
4.SELECTING A MOTOR DESCRIPTIONS FOR FANUC AC SERVO MOTOR β series B-65232EN/03
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4.2.2 Data Sheet
The data sheet gives the values of motor parameters relating to the
performance.
The values of parameters are those under the following conditions.
• The ambient temperature for the motor is 20°C.
• The drive current of the motor is pure sine wave.
Important parameters on the data sheet are defined as follows :
(a) Continuous RMS current at stall : Is (Arms)
This current represents a maximum effective current that can
continuously run the motor at low speed.
(b) Torque constant : Kt (kgf⋅cm/Arms)
This constant represents a torque value generated by the motor
for a phase current of 1 ampere.
The torque constant decreases by 0.19% with the β series or by
0.11% with the βM series for each temperature increase of 1°C
beyond 20°C due to the negative temperature coefficient of the
magnet material.
The torque constant is a function of the total flux and the total
number of conductors in the armature.
The back EMF constant (Kv) and the torque constant are inter-
related as follows :
Kt (kgf⋅cm/Arms)=30.6 Kv (Volt⋅sec/rad)
Thus if Kv is reduced due to demagnetization of the magnet, Kt
is also reduced in the same proportion.
(c) Back EMF (electromotive force) constant : Kv (Volt⋅sec/rad)
A back EMF is a voltage that is generated by rotating a servo
motor with external mechanical force. The back EMF constant
of a motor represents the relationship between the number of
conductors in the armature of the motor and the amount of flux
in the field magnet. The back EMF constant is also a parameter
representing the intensity of a permanent magnet. The
relationship can be given as.
(Volt⋅sec/rad)=(volt/min
-1
) ×9.55
Back EMF constant is indicated as the RMS voltage per phase,
so multiply
3 to get actual terminal voltage.
(d) Mechanical time constant : tm (sec)
This is a function of the initial rate of rise in velocity when a
step voltage is applied. It is calculated from the following
relationship.
tm=(Jm×Ra)/(Kt×Kv)
Jm : Rotor inertia (kgf⋅cm⋅sec
2
)
Ra : Resistance of the armature (Ω)
(e) Thermal time constant : tt (min)
This is a function of the initial rate of rise of winding
temperature at rated current. It is defined as the time required to
attain 63.2 percent of the final temperature rise.
(f) Static friction : Tf (kgf⋅cm)
This is the no-load torque required just to rotate the rotor.