DC motor speed is often regulated with a closed-loop speed
controller using tachometer feedback (Figure 1). It is pos-
sible, however, to control dc motor speed without tachom-
eter feedback.
Figure 2 shows an open-loop type speed control circuit that
drives a dc motor at a speed proportional to a control
voltage, V
IN
. It does this by exploiting a basic characteristic
of dc motors鈥攊ts speed-dependent reverse EMF voltage.
The motor is modeled as a series winding resistance, R
M
,
and a reverse EMF generator. The op amp circuitry provides
a negative resistance drive equal to the winding resistance.
This causes the reverse EMF to be proportional to the input
control voltage. Motor speed and direction are determined
by the magnitude and polarity of the control voltage.
Operation can be visualized by first imagining a perfect
frictionless motor with no mechanical load. An input volt-
age provides a proportional op amp output voltage, V
O
.
Without a mechanical load, the motor draws no current
because the reverse EMF exactly matches motor drive
voltage.
When a mechanical load is applied, current flows through
the motor and the sense resistor, R
S
. This creates a voltage,
V
S
, that is summed with the input control signal at the non-
inverting op amp input. This positive feedback increases the
drive voltage applied to the motor, maintaining constant
speed. Proper speed control is achieved by setting the gain
at the non-inverting input so that it compensates for the
voltage drop in the series winding resistance and the sense
resistor.
Circuit values are calculated with the following design pro-
cedure. Example values correspond to Figure 1.
1. Determine gain. The input control voltage must be capable
of producing the needed output voltage swing to drive the
motor. In the example circuit, a
鹵2V
input must deliver
鹵20V
to the motor with no mechanical load. R
1
and R
2
are
chosen to provide the required gain of 鈥?0. G = 鈥揜
2
/R
1
.
2. Determine the winding resistance, R
M
, by measuring with
an ohmmeter. Use the average of several readings taken at
different rotor positions.
3. Choose the value of the sense resistor, R
S
. Use a convenient
value that is less than R
M
R
1
/R
2
. This assures that a reason-
able value of R
3
can be used to adjust the speed regulation
behavior. In the example (12鈩?(1k鈩?/10k鈩?= 1.2鈩? a
standard value of 1鈩?is chosen.
4. Calculate the nominal value of R
3
:
R
3
=
10 k
鈩?/div>
R
2
=
=
5k
鈩?/div>
R
M
/ R
S
鈭?/div>
R
2
/ R
1
12
鈩?/div>
/ 1
鈩?鈭?/div>
10 k
鈩?/div>
/ 1k
鈩?/div>
+25V
(1)
R
1
1k鈩?/div>
V
IN
R
2
10k鈩?/div>
R
3
5k鈩?/div>
OPA548
R
M
=
12鈩?/div>
R
M
dc
Motor
EMF
鈥?5V
(2)
R
CL
(1)
Control
Voltage
dc
Motor
M
T
Tachometer
1鈩?/div>
R
S
NOTES: (1) 4.7碌F tantalum recommended. (2) Current limit
set resistor 14.7k鈩?= 2.5A.
FIGURE 1. Tachometer-Feedback Speed Controller.
FIGURE 2. Open-Loop Motor Speed Controller.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user鈥檚 own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
1
2
