Page 162 - Servo Motors and Industrial Control Theory

9.4 An Example on Choosing a Servo Motor 157

2
1. The load inertia is 0.1 kg m .
2. Maximum velocity variation ω -105 rad/s. The maximum required settling time
c
is t : = 0.5 s.
a
3. The minimum power requirement is
P: = T .ω c
e
P: = 10. 05 which is appro im tely 1 kW.
a
1
,
x
Therefore, the first choice will be a motor with 1 kW power rating providing it
satisfies the required speed of response.
4(a). Calculation of total settling time for AC motors
From Figs. 9.5, 9.6, 9.7, and 9.8 for a 1 kW power rating motor, the following
settling time can be obtained,
−
3
t : 0.1, t := r 2 = 8, t : = 0.22.10 t : 0.07=
s1
s 2
r1
substituting in equation one yields,
t : = 0.1 0.18 0.22.10 .105+ + − 3 + 0.07.105.0.1
a
t : = 1.66sec
a
Comparing t with the required settling time of 0.5 s, it is obvious that the 1 kW AC
a
motor does not provide the required speed of response.
If the above calculation is repeated for 2, 3 kW or higher power AC motor it
can be found that the 5 kW AC motor with total settling time of 0.49 s provide the
required speed of response. This is because the load inertia is very large for smaller
AC motors.
The dynamic velocity drop for the torque of 10 N m for this motor using eq. (9.4)
and Figs. 9.9 and 9.10 can be calculated as

Dynamic velocity drop = (54*0.1)*10 = 46rpm

4(b). Total settling time from DC motors
From a similar analysis for DC servo motors it is found that that the following mo-
tors will provide a satisfactory performance:

One kilowatt ceramic DC motor controlled with 150 Hz Thyristor Bridge with
total of settling time of 0.45 s and dynamic velocity drop of 39 rpm.
One kilowatt ceramic magnet DC motor with pulse width modulated controller
with total settling time of 0.41 s and dynamic velocity drop of 28 rpm.
One kilowatt rare earth magnet DC motor with total settling time of 0.27 s and
dynamic velocity drop of 19 rpm.
One kilowatt brushless DC motor with total settling time of 0.49 s and dynamic
velocity drop of 16 rpm.