Motor Protection Device For Wound Rotor Induction Machines

Many electric adjustable speed drives in ukrainian industrial enterprises still use wound rotor induction machines (WRIM) regulated by varying rotor resistance. The rotor resistance changing is done by means of mechanical contacts. The presented work focuses on one of the cases of WRIM failure – overheating of rotor windings. This is mainly caused by the rotor circuit unbalance which resulted from misconfiguration of the rotor resistances (e.g. one of the resistors breaks open, mechanical contact fails to close, etc.). In the vast majority of drives with WRIM, its rotor currents are not monitored at all, and rotor unbalance is left “unseen” by the stator protection devices, causing WRIM thermal failure. The paper presents a motor protection device, which is able to detect WRIM rotor currents unbalance while using only stator current and voltage measurements. This allows to improve reliability of WRIM drives while maintaining low cost of protection device itself.