Rockwell Automation 150 MNL/SMC DIALOG+/APP-PROD GUIDE - Manual (Page 168)

10-6 Reference
Torque and Horsepower (cont.) By manipulating the preceding equation for kVA/HP for three-phas
motors, the following equation can be used for calculating locked-
rotor current:
This equation can then be used to determine the approximate starting
current of any particular motor. For instance, the approximate
starting current for 7-1/2 HP, 230 volt motor with a locked-rotor kVA
code letter of G would be:
Operating a motor in a locked-rotor condition for an extended period
of time will result in insulation failure because of the excessive heat
generated in the stator. The following graph illustrates the maximum
time a motor may be operated at locked-rotor without incurring
damage caused by heating. This graph assumes a NEMA Design B
motor with Class B temperature rise.
Figure 10.4 Motor Safe Time vs. Line Current - Standard Induction Motors
➊ Base Amps and Nameplate Amps.
Motor protection, either inherent or in the motor control, should be
selected to limit the stall time of the motor.
LRA
1,00 0H
×
kVA/HP
×
1.73 Volts
×
-------------------------------------------- =
LRA
1,00 07.
Ч
6.0
Ч
1.73 230
Ч
------------------------------------- 113 Amp = =
From Operating
Temperature
From Ambient
Motor
Line
Amps
Per
Unit ➀
8
6
4
2
1
0
10 15 20 100 200 700
Motor Running
Motor Stalled
Time in Seconds