this class have the poorest speed
performance (curves 5,6). Adding a
power resistor in series with each
winding, and using a higher supply
voltage to maintain rated voltage at the
motor.
Can
enhance
high-speed
performance somewhat (curve 4). This
effectively lowers the motor's L/R time
constant, allowing more rapid current
build up. Due to large power losses in
the resistors, this approach is practical
only for resistor values up to a few times
the motor resistance.
Performance of a bipolar voltage
drive is the same whether the windings
are connected in series or parallel,
because the L/R time constant is the
same. But the unipolar connection has
higher speed capability than either
bipolar connection because the unipolar
L/R time constant has half the value.
Constant current drives (curves
1,2,3) use a much higher supply voltage
than the rated voltage of the motor
(typically 10 times higher or more).
They also use current-sensing circuitry
and apply pulse-width modulation
(PWM) to the supply voltage to maintain
the motor's rated current and keep it
from overheating. As noted earlier, a
higher supply voltage allows the motor
to run at higher speeds. Advances in
integrated circuits and power-switching
transistors have made this a very popular
type of drive, even though it is relatively
complex.
The bipolar constant current
drive is the most popular drive type
today today for high-performance
applications. When the same supply
voltage is used a bipolar parallel
connection (curve 1) will result in about
twice the speed of a bipolar series
connection (curve 2). However, the
parallel connection requires twice the
current of the series connection.
Unipolar constant current drives are less
common and their low-speed torque
performance is lower compared to
bipolar drives (curve 3).
Understanding
the
relative
performance of a step motor with
various drive types will aid users in
selecting the most suitable control for a
given application, or provide insight on
how to increase performance of a
stepping
motor
system.