Tech Briefs Magazine - August 2021 - MD-13

For the same joule losses dissipated, the bipolar motor can
produce √2 (≈40%) more torque than the unipolar drive. And
for the same electrical power, the bipolar drive gets better
results than the unipolar drive.
Dynamic Mode
The preceding demonstration shows that when the same power
is dissipated, the bipolar motor can deliver 40% more torque.
However, at high speed in voltage drive, the unipolar motor can
deliver higher torque than the bipolar motor because the current
can flow faster in the coil. Figure 7 provides an example.
Pull-out torque vs Speed
5V, full steps, voltage drive
12
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10
8
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6
Torque 26M048D1U
UNIPOLAR
Torque 26M048D1B
BIPOLAR
4
2
0 100 200 300 400
500 600
Speed (RPM)
Figure 7. Pull-out torque versus speed.
Wires and Motor Connection
A bipolar motor generally has four wires whereas a unipolar
motor has eight wires if the middle point is not connected
(Figure 8).
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Voltage drive
Current drive
Amount of copper
powered
Torque for same joule
losses
700 800 900
1000
A serial assembly has a resistance four times higher than a
parallel assembly. A serial connection requires twice the current
and half the voltage of a parallel connection. A serial or
parallel connection will match the power supply.
Unipolar
Possible with 4 transistors
Complex
Half
Less torque performances for the
same joule losses
Speed in voltage drive Unipolar may present lower
torque at low speed and higher
torque at high speed
Speed in current drive Current drive is complex, not really
used
Coil connection
Number of wires
Variability
No choice
6 or 8 wires
Can be converted into bipolar
mode if 8 wires
Bipolar
Possible with 8 transistors
Current drive is easy if there is a
chip available to do it; possibility
to drive in micro steps
Full
40% more torque performances
for the same joule losses
More torque at low speed, and
speed limited by the highest
torque constant
Full advantage of current drive
Parallel or serial
4 or 8 wires
Can be converted into unipolar
mode if 8 wires
Figure 10. Comparison of unipolar versus bipolar assemblies.
Make Your Comparison
Figure 8. Wire connections.
Eight Wires
If the unipolar motor has eight wires, it can be converted
into a bipolar version by connecting the half-phases. The coils
can connect in series or parallel. Both options have the same
motor regulation (R/k2) and the same torque performances
for the same electrical power.
Motion Design, August 2021
Cov
ToC
The table in Figure 10 presents the advantages of unipolar
and bipolar assemblies. Unipolar control was popular in the
past but bipolar in current drive has become more prevalent
thanks to cost improvements. For voltage drive, unipolar control
is still a cost-effective option.
This article was written by Clemence Muron, Applications Engineer
at Portescap, West Chester, PA. For more information, visit http://
info.hotims.com/79416-281.
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Figure 9. Differences between series and parallel coil connections.
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Torque (mNm)
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Tech Briefs Magazine - August 2021

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