Motor Statring Protection by Impedance Sensing

A linestart induction motor, when protected against locked rotor by a time overcurrent relay, can be tripped off during a normal start under some conditions of load inertia and voltage. Such failure to start can occur even when the relay appears to be correctly set. To prevent it the relay should be disabled at a time after rotation begins and before the relay can operate. A speed sensing switch can supervise in this way but requires direct access to rotating parts, sometimes not feasible or not economical. Impedance sensing offers an accurately setable alternate means by direct relay monitoring of voltage, current and phase angle without requiring access to rotating parts.