Fuji Electric FRENIC-Ace series, User Manual

Page: 596 / 788

10.3 Equations for Selections
10-14
10.3.4 Calculating the RMS rating of the motor
In case of the load which is repeatedly and very frequently driven by a motor, the motor current fluctuates largely
and enters the short-time rating range of the motor repeatedly. Therefore, you have to review the allowable thermal
rating of the motor. The heat value is assumed to be approximately proportional to the square of the motor current.
If an inverter drives a motor in duty cycles that are much shorter than the thermal time constant of the motor,
calculate the “equivalent RMS current” as mentioned below, and select the motor so that this RMS current will not
exceed the rated current of the motor.
Figure 10.3-7 Sample of the Repetitive Operation
First, calculate the required torque of each part based on the speed pattern. Then using the torque-current curve of
the motor, convert the torque to the motor current. The “equivalent RMS current, Ieq” can be finally calculated by
the following equation:
) A (
t t t t t t
t I t I t I t I t I
I
6 5 4 3 2 1
5
2
5 4
2
4 3
2
3 2
2
2 1
2
1
eq
+ + + + +
⋅ + ⋅ + ⋅ + ⋅ + ⋅
= (Equation 10.3-26)
The torque-current curve for the dedicated motor is not available for actual calculation. Therefore, calculate the
motor current I from the load torque τ
1
using the following equation (Equation 10.3-27). Then, calculate the
equivalent current Ieq:
) A ( I I
100
I
2
100 m
2
100 t
1
+






×
τ
=
(Equation 10.3-27)
Where,
τ
1
is the load torque (%), I
t100
is the torque current, and I
m100
is exciting current.