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Pancake Resolvers >> Brush Type Motors >>
Brushless DC Motors
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S1
+V S
S2
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S3
Direction Control
Input
PWM INPUT FROM
VELOCITY CONTROL CIRCUIT
portional to the input control voltage.The
same control scheme can be applied to a
three phase brushless DC torque motor as
shown in Figure 10. The Hall devices are
mechanically displaced 120 electrical degrees
from each other and are aligned with the
BEMF waveform of each phase.The control
equations are as previously derived for the
two phase system, except the output torque
is:
T = VC KC (Sin2 + Sin2 (0 + 120) +
Sin2 ( - 240))
T = 1.5 VC KC and, as before, the output torque is directly
proportional to the input control voltage.
Square Wave Drive
A square wave drive system provides the
most efficient utilization of the control elec-
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or
Custom
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tronics, and yields the lowest system cost.
Power dissipation in the output stage is
minimized by operating the power switch
devices in the complete OFF or fully ON
stage.The output stage can be interfaced
directly with the commutation logic circuit,
and the whole system can be digital.
This allows the use of lower cost digital
integrated circuits to decode the position
sensor output and to sequence the drive
circuit power semiconductors.
Figure 11 shows a simplified drive circuit
for bi-directional control of a two phase
brushless DC motor.The timing and excitation
waveform for this motor were shown
in Figure 7.The rotor position sensors are
Hall effect switches and their outputs are
processed by Exclusive OR gates. This feature
allows the direction of rotation to be
reversed. A logic “1” on the direction con-
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Q1
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Q2 Q3
Q4 Q5 Q6
CURRENT
SENSE
OUTPUT
Figure-13. Functional drive circuit for bi-directional control of a three phase brushless DC motor.
0B
0A
0C
Commutating Brushless DC Motors