Very Fast Measurement of Low Speed of Rotating Machines Using Rotating Magnetic Field

The measurement at low speed becomes very difficult due to the slow response of transducers. At low speed (below 10 r/min), normally, transducers take several seconds to give the required output signal. In the proposed technique, a fast rotating magnetic field (RMF) is used to measure the speed as well as deviation in speed for low-speed machines. A sine wave oscillator (single-phase ac source) is used to generate a balanced three-phase ac voltage, which is applied to the stator windings of a synchro whose rotor is coupled with the rotating member or rotor of a motor. The RMF in the air gap generates emf in the rotor of the synchro whose frequency depends upon the slip speed. Since the RMF revolves at a speed several times the speed of the rotating member or the rotor of the motor, hence the measurement becomes very fast. The proposed scheme is tested successfully even at 1 r/min, and the overall performance is recorded at dynamic conditions. The results of the measurements are compared with a conventional method (tachogenerator), which shows the accuracy of the proposed method. The output is also obtained in terms of dc voltage and dc current, which can be used for feed-back and control applications.