3.SELECTING A MOTOR SPECIFICATIONS FOR THE βiS SERIES B-65302EN/01
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Inertia of a cylindrical object in which the center of rotation is displaced
Center of rotation
2
0
MRJJ +=
J
0
: Inertia around the center of the object
M : Weight of the object
R : Radius of rotation
The above equation is used to calculate the inertia of, for example, a
large gear which is hollowed out in order to reduce the inertia and
weight.
The sum of the inertia values calculated above is J (load inertia) for
accelerating the motor.
Cautions as to the limitations on load inertia
The load inertia has a great effect on the controllability of the motor
as well as the time for acceleration/deceleration in rapid traverse.
When the load inertia is increased, the following two problems may
occur: When a command is changed, it takes more time for the motor
to reach the speed specified by the new command. When a machine
tool is moved along two axes at a high speed to cut an arc or curve, a
larger error occurs.
When the load inertia is smaller than or equal to the rotor inertia of
the motor, those problems will not occur. When the load inertia is up
to three times the rotor inertia, the controllability may have to be
lowered a little. Actually, this will not adversely affect the operation
of an ordinary metal cutting machine. If a router for woodworking or
a machine to cut a curve at a high speed is used, it is recommended
that the load inertia be smaller than or equal to the rotor inertia.
When the load inertia is greater than the rotor inertia by a factor of
more than 3 to 5, the controllability of the motor will be adversely
affected.
If the load inertia much larger than three times the rotor inertia, an
adjustment in the normal range may be insufficient. Avoid using a
machine with such a great load inertia.