Why coil is not getting burned on the flow of lra?

If I may be so bold as to respond to what I think you are asking...

LRA (Locked Rotor Amps) is the value of the expected current that will flow in the motor circuit if the rotor is not rotating. In this condition, there is very little inductance to inhibit the flow of current, mainly only the winding conductor resistance is impeding the flow, so the current values are very high. The expected range can be anywhere from 400 to 800% of rated Full Load Amps (FLA) with the generally accepted value of 600% being considered the norm.

This condition only happens under two conditions;

1. The motor has been stopped while energized, i.ew. a mechanical jam on the motor shaft by the load or power transmission components.
2. The instant when the motor has been initialized while standing still, until the motor reaches approximately 80% speed. This can be for anywhere from a fraction of a second to a few seconds, depending on the load inertia, friction etc.

Protection in the form of preventing damage to the motor windings (and the conductors feeding the motor) is done based upon a curve, defined as an "I²t Overload Curve", where the current (I) value at any point on a curve is the square of the time (t) increment compared to the previous time point. So the more current you have, the less time it will take for the tripping device to act. For motors, this curve is designed in "Classes", such as Class 10, Class 20, Class 30 etc., wherein the Class number defines the amount of time the device will trip in for a current value of 600%, LRA. So when you pick a Class 10 Overload Relay, that means it will trip in no more than 10 seconds at LRA. This Class value is selected by the motor manufacturer to coincide with the Thermal Damage Curve of the motor itself. So when a motor manufacturer designs a motor, they know that it can withstand at no more than 10 seconds at LRA, they will insist that the motor be protected by a Class 10 OL relay. If the motor is designed with heavier conductors in the stator and can withstand high current for a longer time without burning, then they will designate it for Class 20, or Class 30. In general, IEC motors are almost all Class 10, NEMA motors are almost all Class 20, and Class 30 are a rare class of what we call "Mill Duty" motors here in North America.

So for you, that is all you really need to know. The exact details of the motor LRA are covered by the OL class that the motor manufacturer recommends.

LRA is not factored into any other protection values on a standard motor circuit. Fusing (or a circuit breaker) are intended to provide Short Circuit protection, not I²t protection, so LRA is irrelevant to their selection.

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