2 - Fagor Automation

Selection criteria
5.
SELECTING CRITERIA
Selection of the synchronous motor and its associated drive
232
DDS
HARDWARE
Ref.1307
208
Second motor pre-selection
Calculation of inertia (J)
The next step is to calculate the load that the motor has to move when
accelerating; that is the moment of inertia of all the elements it moves.
Total inertia (from now on inertia) JTOTAL is due to the load JLOAD and to
the rotor of the motor itself JMOTOR .
J
TOTAL
=
The inertia due to load may be divided into that of the table + that of the
ballscrew + that of the system used to compensate for non - horizontal
axes + that of the pulley or gear used for transmission and which turns
with the ballscrew (pulley 1). All these elements are affected by the reduction
factor i as shown by the following equation.
The inertia due to the pulley that turns with the motor (pulley 2) is not affected
by the i factor.
The inertia of each element is:
The resulting inertia will be in kg·m².
L Leadscrew length in m.
L1 Width of pulley 1 in m.
L2 Width of pulley 2 in m.
Dp1 Diameter of pulley 1 in m.
Dp2 Diameter of pulley 2 in m.
Material density:
7700 kg/m³ for iron/steel
2700 kg/m³ for aluminum
i, h are data used earlier.
See previous sections.
The inertia of the motor J MOTOR is:
J
LOAD
+ J
MOTOR
J TABLE + J BALLSCREW + J PULLEY1 + J COMPENSATION
J LOAD = ----------------------------------------------------------------------------------------------------------------------------------------------- + J PULLEY2
2
i
J TABLE = m
J PULLEY1
=
h
------
2
2
4
Dp1 L1
------------------------------------
32
d
J BALLSCREW
4 L
= ------------------------------
32
J PULLEY2
J MOTOR = J ROTOR + J BRAKE
4
D
p2
L 2
--------------------------------------
32
this data may be obtained from the characteristics table of the corresponding
motor manual.
Verify that in the characteristics table the rotor of the motor chosen in the
1st selection has an inertia JMOTOR that meets the following condition:
J MOTOR
J
LOAD
K
where k is a factor whose value depends on the application given to the
motor.
The ideal will be to obtain a JMOTOR = JLOAD For a positioning axis, the typical value of “K” will be between 1 and 3.
WARNING. Note that if this requisite is not met, a new motor must be selected
which meets the conditions of the 1st selection and the 2nd one.
=