
3.SELECTING A MOTOR SPECIFICATIONS FOR THE αiS/αi SERIES B-65262EN/03
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3.2.5 Calculating the Root-mean-square Value of the Torques
A motor gets hot in proportion to the square of the torque. For a servo
motor for which the load condition always changes, the calculated
root-mean-square value of torque in a cycle must be sufficiently
greater than the stall torque.
Root-mean-square value of torque in acceleration/deceleration in rapid
traverse
First, generate an operation cycle which performs acceleration/
deceleration in rapid traverse with a desired frequency of positioning
in rapid traverse. Write the time-speed graph and time-torque graph as
shown below.
Speed
Time Time
Torque
From the time-torque graph, obtain the root-mean-square value of
torques applied to the motor during the single operation cycle. Check
whether the value is smaller than or equal to the torque at stall.
() ()
0
30121
2
2
2
2
t
tTtTTtTtTT
T
mamma
rms
+−+++
=
T
a
: Acceleration torque
T
m
: Friction torque
T
o
: Torque when stopped
When T
rms
falls within 90% of the stall torque T
s
, the servo motor can
be used. (The entire thermal efficiency and other margins must be
considered.)
NOTE
The motor actually rotates, but the determination
must be based on the stall torque.
When the motor is being operated at high speed for
a comparatively large proportion of the time, you
must take the rotating speed of the motor into
consideration and evaluate whether output can be
specified in terms of a continuous operation torque.