The three current industry standards, NEMA MG 1, ANSI C50.41, IEEE C37.96, include
information about induction motors that are subjected to momentary service interruptions.
NEMA MG1 and ANSI C50.41 suggest that peak torque and peak current magnitudes in an
induction motor can reach values of 2 20 pu during momentary service interruptions. IEEE
C37.96 suggests that peak current magnitudes in an induction motor can reach 2.5 times
locked rotor current during momentary service interruptions. All three standards provide
guidelines for avoiding the potentially damaging transients due to momentary service
interruptions, but independent research shows that significant torque and current transients
are still possible even if the recommendations in the standards are followed.
Industry standards and previous analytical data suggest a wide range of peak current and
peak torque magnitudes. Industry standards claim that the peak torque and peak current
magnitudes can be as low as 2 pu and analytical studies claims that the peak torque
magnitude can be as high as 44 pu. Other studies claim peak torque magnitude values
between these two extremes, but the results still vary greatly. Little experimental data is
available to determine which studies are most accurate. However, important information
found in the literature can help guide this study to determine the worst case shaft torque
transients. This information is as follows.
The heating effect of the interruption currents are less than the heating effects of the startup
currents and should not cause any adverse problems .
Momentary service interruptions produce a higher shaft torque than supply fa ult disturbances
and supply fault disturbances produce higher shaft torque than voltage sags.
The interruption duration during a momentary service interruption has a significant effect on
the shaft torque .
The point on wave "drop-out" point of a momentary service interruption has no effect on the
shaft torque .