Very Fast Measurement of Speed by Rotating Magnetic Field

For very low speed measurement, the output voltage of a tachometer type transducer and the voltage/noise ratio become very low. Moreover, it becomes stable only after several revolutions and it causes a delay of the order of several tens of second. In this work, a synchro based fast speed measurement technique is proposed. A balanced three-phase voltage is applied to the stator windings of synchro. It produces rotating magnetic field (RMF) which rotates at synchronous speed in the air gap. The induced emf generated in rotor of synchro and its frequency depend upon slip. The fast revolving RMF is used to measure the speed at very fast rate. A frequency meter connected at the output of rotor measures the frequency and hence speed. When RMF revolves at a speed, several times the speed of the rotor (which is physically coupled to the rotating member), the measurement becomes very fast. For very low speed of the rotor, the RMF rotates several times even before completion of one revolution of the rotor. Thus the output of synchro varies immediately and several times faster than the rotor itself. For lower speed and higher slip, the magnitude of the output voltage is unaffected, even at zero rpm. The scheme is very attractive for very low or super low speed for fast tracking and generation of feedback control signals.