Objectives  In order to detect the weak stray magnetic field signal of the induction machine during operation, this paper presents an optimized design method of low noise Induction Magnetometer (IM).

  Methods  By analyzing the frequency response and noise model of the IM, the main noise sources that affect the performance of the IM at different frequency bands are proposed, and the formula of the equivalent input magnetic field noise level is obtained. Based on this, a special low-noise amplifier circuit is designed. And the optimal design method of the IM is proposed. Besides, for different restrictions, the optimization formulas and results are given. In addition, a small IM with a linear response to 10 kHz is fabricated, and the magnetic field of an induction machine under normal and fault condition is tested.

  Results  The test results show that the equivalent magnetic field noise level is 2 pT/\sqrt \rmHz at 10 Hz, and 0.7 fT/\sqrt \rmHz at 20 kHz.

  Conclusions  Through optimized design, the IM can effectively monitor the magnetic field change of the induction machine, and can be used for online monitoring.