There are Negative Sequence Current Ratings of generators because it is EXPECTED that a generator will not be connected to a perfectly balanced load, hence the 51Q protection scheme found in Generator Relay systems. That rating of the generator is all about the extra cooling techniques used in the generator construction.
There are no "Negative Sequence Current Ratings" of induction motors per se because they are MOTORS, not generators, so they are expected to deliver mechanical power, not produce electrical power.
But Negative Sequence Current caused by voltage imbalance does affect induction machines in that it heats up the rotor at a rate disproportionate to shaft load, meaning that given the same shaft load, a motor running from an unbalanced line voltage will consume more energy and heat the rotor faster than it would if the line voltage was balanced. How that affects the motor will therefore vary by how much mechanical load is connected to it and how much voltage imbalance exists; if the motor is fully loaded to its maximum mechanical kW, then even a small amount of NSC due to a small voltage imbalance will cause severe rotor heating, but if the shaft kW load is only a small fraction of the motor rating, the effect of a small level of NSC may be inconsequential, but the effect of NSC from a high voltage imbalance will be. So the way induction motors are "rated" for this is in the basic design considerations. So for example in NEMA MG-1, the "service conditions" for which a standard motor is designed allow for up to 1% voltage imbalance. More than that (referred to as "Unusual Service Conditions" in MG-1) and the user must consult the motor mfr for de-rating instructions.
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