DC Motor Drives
LM 317 Voltage Regulator
DC Motor Drives
Current Rating:
LM317T – 1.5 A
(with heat sink)
• The current used to drive the DC motor typically
come from one of these sources:
Fixed voltage:
– a ____________ (voltage changes with discharge)
– a voltage ___________ (from rectified AC or battery)
Adjustable voltage:
LM317 ~ $0.75
+
+12 to +24 VDC
R1 = 260Ω
R2 = 1kΩ
– a linear power _______________ (or power op-amp),
– a pulse-width-modulated (______) current source, or
– a silicon-controlled-rectifier (________) modulated AC
source.
-
Motor Leads
1 kΩ potentiometer
Linear Power Op-Amp
• Linear power transistors and power op-amps
are generally limited to low power (<100 W)
applications
27.1
Pulse-Width Modulation
+
+
Va
Vmax
-
From: Apex Microtechnology, Tuscon, AZ
Pulse-Width Modulation
+
+
Pulse-Width Modulation
+
+
Va
Vmax
-
Base voltage
turned “ON”
Va
-
Transistor “switch” turns ON,
allows current to flow
Vmax
Base voltage
turned “OFF”
Transistor “switch” turns OFF,
no current flows
-
DC Motor Drives
100
Average Va
is 80% of
Vmax
80
60
40
20
0
0.00008 sec
PWM - 60% Duty Cycle
Armature Voltage, Va
Armature Voltage, Va
PWM - 80% Duty Cycle
27.2
100
80
40
20
0
0.00006 sec
0.00010 sec
0.00010 sec
100
80
60
Average Va
is 40% of
Vmax
40
20
0
0.00004 sec
PWM - 20% Duty Cycle
Armature Voltage, Va
PWM - 40% Duty Cycle
Armature Voltage, Va
Average Va
is 60% of
Vmax
60
100
80
60
40
Average Va
is 20% of
Vmax
20
0
0.00002 sec
0.00010 sec
PWM Motor Drive
• If the time period T is ________ compared
to the _________________ of the system,
the motor response will be the same
– PWM switching frequencies in the 10 kHz (or
higher) range are frequently used.
• Relatively high drive efficiency (up to 80%)
– inefficiency creates heat in the amplifier that
must be dissipated!
0.00010 sec
Low Cost, Pulse Width Modulator for
12 to 24 volt Applications
U2: 7812 Voltage Regulator
U1: LM324 Quad Op-Amp
R1-R4: 100kΩ
R5: 47kΩ
R6-R7: 4.7kΩ
C1: 0.001 to 0.01 µF
Q1: IRF521 FET (3 amps)
Q1: IRFZ34N FET (10 amps
with heat sink)
DC Motor Drives
HalfHalf-Wave Rectifier
27.3
FullFull-Wave Rectifier
+
+
Va
VAC
Va
-
-
VAC
0
Silicon Controlled Rectifier
SiliconSilicon-Controlled Rectifier Drive
Gate
+
+
Va
VAC
Gate
Va
Gate
-
0
VAC
Delay time is
adjustable by
gate signal
Form Factor
Form factor (FF) is a measure of
departure from pure direct current (DC)
halfhalf-wave rectified AC
unfiltered FF = 1.61.6-2.0, filtered FF = 1.11.1-1.5
-
fullfull-wave rectified AC
unfiltered FF = 1.11.1-1.6, filtered FF = 1.01.0-1.1
DC Motor Current Drives
Most higher power application use
either the PWM or SCR drive methods
SCR drives are rated at 90 VDC (for
use with 115115-120 VAC) or 180 VDC (for
use with 230 VAC)