Precision Low Cost Electric Motor Flux and Torque Measurement Method

Torque and flux measurements in ac motor are often a complicated matter. For flux measurement, a coil or a field sensor must be inserted inside stator windings causing relevant costs especially for big machines. Anyway, measurement has not great accuracy since calibration is not easy. For torque measurement, a telemetry system can be installed on rotor axle but, especially for notable accuracy, this system has high costs. Sometimes is not possible to install a telemetry directly on rotor axle and a gear box must be interposed, losing every chance to obtain a precise measurement. Cost and measurement uncertainties increase with machine size so that, for high power (more than 500 kW), this is about an insoluble matter. In this paper, a method based on a state observer is proposed, that overcomes all disadvantages of other methods because only voltages and current measurement is needed. For more accuracy, only rotor speed measurement is needed in addition. Introduction Nowadays, new technologies applied in the field of power electronics, computer science and control systems have produced great performance improvements in electric drives. In fact new high speed power devices are today available, microprocessor computational capability has considerably increased and new control approaches have been introduced. All these, applied on electric drives, caused a notable progress so that field oriented electric drives with very compact weight and size, excellent fatigue characteristics and fast dynamic response are commercially available today. Although drive regulation schemes have relevantly changed with respect to the past, variables to be regulated remained essentially the same: flux and torque. Since flux and torque are the regulated quantities, an estimate of the performances of an electric drive needs an accurate examination of flux and torque waveforms. Unfortunately flux and torque measurements, as will be explained, are not an easy matter. Flux measurement can be done inserting a certain number of coils inside the stator windings and taking the resulting voltage. This practice is yet afflicted by the following counterparts: Mechanical access difficulty, especially on small size machines and in already installed big machines. Air gap measurement only. Torque measurement is normally performed employing a sensor, that is often a strain gauge telemetry system, directly mounted on the rotor axle. This practice has yet the following counterparts: High noise level due to rotating part coupling. Calibration difficulties especially for high power machines. Presence of gear box that introduces hysteresis and low pass characteristics. High costs. For this reasons, flux and torque measurements don’t give today sufficiently accurate results and great efforts are directed towards the research of new methods. In this paper a precision low cost flux and torque measurement method is described, based on the technique of the state observer. As it is well known, in an electric motor, torque is a function of rotor and stator flux. The proposed method consists in estimating rotor and stator flux of an induction motor and then calculating the torque as the result of these estimations. State observer Considering a dq fixed stator reference frame, the induction motor model can be expressed with the following matrix equations. ø C i V ø A ø s ù (1)