Tech Briefs Magazine - August 2021 - MD-12

Bipolar Motor Construction
Bipolar motors need only one coil winding per phase and
the current can flow in both directions per coil. Eight transistors
with two H-bridges are required to control bipolar motors,
as shown in Figure 4.
+V
Q1
Q3
Q1
Q
Q2
Q
Q2
Q5
Q4
Q7
+V
Q3
Q6
Q8
Q4
Q4
U
Q1
Q2
A
Q3
Q4
Phase A
Q5
Q6
B
Q7
Q8
Phase B
the torque is proportional to the current, the motor will deliver
less torque. At high speed, it results in a higher torque. Even if
some joule power is dissipated in the external resistance, the
motor will be able to deliver more torque thanks to the lower
electrical time constant. This allows the current to rise more
quickly in the coil. (Note: With an increase in supply voltage,
you can compensate for the lower current; however, the overall
energy efficiency will be lower. The torque is improved at high
speed and maintained at low speed.)
Holding Torque
Holding torque is the maximum torque that the motor can
hold. Holding torque is proportional to the torque constant
and the current in the phase.
Phase A
Phase B
Figure 4. Bipolar motor and drive construction.
In Figure 5, the transistors are alternately closed and opened
to provide commutation. Bipolar drives have the advantage of
using all the copper per phase.These bipolar drives are used
either in the motor's voltage drive or in current source. For the
current source, the current in each phase is controlled with a
pulse width modulation (PWM). Two techniques are used for
PWM: slow decay or fast decay, depending on whether the current
is supposed to decrease slowly or quickly through the
motor phase during the PWM " off " time.
A
A
Tholding max = k * i
where
* Tholding max is the holding torque (Nm)
* k is the torque constant (Nm/A)
* i is the current in the phase (A)
Higher torque can be generated by increasing the number
of coil turns or by increasing the current flow. Increasing the
current caused extra heat due to the joule loss dissipation
(Pjoule = R * i2 ). The current supply is limited by the thermal
capability of the coil. The coil temperature can generally reach
the coil's maximum admissible temperature - typically 100 °C
or 150 °C, depending on the motor type.
Let's look at the joule losses in both combinations (Figure 6
below), considering one phase ON:
+V
Q1
Q2
Q1
+V
Q2
Q3
Q4
Q3
Q4

Figure 5. Bipolar drive electronics. (Left) A+, Q2, Q3 closed; Q1, Q4 opened.
(Right) A-, Q1, Q4 closed; Q2, Q3 opened.
Advantages and Limitations
Voltage drive. A simple circuit with four transistors provides
cost-effective unipolar control. A voltage drive for bipolar
motors requires two H-bridges (eight transistors).
Current drive. A bipolar mode is preferable for current drives
because unipolar technology requires more complex electronics
to achieve less motor performance.
Voltage drive precaution. Due to the inductance effect, the current
needs some time to rise in the coil. For either unipolar or
bipolar drives, you can add a serial resistance to decrease the
electrical time constant (L/R). By adding an external resis -
tance, the current decreases (i = U/(R+r)).
In summary, adding resistance for the same power supplied
results in a lower torque at low speed. The current is lower due
to the joule power dissipated in the external resistance. Because
12
Cov
ToC
Taking into account that each individual coil has its own resistance,
inductance, and torque constant, and if the joule losses
are the same for both cases, Pjoule bi = Pjoule uni = P0, we will have:








  
Figure 6. Joule loss calculations.
Motion Design, August 2021
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