
3.SELECTING A MOTOR SPECIFICATIONS FOR THE αiS/αi SERIES B-65262EN/03
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3.1 CONDITIONS FOR SELECTING A SERVO MOTOR
The conditions for selecting a servo motor are given below.
[Selection condition 1] Constant load torque
- The constant load torque including mechanical friction and
gravity must fall within approximately 70% of the stall
torque of a motor.
If the constant load torque is close to the stall torque, the root-
mean-square value of the total torque including the acceleration
torque is more likely to exceed the stall torque.
Along the vertical axis, the load may be increased during lifting
and at stop due to a mechanical factor. In this case, the
theoretically calculated gravity retaining torque must be 60%
(less than 60% in some cases) of the stall torque of a motor.
This figure of "within 70% of the continuous torque rating" is
for reference only. Determine the appropriate torque based upon
actual machine tool conditions.
[Selection condition 2] Motor speed
- The motor speed must not exceed the maximum motor speed
(rated speed during continuous operation).
Calculate the motor speed and check that the speed does not
exceed the maximum motor speed. For continuous operation,
check that the speed does not exceed the rated speed.
[Selection condition 3] Load inertia ratio
- The load inertia ratio must be appropriate.
The ratio of motor inertia and load inertia (load inertia ratio)
greatly affects the controllability of the motor as well as the
acceleration/deceleration time in rapid traverse.
When the load inertia does not exceed three times the motor
inertia, an ordinary metal cutting machine can be used without
problems, while the controllability may have to be lowered a
little in some cases.
For a machine for cutting a curve at a high speed, such as a
router for woodworking, it is recommended that the load inertia
be smaller than or equal to the motor inertia.
If the load inertia is greater than the motor inertia by a factor of
more than 3 to 5, the controllability of the motor may be
adversely affected. If the load inertia is much larger than three
times the motor inertia, adjustment within the normal range may
be insufficient. It is desirable to avoid using a motor with such
inertia.