Page 153 - Servo Motors and Industrial Control Theory

148 9 The Choice and Comparison of Servo Motors

Fig. 9.3 A comparison of 50 × 1000
volume (proportional to
weight) of different servo
motors
40
(7)
(2)
30
(6)
Volume (cm 3 ) 20
(3)

10 (5)

(4)
0
0 2 4 6 8 10
Power (kw)

and it seems that ceramic magnet DC servo motors have a higher size to power ratio
than induction AC servo motors. But the power unit of AC induction servo motors
have a larger unit with complex circuitry. The stepping motors are competing with
other types of servo motors at low power range but at high power requirements,
they are not very efficient. At very low power requirements, all servo motors have
similar power to weight ratio and in this range the speed of response and stiffness to
external torque become more important.
The speed of response strongly depends on the inertia of servo systems. It is use-
ful to compare the inertia of the motor without any inertia referred to motor shaft.
This is shown in Fig. 9.4. It can be seen that hydraulic motors have the lowest inertia
for a given power. The brushless DC motors having lower inertia than other electric
motors, and they are a good replacement for hydraulic motors as speed of response
is concerned because lower inertia means the possibility of fast response. Stepping
servo motors are only available at low power range and their inertia is higher than
hydraulic and brushless and rare earth magnet DC motors. AC induction motors
have low inertia at lower power range and the inertia increases rapidly as the power
rating is increased. Ceramic magnet DC servo motors have similar inertia to other
motors at low power range but increases rapidly as the power rating increases. It
should be noted that in order to keep similarity, all motors up to the velocity of
1,500 rpm are compared in this section. Because as rotating velocity is increased,
the inertia reduces but the maximum torque also reduces. This is because of design
of motors. As rotating speed increases, the inertia reduces which means that greater
power can be transmitted at higher velocity and at lower maximum torque.