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CUSTOMER CENTRE >
TECHNICAL SUPPORT > FAQs
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Q.
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Why do step
motors run hot? |
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A. |
Two reasons:
A. Current flows through the motor windings even at
standstill.
B. PWM drive designs tend to make the motor run
hotter. Motor construction, such as lamination
material and riveted rotors, will also affect
heating. |
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Q.
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What
is a safe operating temperature? |
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A. |
Most motors have class B
insulation, which is rated at 130°C. Motor case
temperatures of 90°C should not cause thermal
problems. However, motors should be mounted away
from where operators can come into contact with the
motor case. |
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Q.
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What
can be done to reduce motor heating? |
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A. |
Many drives feature an automatic
reduction of current at standstill set by a command
or jumper link. This reduces current when the motor
is at rest without positional loss. |
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Q.
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How
can end of move “ringing” be reduced? |
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A. |
Friction in the system will help
damp this oscillation. If a start/stop velocity is
used, lowering it will help. The additional features
in the SILENTstep drive almost eliminates this
problem. |
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Q.
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How
can you reduce the chance of resonance? |
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A. |
This is most likely to happen
with full step systems. Adding inertia lowers the
resonant frequency, and friction tends to damp out
resonance. Start/stop velocities higher than the
resonant speed will help. Changing to a half step,
ministep or microstep drive will also minimise any
resonance.
An additional phenomena of stepper systems can be
midrange instability. This varies in severity with
the nature of the load being driven. It generally
sets in at step rates of 1200 to 3000 full
steps/second (6 to 15 revolutions per second). The
oscillation itself is in the 50-150 Hz range, and
often builds in amplitude over a number of cycles,
causing a stall condition within 0.1 to 1 second.
Unlike the fundamental motor resonance, half
stepping and microstepping do not alleviate the
problem.
Our bipolar microstepping drives completely suppress
midrange resonance, by sensing the deviation from
intended position and electronically introducing
viscous damping to counteract the effect. This
'injection' allows the motor torque to be used for
accelerating the load, instead of being wasted on
spurious oscillations. |
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Q.
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What is
micro-stepping? |
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A. |
Conventional bipolar drives
alternate the current direction in one coil at every
step, resulting in a rotor displacement of 1.8
degrees. In microstepping, the coil current is
changed in much smaller increments, increasing in
one coil as it decreases in the other. The rotor
responds by swinging to its new magnetic
equilibrium, which can be a small fraction of a full
step.
Microstepping has two principal benefits:
a. it provides increased resolution without a
sacrifice in top speed
b. it provides smoother low speed motion.
For example, to achieve a resolution of 5 microns
with a full step system requires the use of a screw
with a 1.0mm lead. This places substantial
constraints on top speed. A shaft speed of 40
revolutions per second results in a linear velocity
of only 40mm per second. Use of a divide-by-10
microstepper provides the same 5 micron resolution
with a 10mm leadscrew, but the linear velocity in
this case is now 400mm per second. Alternately, the
resolution can be increased, to 0.5 micron with a
1mm lead, or 1.0 micron with a 2mm lead.
Since stepping motors, by definition, move in
discrete angular increments, operation at low step
rates (especially near the fundamental resonance)
generates noise and vibration. Microstepping
decreases the size of these increments, and
increases their frequency for a given rotation rate.
This results in significantly smoother low speed
operation. A laser interferometer was used to
produce the graphs in Figures 33a-d, which show the
reduction in positional oscillations and velocity
ripple for a positioner at a low (0.05
revolution/second) step rate.
Despite the apparent benefits of microstepping, it
is frequently implemented with excessive degrees of
subdivision. A key attribute of many commercial
systems is “empty resolution”, where the apparent
microstepping resolution can not be achieved. The
torque which any microstep generates is found as
follows:
torque per microstep = motor holding torque x
sine(90 degrees/SDR), where SDR is the step division
ratio.
In the case of 50,000 step/revolution systems, the
SDR is 250, and each microstep produces a torque
change of 0.3 oz-in (with a standard 53 oz-in
motor). Most high repeatability, preloaded leadscrew
systems have torques in the 3-6 oz-in range;
accordingly, 10-20 microsteps must be taken before
the torque builds to a level which results in
leadscrew motion. Direction reversals behave
similarly; what appears to be backlash in the
positioning table is actually excessive
microstepping resolution. High division ratios have
the additional effect of limiting the achievable top
speed, by the inability to produce very high speed
pulse trains (100 revolutions per second at 50,000
steps per revolution requires 5 MHz ramped pulse
trains). |
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Q.
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A move is made in one direction, and then the motor
is commanded to move the same distance but in
the opposite direction. The move ends up short, why? |
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A. |
Several factors could be
influencing the results.The motor has magnetic
hysteresis that is seen on direction changes, this
is of the order of 0.03°. Any mechanical backlash in
the system will create positional errors. A step
pulse occurring before the direction signal is
established will cause an error. |
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Q.
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Why are some motors constructed as eightlead motors? |
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A. |
This allows greater flexibility.
The motor can be run as a six-lead motor with
unipolar drives. With bipolar drives, the windings
can then be connected in either series or parallel
to give different torque-speed characteristics. |
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Q.
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How long can
the motor leads be? |
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A. |
Under normal conditions, 15m for
unipolar drives and 30m for bipolar drives should be
OK. Shielded cables are recommended. |
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