Page 112 - Servo Motors and Industrial Control Theory -

106 6 AC Servo Motors

INDUCTANCE (HENRY)
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1 f = 5 f = 75
0.05
0
0 6 12 18 24 30 36 42 48 54 60
TORQUE (N-M)

Fig. 6.4 Variation of inductance the torque and frequency change

values that the motor generate sufficient torque to overcome the applied torque. The
torque must not be very small so that the resistance is low and it should not be too
large when the resistance is small. It would be interesting to calculate the current as
the torque is increased.
It is possible to predict the variation of the current as the external torque in-
creases from the variation of resistance and inductance. This is shown in Fig. 6.5.
The figure shows that as the torque increases the current also increases. As the
torque increases, the current almost linearly increases. At large values of current,
the torque reduces rapidly. This is a disadvantage for AC servo motors. Therefore,
the AC motors as mentioned previously do not have large starting torque. It would
be interesting to observe speed-torque characteristic of AC motors. In the above
discussion, it was shown that the equivalent resistance and inductance can be calcu-
lated from the data provided by manufacturers. Then the current can be calculated.
Once the current is known, the torque can be calculated accordingly from the torque
equation given above. This is shown in Fig. 6.5.
It can be seen that the speed-torque characteristic of AC motors is linear over
a wide range of speed and torque. This is shown by the dotted line in Fig. 6.6. At
large torque, the speed drops rapidly and the output torque also drops rapidly. This
diagram must be provided by manufacturers. As the frequency (speed) reduces the
output torque also reduces. The torque equation can be assumed linear and as previ-
ously stated it can be written as

T : K ·I= (6.5)
m t