Stepper Motor Applications and Advantages Disadvantages
• Lowest-cost solution
• A stepper motor will always offer the
cheapest solution. If a stepper will do the job, use it.
Rugged and Reliable:
Steppers are mechanically very simple
and apart from the bearings (like in servos) there is nothing to deteriorate or
fail.
No Maintenance:
There are no brushes or other wearing
parts requiring periodic checking or replacement.
Industry-standard ranges (Nema or metric):
Steppers are produced to standard flange
and shaft sizes so finding a second source is not a problem.
Few environmental constraints:
A stepper may be used in just about any
environment, including in a vacuum. Special magnets may be needed if there are
very large magnetic fields around, e.g. in evaporation chambers.
Watch heat dissipation in a vacuum
(there is no convection cooling).
Inherently failsafe:
There are no conceivable faults within
the drive to cause the motor to run away. Since current must be continually
switched for continuous rotation most faults cause the motor to stop rotating.
A brush motor is internally-commutated and can run away if continuous current
is applied. A brushless servo relies on the feedback signal. If the signal is
damaged, or absent the motor will run away.
Not easily demagnetized by excessive current:
Owing to the perpendicular planes of the
permanent magnet and alternating flux paths stepper motors will more often melt
the windings before demagnetizing the permanent magnet, as would happen in a
brushed motor.
Inherently stable at standstill:
With DC flowing in the winding, the rotor
will remain completely stationary. There is no tendency to jitter around an
encoder or resolver position. This is useful in applications using vision
systems.
Can be stalled indefinitely without damage:
There is no increase in motor current as
a result of a stall or jam as in a servo system. There is no risk of
overdriving a stepper system due to large loads, or high speeds.
High continuous torque in relation to size:
Compared with brushed servos of the same
size, a stepper can produce greater continuous torque at low speeds.
Only 4 leads required:
This minimizes the installed cost,
particularly important in applications where connections are expensive (e.g.
vacuum chambers).
Stepper Motor Drawbacks:
Ringing, resonance and poor low speed
smoothness:
These are criticisms generally leveled
at full-step drives. These problems may be almost wholly overcome by the use of
a higher-resolution drive.
Undetected position loss in open loop:
This should only occur under overload
conditions and in many applications it causes few problems. When position lost
must not go undetected, a check encoder may be fitted which then results in a
very secure system. The encoder is not needed for positioning, only for
confirmation. If a positioning encoder is desired a servo system should be
used.
Uses full current at standstill:
Since current is needed to produce
holding torque, this increases motor heating at standstill.
Noisy at high speeds:
The 50-pole rotor has a magnetic
frequency of 2.5 kHz at 3000 rpm. Magneto-striction causes a correspondingly
high-pitched sound.
Excessive iron losses at high speed:
Again due to the high pole count,
hysteresis and eddy current losses are higher than in a servo. A stepper is
therefore not recommended for continuous operation at speeds approximately
above 2000 rpm.
