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High-order induction machine thermal models have been widely studied for the analysis of the thermal behavior of induction machines. However, the real-time implementation of such models for the online thermal protection of induction machines is very difficult due to the lack of accurate information on the machines. This paper proposes a novel simplified thermal model of induction machines for thermal overload relays. Instead of using a lumped thermal network, a transfer-function-based approach is proposed for reducing the order of induction machine thermal models. The proposed thermal model requires significantly fewer thermal parameters for the accurate modeling of the thermal behavior of the machine. The proposed thermal model is validated via experimental results on a 7.5-hp open-drip-proof induction machine under various load conditions. The major features of the proposed thermal model are the following: 1) high accuracy of the stator winding temperature estimation, with a root-mean-square error within 3°C; 2) low computational requirement, which reduces the cost of thermal overload relays; and 3) easy implementation since only current sensors are required.