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Abb AC Servo motors
| Motors and Generators
| ABB
75
5
A move profile defines the desired acceleration rate, run time, speed and
acceleration rate of the load. For example, suppose with a system at rest
(time = 0 in Figure 6), the positioning controller issues a command to the
motor to start motion. At t = 0, with full power applied, the motor has not
yet started to move. At this instant, there is no feedback signal, but the error
signal is large.
As friction and inertia torques are overcome, the motor and load begin to
accelerate. As the motor reaches the commanded speed, the error signal is
reduced, and in turn the voltage applied onto the motor is reduced. As the
system stabilizes at running speed only nominal power (voltage and current)
are required (to overcome friction). At time t = 1, the load approaches the
desired position and begins to decelerate.
Applications with these move profiles result in most of the input energy
dissipated as heat. Such packages are therefore limited by the maximum
power dissipation capacity of the motor. In order to guarantee that maximum
power handling capability of the motor is not exceeded, each application
must be investigated individually. Basic motor dynamic equations must be
solved and power calculated for each motor in order to determine whether
the application will be handled
successfully.
The first step in the process is identifying the acceleration rate. For an
example, let’s assume that our application has a move profile as identified in
Figure 7. The acceleration rate can be determined from the speed and the
acceleration time, as follows:
accel rate
=
speed ÷ accel time
accel rate (rad/sec
2
)
=
Wm (rad/sec)
t
acc
(sec)
To convert from RPM to rad/sec divide by 9.55.
For our example the acceleration rate is:
accel rate (rad/sec
2
) = 209 (rad/sec) = 1741.6 rad/sec
2
.12 (sec)
Motor parameters
Inertia = .0037 lb-in-s
2
(4.26 gm-cm-s
2
)
Continuous stall torque = 14.4 lb-in (1.6 Nm)
Torque constant = 4.8 lb-in/amp (0.54 Nm/amp)
Resistance = 4.5 ohms
Thermal resistance = 1.56 ˚C/watt
Load conditions
Load = 200 lbs (90.6 Kg)
Ball screw inertia = .00313 lb-in-s
2
(0.61 gm-cm-s
2
)
Friction torque = .95 lb-in (1094 gm-cm)
The move profile
Example
Example parameters
Figure 6
Figure 7
Figure 8
Motor
Load
Abb AC Servo motors