Page 156 - Servo Motors and Industrial Control Theory -

9.3 Comparison of Results and Design Procedure 151

Fig. 9.6 Dynamic settling 1.0
time of servo motors
without any load inertia
0.8

3
0.6 1a
tr1 (sec)

0.4 1b
2
1c
0.2
5
6
4
0
0 2 4 6 8 10
Power (kw)

The stepping servo motors have the largest settling time at all power ranges. The
stepping motors operating on the principle of magnetic pull inherently oscillatory
and the controller must take this into account.
The approximate saturated settling time due to additional load inertia is similar
to that for motor inertia, that is,
I
t : = 1 (9.3)
2 s
T
m
The above equation can be calculated for a unit load inertia and a unit velocity
change. This gives the eq. (9.3) a generality for all types of servo motors and can be
used in eq. (9.1) to obtain the total settling time for a given application.
Figure 9.7 shows this settling for a unit load inertia and for a unit of velocity
change.
In Fig. 9.7 the saturated settling time of electrical AC induction, stepping and
rare earth magnet DC motors are very large at low power rating. This means that
a unit load inertia is very large compared to the inertia of the rotor. Hydraulic mo-
tor has an average saturated settling time slightly larger than rare earth magnet and
ceramic magnet DC servo motors. At high power range electrical motors have the
lowest saturated settling time. This means electrical motors must be matched to the
load inertia referred to motor shaft.
The dynamic settling time due to the load inertia is greatly influenced by the
maximum torque capability of the motor. A series of simulations were carried out
to obtain the dynamic settling time for all types of servo motors at different load