Page 160 - Servo Motors and Industrial Control Theory -

9.3 Comparison of Results and Design Procedure 155

Fig. 9.10 Reduction in 10
dynamic velocity drop for
an addition of a unit load
inertia at a unit load torque
8 4

2
6
W I (rpm / n - m / kg-m 2 ) 4 5 6

2
1b k

0
2 4 6 8 10
–1
1a Power (kw)
–2

5. From Figs. 9.9 and 9.10 calculate the velocity drop due to the applied torque and
compare with the permissible value.
6. If from the above analysis none of the motors provide a satisfactory performance,
increase the power rating of the motor, and repeat the analysis from step 4.
7. If the higher power rating motor does not produce a satisfactory result then one
must use acceleration feedback (or current feedback). Use Figs. 9.11 and 9.12
instead of Figs. 9.6 and 9.7 and repeat the search from step 4.
From the above analysis, one might find that a number of types of servo motors
provide the required performance. In this case, the final decision might be based on
other criteria such as capital cost, reliability and availability.
To compare capital costs, the complete drive unit must be considered, including
the capability of the controller, the number of axes to be driven, the compatibility
of possible interface with micro-computer, the addition of velocity and position
transducers, and so on. There are indications that for low power rating the hydraulic
servo motor drive units are the most expensive. Pulse width modulated systems are
the second most expensive and they are followed by thyristor controlled DC motors
and AC and stepping motors. At higher power rating, that is, above 10 kW power
rating, hydraulic motors become cheaper than electrical motors.
The AC and stepping motors are likely to be the most reliable because both are
brushless. Hydraulic motors are reliable but require more maintenance care. The
development of brushless DC motors show considerable promise for the future.