Page 158 - Servo Motors and Industrial Control Theory -

9.3 Comparison of Results and Design Procedure 153

Fig. 9.8 Dynamic settling 16
time of servo motors due
to a unit load inertia 15 4
8

t2 (sec / kg-m 2 ) 6 2 3

4
1a

2
1b
K
6 5
0
0 2 4 6 8 10
Power (kw)

dynamic velocity drop usually reduces as the load inertia increases. The rare earth
magnet DC motors and the brushless magnet DC motors have the lowest dynamic
velocity drop. This is because they can generate a high value of torque for a short
period of time.
The type of power unit for DC motors have also influence on Dynamic velocity
drop. It can be seen that PWM DC motors have lower dynamic velocity drop com-
pared with thyristor controlled DC motors. It can be seen that the dynamic velocity
drops for electrical motors reduces as the power rating increases. This is because the
larger motors have higher inertia and larger torque capability.
The dynamic velocity drop usually reduces as load inertia is increased. The re-
duction in dynamic velocity drop for a unit load inertia and unit torque is shown in
Fig. 9.10.
It can be seen that the load inertia reduces the dynamic velocity drop of hydraulic
motor considerably, and to a lesser degree for AC and DC motors. The effect of load
inertia on stepping motors is negligible, since the error is limited to a maximum of
one step, and they are not shown in these graphs. The total dynamic velocity drop,
therefore, can be obtained as
∆ω : = (ω − ω )T (9.4)
I
c r 1 1 c
where ∇ ω , ω , ω , T , I are the total dynamic velocity drop (rpm), dynamic veloc-
r
c
l
l
e
ity drop for a unit external torque with no load inertia obtained from Fig. 9.9 rpm/
(N M), the reduction in dynamic velocity drop for a unit load inertia and a unit