Biomedical Engineering Reference
In-Depth Information
constant; therefore, the motor speed is
n
=
n
20
−
IRK
e
10
−
3
=
6041
−
114
.
6
×
×
42
.
8
×
304
=
4550 rpm
This is reasonably close to the desired value; therefore, the motor will probably be suitable.
One final check is to determine whether the motor will be able to accommodate the heat
rise in the coils. The power dissipated in the motor is
I
2
R
=
(
P
dis
=
10
−
3
2
114
.
6
×
)
×
42
.
8
=
0
.
6W
The heat rise under steady-state conditions is determined by the product of the dissipated power
and the thermal resistance from the rotor coils to the air.
=
R
th
1
+
T
P
dis
(
R
th
2
)
=
.
×
(
+
.
)
0
6
30
8
5
=
23
.
1
◦
C
Hence, the motor winding temperature will be the sum of the ambient plus the temperature
rise.
T
mot
=
T
amb
+
T
=
20
+
23
.
1
1
◦
C
=
43
.
This is well below the 85
◦
C maximum operating temperature for the motor, so the motor
will be capable of running indefinitely under these conditions (MicroMo, 2008a).
3.2.2.6 Powering DC Motors
The simplest form of motor control is openloop. The voltage, or sometimes the current,
is set to some preset value determined by the required output speed given a specific load
torque. The voltage can be provided by a general purpose linear or switched mode regulator
as discussed in Chapter 2, but it is more usual to develop specific electronics for motor
control. Linear regulators are very inefficient, particularly if the motor speed needs to be
controlled over a wide range. Thus, most motor controllers use pulse-width modulation
to provide the required average DC voltage, as shown in Figure 3-18.
PWM is achieved by connecting the DC supply to the motor at regular intervals
to provide the required mean voltage. Because the switching rate is very high, motor
inductance and its rotational inertia ensure that it runs at a constant speed. The relationship
among the switching frequency,
f
(Hz), the period,
T
(s), and the duty cycle,
η
%
,isas
follows:
1
f
T
=
(3.36)
η
%
=
100
t
T
(3.37)
where
t
(s) is the on period as shown in Figure 3-18.
