Protection Engineering Basics and Schemes Protection Engineering | Page 277
9.5 Locked Rotor Protection When motor starts at full voltage, it draw about 300 to 600 % of their full load running ampere, this inrush current is called locked rotor current. It is necessary for rotor motion.
Starting current of motor is very high, so special kind of setting is provided in motor relay which allow over load for certain period, after which if overload sustain then protection relay trip the circuit to isolate and protect motor from damage.
Starting current( I s) and starting time( T s) must be set in relay, relay will calculate thermal stress
thermal stress = I s 2 T s
And operates when value of thermal stress increase from set values