Design Note 26
Issue 2 January 1996
D.C. Motor Speed Control using the ZTX450
Introduction
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.
D1
1N4000
Theory
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).
The value of r
a
varies from motor to
motor, so the bridge must be balanced
to suit the motor employed.
+6V
100
1
U1 ZRA125
M
R3
22
C2
RV1
Q3
Q2
220
C3
100µF
Q2,3 - ZTX214C
330
ra
0.1µF
C1
220pF
R1
270
R2
680
Q1
ZTX450
0V
Figure 1
DC Motor Speed Control Circuit.
DN26 - 1
Design Note 26
Issue 2 January 1996
+6V
Vx
R1
M
R3
Vm
Vy
Vc
Vz
0V
ra
R2
R3
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.
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 3
Current/Speed Characteristics of a Test
Motor.
Figure 4
Torque/speed Characteristics of a Test
Motor.
DN26 - 2
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