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4.5.2 Starting and Stopping Time
The motor starting time (tr) and stopping time (tf) under a constant load are calculated using the following for-
mulas. Motor viscous torque and friction torque are ignored.
Starting time:
tr =
Stopping time:
tf =
N
:
Motor speed (min
M
J
:
Motor rotor moment of inertia (kg m
M
J
:
Load converted to shaft moment of inertia (kg m
L
T
:
Instantaneous peak motor torque when combined with a SERVOPACK (N m)
PM
T
:
Load torque (N m)
L
Calculate the torque from the motor current using servomotor torque constant × motor current (effective value).
The following figure shows the motor torque and motor speed timing chart.
Motor torque
(current amplitude)
Motor speed
4.5.3 Load Moment of Inertia
The larger the load moment of inertia, the worse the movement response of the load.
The size of the load moment of inertia (J
limited to within 5 to 30 times the moment of inertia of each servomotor (J
guideline and results may vary depending on servomotor drive conditions.
An overvoltage alarm is likely to occur during deceleration if the load moment of inertia exceeds the allowable
load moment of inertia. SERVOPACKs with a built-in regenerative resistor may generate a regeneration overload
alarm. Take one of the following steps if this occurs.
• Reduce the torque limit.
• Reduce the deceleration rate.
• Reduce the maximum motor speed.
• Install an externally mounted regenerative resistor if the alarm cannot be cleared. Contact your Yaskawa
Application Engineering Department.
Regenerative resistors are not built into 200 V SERVOPACKs for 30 W to 400 W or 100 V SERVOPACKs for
30 W to 200 W. The following figures show the tentative relationship between the load moment of inertia and
motor speed using an example with a load moment of inertia 10 to 30 times the rotor moment of inertia at the
motor shaft.
External regenerative resistors are required when this condition is exceeded or if the allowable loss capacity (W)
of the built-in regenerative resistor is exceeded due to regenerative drive conditions when a regenerative resistor
is already built in.
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4.5 SERVOPACK Overload Characteristics and Allowable Load Moment of Inertia
2
N
(J
+ J
)
M
M
L
[s]
60
(T
T
)
PM
L
2
N
(J
+ J
)
M
M
L
[s]
60
(T
T
)
PM
L
-1
)
2
)
tf
tr
) allowable when using a servomotor depends on motor capacity and is
L
2
)
Time
Time
). This value is provided strictly as a
M
4
4-13
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