# Tech Briefs Magazine - August 2021 - MD-11

```Bipolar and Unipolar Drives
for Stepper Motors:
A Comparison
A
stepper motor is a type of brushless DC motor comprised
of connected coils called " phases. " These electromechanical
devices are generally driven in open loop
without a feedback sensor, with current applied on the phases
without knowing the rotor position. The rotor moves into
alignment by means of the stator's magnetic flux, generated by
the current flowing in the phases. At each pulse, the current
can be supplied to the next phase, allowing incremental rotational
movements, or steps.
There are two methods to supply current in the coils: bipolar
between bipolar and unipolar motors, their driving methods,
Figure 1 shows a four-step, permanent magnet stepper
motor. The rotor is made with a one pole-pair magnet and the
stator consists of two phases: Phase A and Phase B. In the
unipolar method, the current always flows in the same direction.
Each coil is dedicated to one current direction, so either
coil A+ or A- is powered; the A+ or A- coils are never powered
together. In the bipolar method, the current can flow in both
directions in all coils. Phases A+ and A- are powered together.
A bipolar motor requires one coil minimum per phase and a
unipolar motor requires two coils minimum per phase. Here's
a detailed look at both options:
Unipolar Motor Construction
In the unipolar configuration, each motor phase is composed
of two coil windings. With a two-phase motor composed
of Phases A and B, the motor has four coil windings, as shown
in Figure 2.
* Phase A is composed of A+, A*
Phase B is composed of B+, BA+
AB+
BU
Q1
Q2
Q1
Q2
Q3 Q4
Q3
Phase A
Q4
Phase B
A+ AB+
BPhase
A
Phase B
Figure 2. Unipolar motor and drive construction.
Phase B+
Phase AS
N
APhase
A+
A+
A+
The current in each coil can only flow in a single direction,
making it unipolar. In voltage drive, the control system is easy,
with only one switch, or transistor, per coil. When the transistor
is closed, the coil is powered. To commutate the motor, the
transistors are alternately closed and opened.
In Figure 3, transistors Q1 and Q2 cannot be closed at the
same time. To power Phase A, you must close either transistor
Q1 or Q2, depending on the direction you need the current to
work. With unipolar control, only half of the phase is powered
at a time, so the current only uses half of the copper volume.
With voltage drives, serial resistances are typically applied to
decrease the electrical time constant. This scenario will be
explained later in the article.
APhase
BQ1
Q2
Q1
Q2
Figure
1. A four-step stepper motor.
Figure 3. Unipolar drive electronics. (Left) A+ Q1 closed and (right) Q2 opened.
Motion Design, August 2021
Cov
ToC
11
Background image: Edilus/Shutterstock.com
YEL
RED
ORN
YEL
ORN
RED
BRN
GRN
BLK
YEL
RED
ORN
```
http://www.abpi.net/ntbpdfclicks/l.php?202108MDNAV

# Tech Briefs Magazine - August 2021

Tech Briefs Magazine - August 2021 - Intro
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