DN26

Design Note 26
Issue 2 January 1996
D.C. Motor Speed Control using the ZTX450
Introduction
Theory
The function of the following circuit is to
improve the load/speed regulation of a
d.c. machine. One of the main reasons
why the speed of a permanent magnet
field d.c. motor varies with load, is that
a voltage drop is current and hence load
dependent. The circuit given in Figure 1
stabilises the speed of the motor by
cancelling out the effect of the motor
rotor resistance using a bridge circuit.
If the bridge and variable resistor are
analysed on their own (Figure 2), a proof
can be derived which shows that the
control voltage (i.e. the speed) is not
directly dependent on the motor current.
(Please refer to Appendix).
D1
The value of ra varies from motor to
motor, so the bridge must be balanced
to suit the motor employed.
1N4000
+6V
R3
22
100
1
U1 ZRA125
M
330
C2
ra
0.1µF
RV1
C3
Q3
Q2
220
Q1
C1
220pF
100µF
Q2,3 - ZTX214C
ZTX450
R1
R2
270
680
0V
Figure 1
DC Motor Speed Control Circuit.
DN26 - 1
Design Note 26
Issue 2 January 1996
+6V
Appendix
Derived Proof
R3
Vx
R1
Vm
M
Vy
ra
R2
Design Note 26
Issue 2 January 1996
Vc
Vz
R3
0V
Figure 2
Bridge Circuit.
This can be done by calculation and
measurement of the motor resistance, or
by adjusting the value of the 100Ω preset
resistor until the motor speed just
becomes unstable and then backing off
a fraction.
Figure 3
Current/Speed Characteristics of a Test
Motor.
The current/speed and torque/speed
characteristics of a test motor have been
plotted in Figures 3 and 4, with and
without the control circuit, to show the
improvement in speed stability.
Variations in motor speed due to supply
changes are also greatly reduced by the
circuit. On the test motor a speed change
o f ± 2% was recorded for a supply
change of ± 20%.
Larger motors can be accomodated by
using higher current capable transistors,
such as the ZTX689B - a high gain, 2A DC
rated device, or the ZTX869 - a high gain
5A DC rated transistor.
Figure 4
Torque/speed Characteristics of a Test
Motor.
DN26 - 2
DN26 - 3
Design Note 26
Issue 2 January 1996
+6V
Appendix
Derived Proof
R3
Vx
R1
Vm
M
Vy
ra
R2
Design Note 26
Issue 2 January 1996
Vc
Vz
R3
0V
Figure 2
Bridge Circuit.
This can be done by calculation and
measurement of the motor resistance, or
by adjusting the value of the 100Ω preset
resistor until the motor speed just
becomes unstable and then backing off
a fraction.
Figure 3
Current/Speed Characteristics of a Test
Motor.
The current/speed and torque/speed
characteristics of a test motor have been
plotted in Figures 3 and 4, with and
without the control circuit, to show the
improvement in speed stability.
Variations in motor speed due to supply
changes are also greatly reduced by the
circuit. On the test motor a speed change
o f ± 2% was recorded for a supply
change of ± 20%.
Larger motors can be accomodated by
using higher current capable transistors,
such as the ZTX689B - a high gain, 2A DC
rated device, or the ZTX869 - a high gain
5A DC rated transistor.
Figure 4
Torque/speed Characteristics of a Test
Motor.
DN26 - 2
DN26 - 3
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