
B–65142E/04
4. SELECTING A MOTORDESCRIPTIONS FOR THE α series
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4
SELECTING A MOTOR
When selecting an applicable motor, the load, rapid traverse feedrate,
increment system, and other conditions must be considered. This section
describes how to calculate the load and other conditions, showing an
example of a table with a horizontal axis.
Motors are subjected to two types of torque: constant load torque
(including friction), and cutting power and acceleration/deceleration
torque. Calculate the two loads accurately and select a motor that satisfies
the following conditions:
Condition 1
When the machine is operating without any load, the torque is
within about 70% of the continuous torque rating.
When the machine tool is stopped, the motor is generating torque in
a balanced state with the friction–induced load. If acceleration/
deceleration torque required for actual operation is added when this
value is close to the rated torque, the rated torque may be exceeded as
the average torque, and the motor is more likely to overheat.
This figure of “within 70% of the continuous torque rating” is for
reference only. Determine the appropriate torque based upon actual
machine tool conditions.
Condition 2
Acceleration can be made with a desired time constant.
Generally, the load torque helps deceleration. If acceleration can be
executed with a desired time constant, deceleration can be made with
the same time constant. Calculate the acceleration torque and check
that the torque required for acceleration is within the intermittent
operating zone of the motor.
Condition 3
The frequency of positioning in rapid traverse is set to a desired
value. The greater the frequency of positioning in rapid traverse, the
greater the ratio of acceleration time to the entire operation time. This
may overheat the motor. When the acceleration time constant is
increased according to the rapid traverse feedrate and positioning
frequency constant, the amount of produced heat decreases in inverse
proportion to the acceleration time constant.
Condition 4
If the load condition varies during a single cycle, the
root–mean–square value of the torques is smaller than or equal to
the rated torque.
Condition 5
The time for which the table can be moved with the maximum
cutting torque (percentage duty cycle and ON time) is within a
desired range.
The procedure for selecting a motor is described below: