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Stepper Motors An Overview

Aliasgar Kutiyanawala
Utah State University

Aliasgar Kutiyanawal

Whats a Stepper Motor?

A stepper motor is a
motor that, as the
name suggests, moves
in steps. Stepping
motors are known in
German as
Schrittmotoren, in
French as moteurs pas
pas, and in Spanish
as motor paso paso.
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Compare With a Servo

Stepper motors are similar to servo motors as we
can perform position control with both.
However, Servo motors require some form of
analog feedback whereas stepper motors are often
open loop.
Stepping motors can be used in simple open-loop
control systems; these are generally adequate for
systems that operate at low accelerations with static
loads, but closed loop control may be essential for
high accelerations.
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Compare With a Servo (Contd.)

For high accelerations with variable loads,
all rotor information is lost, and we require
closed loop for accurate control
Servo motors are not subject to this problem

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Types of Stepper Motors

Permanent Magnet
Employ permanent magnet
Low speed, relatively high torque

Variable Reluctance
Does not have permanent magnet
Low torque

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Types of Stepper Motors

multi-toothed stator poles and a permanent
magnet rotor
High static and dynamic torque

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Variable Reluctance Motors

The variable reluctance motor in the
illustration has four "stator pole sets"
(A, B, C,), set 15 degrees apart.
Current applied to pole A through the
motor winding causes a magnetic
attraction that aligns the rotor (tooth)
to pole A. Energizing stator pole B
causes the rotor to rotate 15 degrees in
alignment with pole B. This process
will continue with pole C and back to
A in a clockwise direction. Reversing
the procedure (C to A) would result in
a counterclockwise rotation.

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Permanent Magnet Motors

Unlike the other stepping motors, the
PM motor rotor has no teeth and is
designed to be magnetized at a right
angle to it's axis. The above
illustration shows a simple, 90 degree
PM motor with four phases (A-D).
Applying current to each phase in
sequence will cause the rotor to rotate
by adjusting to the changing magnetic
fields. Although it operates at fairly
low speed the PM motor has a
relatively high torque characteristic.

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Hybrid Motors
They are constructed with multitoothed stator poles and a permanent
magnet rotor. Standard hybrid motors
have 200 rotor teeth and rotate at 1.80
step angles. Other hybrid motors are
available in 0.9and 3.6 step angle
configurations. Because they exhibit
high static and dynamic torque and
run at very high step rates, hybrid
motors are used in a wide variety of
industrial applications.

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Types of Windings

Unipolar motors
Bipolar motors
Bifilar motors
Multiphase motors

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Unipolar Motors
Unipolar stepping motors, both
Permanent magnet and hybrid
stepping motors with 5 or 6 wires are
usually wired as shown in the figure,
with a center tap on each of two
windings. In use, the center taps of
the windings are typically wired to
the positive supply, and the two ends
of each winding are alternately
grounded to reverse the direction of
the field provided by that winding. As
shown in the figure, the current
flowing from the center tap of
winding 1 to terminal a causes the top
stator pole to be a north pole while
the bottom stator pole is a south pole.

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Unipolar Motors
This attracts the rotor into the
position shown. If the power to
winding 1 is removed and winding 2
is energized, the rotor will turn 30
degrees, or one step.
To rotate the motor continuously, we
just apply power to the two windings
in sequence. Assuming positive logic,
where a 1 means turning on the
current through a motor winding, the
following two control sequences will
spin the motor illustrated in the

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Bipolar Motors
The bipolar motor is similar
inc construction to the
unipolar motor except that it
does not have any center
taps.The advantage gained in
the simplicity of construction
of the motor is lost in the
control circuitry as that
becomes more complex. A Hbridge is required for the
control of the motor.

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Bifilar Motors
Bifilar windings on a
stepping motor are
applied to the same rotor
and stator geometry as a
bipolar motor, but
instead of winding each
coil in the stator with a
single wire, two wires
are wound in parallel
with each other.
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Multiphase Motors
This permanent magnet
stepping motor is wired
with all windings of the
motor in a cyclic series,
with one tap between
each pair of windings in
the cycle. The most
common designs in this
category use 3-phase and
5-phase wiring.
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What Is Micro Stepping?

Micro stepping is a way of moving the stator flux of a stepper

more smoothly than in full- or half-step drive modes. This
results in less vibration, and makes noiseless stepping
possible down to 0 Hz.
It also makes smaller step angles and better positioning
In many applications microstepping can increase system
performance, and lower system complexity and cost,
compared to full- and half-step driving techniques.
Microstepping can be used to solve noise and resonance
problems, and to increase step accuracy and resolution.

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Controlling A Stepper Motor

H Bridges
UC3717A from TI and other similar devices
from vendors

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Jones on Stepper Motors
Advanced Micro Systems

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