Estimation of the Sag Severity

Estimation of the Sag Severity:

. If full-voltage starting is used, the sag voltage, in per unit of nominal system voltage, is

Where 

V(pu) = actual system voltage, in per unit of nominal, 

kVA_LR = motor locked rotor kVA

 kVA_SC = system short-circuit kVA at motor

If the result is above the minimum allowable steady-state voltage for the affected equipment, then the full-voltage starting is acceptable. If not, then the sag magnitude versus duration characteristic must be compared to the voltage tolerance envelope of the affected equipment. The required calculations are fairly complicated and best left to a motor-starting or general transient analysis computer program. The following data will be required for the simulation:

Fig Typical motor versus transformer size for full-voltage starting sags of 90 percent.

^Ø   Parameter values for the standard induction motor equivalent circuit: R1, X1, R2, X2, and XM.

^Ø   Number of motor poles and rated rpm (or slip).

^Ø   WK2 (inertia constant) values for the motor and the motor load.

^Ø   Torque versus speed characteristic for the motor load.