Compensation for supply and motor unbalances and line resistance for a DC signal injection-based thermal protection method

Thermal protection is one of the most important aspects in an electric motor protection system. A dc signal injection-based stator winding temperature estimation technique has been proposed for soft-starter-connected electric motors by the authors. As a continuation of the previous work, this paper proposes a compensation approach for power supply unbalance, motor internal unbalance, and line resistance. The proposed method is validated from experimental results on a 7.5-hp TEFC induction motor under variable load levels with an unbalanced power supply. The stator winding temperature estimation error is within 10degC in face of power supply unbalance.