Abstract
Electric motors need protection against three major conditions: unfavorable operating conditions, fault conditions, and system disturbances and switching surges. Unfavorable operating conditions may arise due to one or more of the following factors, such as overloading, undervoltage, reverse rotation, protection from harmonic effects, and voltage unbalance (negative phase sequence). Some of the fault conditions are: a fault condition, such as short-circuit between phases; a ground fault condition; a prolonged starting time; a stalling or locked rotor condition; frequent starts; and single phasing. The motor details and working conditions essential for making a proper selection of a protective scheme include type of motor- squirrel cage or slip-ring, ratings, rated voltage and current, type of starting, starting current versus time characteristics, locked rotor current and corresponding “hot” thermal withstand time, motor thermal withstand characteristic curves, number of starts or reversals, if required, and their frequency, ambient temperature, and maximum fault current.
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