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7 Selection
MITSUBISHI CNC
(2) Short time characteristics
In addition to the continuous operation range, the servomotor has the short time operation range that can be
used only in a short time such as acceleration/deceleration. This range is expressed by the maximum torque
and the torque characteristics. The maximum torque or the torque characteristics differ according to each
motor, so confirm the specifications in section "2-1 Servomotor".
The torque required for the servomotor’s acceleration/deceleration differs according to the CNC’s command
pattern or the servo’s position control method.
Determine the required maximum motor torque from the following expression, and select the servomotor
capacity.
(a) Selection with the maximum torque characteristics
In a low-speed rotation range (approximately less than half of the servomotor maximum speed), the
linear acceleration/deceleration time constant "ta" that can be driven depends on the motor maximum
torque. That can be approximated from the machine specifications using the expression (7-2).
••• (7-2)
Using the approximate linear acceleration/deceleration time constant "ta" calculated above, confirm the
torque characteristics of the high-speed rotation range in the CNC’s command pattern or the servo’s
position control method.
N : Motor reach speed (r/min)
J
L
: Motor shaft conversion load inertia
(kg•cm
2
)
J
M
: Motor inertia
(kg•cm
2
)
η : Drive system efficiency (Normally 0.8 to 0.95)
T
MAX
: Maximum motor torque (N•m)
T
L
: Motor shaft conversion load (friction, unbalance) torque (N•m)
ta =
(0.8TMAX-TL)
1.0510
-2
(JL/
η+
JM)N
(ms)