Page 148 - Servo Motors and Industrial Control Theory -

Chapter 9
The Choice and Comparison of Servo Motors

9.1 Introduction

In making a choice of the correct motor for a particular need, there are a number
of factors to be considered; for example, speed of response, accuracy, and dynam-
ic error due to an external disturbance, coupled with capital costs, reliability, and
availability. In the previous chapters, dynamic properties of several types of servo
motors were discussed and by obtaining the mathematical model the dynamic per-
formance may be studied. In this chapter, a systematic approach is given for making
the choice on the basis of a number of simplified graphs of performance.
Servo motors are manufactured in various forms, speed ratings, and weights.
In some applications, it is required to use a gearbox for two reasons of increasing
the output torque and reducing the effect of load inertia referred to the motor. It is,
therefore, difficult to compare them. To help the designer to select a servo motor,
this chapter tries to introduce some common variables so that it is possible to com-
pare them.
In recent years, considerable effort has gone into improving servo motor perfor-
mance, and a wide variety of servo motors are available with comparable character-
istics over a range of power ratings. Hydraulic servo motors have been common in
the past mainly because of their high power to weight or size ratio. Problems such
as the effect of contamination and the necessity for a hydraulic power unit have
diverted the attention of designers from hydraulic to electric motors. At the same
time, electric motors capable of greater overload for reasonable periods of time
have resulted in more compact electric drives, and better design and packaging have
made them more attractive to the user.
Motors of up to 10 kW power rating of the following type have been compared
in this chapter.
1. Ceramic magnet DC motors
2. Rare-earth magnet DC motors
3. Brushless DC motors
4. Stepping motors
5. Variable frequency induction AC servo motors

R. Firoozian, Servo Motors and Industrial Control Theory, Mechanical Engineering Series, 143
DOI 10.1007/978-3-319-07275-3_9, © Springer International Publishing Switzerland 2014