The design philosophy of non-invasive has been introduced in motors monitoring system, in which used air-gap torque to measure efficiency. Zoom-FFT is required for the testing of speed which very few assumed values are required. An efficient implementation of cosine modulated FIR analysis synthesis filter banks is derived. The structures for the analysis and synthesis sections consist of polyphase structure and discrete sine and cosine transform networks. The redundance of computation is removed and the computational efficiency is raised. The implementation structure is simpler. The efficiency of the whole system has been improved, and approved that this algorithm has a good value.