Biomedical Engineering Reference
In-Depth Information
FIGURE 3-15
Maxon 2 W motor
characteristics.
power, and rotation speed is shown in Figure 3-15. Note that the actual output power for
this motor reaches a maximum of just over 1 watt when
ω
=
ω
n
/
2 and
τ
=
τ
s
/
2as
predicted.
The governing equation for the power curve can be obtained by substituting equa-
tion (3.22) or (3.23) into equation (3.24).
P
mot
(ω)
=−
τ
s
2
ω
n
ω
+
τ
s
ω
(3.25)
P
mot
(τ )
=−
ω
n
2
τ
s
τ
+
ω
n
τ
(3.26)
The current drawn by the motor is proportional to the output torque. The motor
specifications include the no-load current,
I
o
=
4
.
91 mA, and the starting current,
I
a
=
281 mA.
The no-load current is equivalent to the friction torque
τ
f
τ
f
=
K
t
I
o
(3.27)
where
K
m
(Nm/A) is the torque constant described earlier.
Motors develop the highest torque when starting, and this is much larger than the torque
available when running. Therefore, the current required on startup is also a maximum
τ
s
=
K
m
I
a
(3.28)
From the motor specifications the torque constant
K
m
=
19
.
6 mNm/A. This is in fact just
the ratio of the stall torque and the starting current,
K
m
=
τ
s
/
I
a
.
