The theory of magnetic circuit design,the constitutive equations of a magneto-rheological fluid,and the load properties of belt conveyors were used to design a magneto-rheological soft starter test-bed.The magnetic field distribution in the working gap was analyzed and the current-speed relationship was investigated.A mathematical model for the time response was deduced.The results show that a linear relationship between current and magnetic field is seen when the magnetic materials are not saturated and the magnetic field is uniform in the working section.The rotation speed of the driven shaft changes linearly with increasing time.The response is rapid and can be as short as milliseconds.This meets the starting requirements of belt conveyors.