Julio C. Moreira

Indirect sensing for rotor flux position of permanent magnet AC motors operating over a wide speed range

This paper describes an indirect sensing, or sensorless, method for rotor flux position for brushless permanent magnet (BPM) motors operating over a wide speed range, while keeping maximum torque per ampere and/or maximum efficiency capabilities. The method described is particularly applicable to trapezoidal back emf type of BPM motors. The typical trapezoidal waveform of the motor internal voltages (or back emf) contains a fundamental and higher order frequency harmonics. In particular, the third harmonic component is extracted from the stator phase voltages while the fundamental and other polyphase components are eliminated via a simple summation of the three phase voltages. The resulting third harmonic signal keeps a constant phase relationship with the rotor flux for any motor speed and load condition, and is practically free of noise that can be introduced by the inverter switching, making this a robust sensing method. In contrast with indirect sensing methods based on detection of the back-emf signal that require heavy filtering, the third harmonic signal needs only a small amount of filtering to eliminate the switching frequency and its side bands. As a result, the method described here is not sensitive to filtering delays, allowing the motor to achieve a good performance over a wide speed range. Motor starting is also superior with this method since the third harmonic signal can be detected and processed at lower speeds than for the conventional method of back-emf sensing.

Modelling of saturated AC machines including air gap flux harmonic components

A new saturation model for induction machines is presented which can be easily extended to other types of AC machines. It is shown that saturation is responsible for the generation of flux space harmonic components traveling in the air gap with the same synchronous speed as the fundamental flux component with the third being the dominant harmonic component. Superposition of the effects of the fundamental and third-harmonic components of the air gap flux is utilized in order to model the saturation of the ferromagnetic parts of the machine. The concept of winding functions is used to derive the inductance terms relating both stator and rotor winding components. In this approach the air gap length is assumed to be variable, being a function of the position and level of the air gap flux. Terminal and torque values for steady and transient states are obtainable from the proposed model, with experimental results showing that the model predicts spatial saturation effects with good accuracy.<<ETX>>

Torque ripple minimization in switched reluctance motors via bi-cubic spline interpolation

A method for minimizing the instantaneous torque ripple in switched reluctance (SR) machines is investigated and implemented. The method is based on estimating the instantaneous SR motor torque from the flux linkage versus current and rotor position characteristic curves via a bi-cubic spline interpolation. These coefficients are computed offline, stored in a given memory location of the control processor, and used by two routines that are capable of estimating the rotor position and electromagnetic torque from the phase voltages and currents. The estimated output torque is then compared to a constant reference value, and the result of this comparison drives a current regulator that generates the proper motor phase currents. The ripple minimization scheme is simple and does not require a very fast processor. Its feasibility is confirmed via simulation and some preliminary experimental results.<<ETX>>

Simple efficiency maximizer for an adjustable frequency induction motor drive

A new method of efficiency maximization that utilizes sensing of the third-harmonic component of air-gap flux is proposed. This signal is used to determine the resulting instantaneous position of the fundamental component of the air gap flux and, consequently, the torque- and flux-producing components of the stator current. In addition, the third harmonic signal is also used to determine the rotor speed. Hence, the output power of the machine can be calculated with only a single sensor wire attached to the neutral point of the machine. The flux-producing component can be readily adjusted to produce the minimum input power for a fixed amount of output power (fixed speed). >

SIMULATION OF A FOUR PHASE SWITCHED RELUCTANCE MOTOR INCLUDING THE EFFECTS OF MUTUAL COUPLING

ABSTRACT Switched reluctance motors are typically constructed such that the coupling between phases is relatively small compared to conventional induction and synchronous machines. While small, this coupling is not negligible and could have important influence on transfer of current between motor phases during both normal and abnormal conditions. This paper presents, for the first time, a coupled circuit model which allows for mutual coupling between motor phases. Comparison is made between simulation and test results and good correlation is obtained.

Direct field orientation controller using the stator phase voltage third harmonic

A novel direct field orientation controller for induction machines based on the determination of the spatial position of the air gap flux from the third-harmonic component of the stator phase voltages is presented. The control utilizes spatial saturation harmonic components rotating at synchronous frequency which are generated in the air gap flux when the machine operates in a saturated condition. When the machine is wye connected, the sum of the three phase voltages results in a signal dominated by the third harmonic and a high-frequency component due to the rotor slot ripple. Extensive experimental results showing field-oriented operation of an induction machine are presented.<<ETX>>

A simple and robust adaptive controller for detuning correction in field-oriented induction machines

An adaptive controller for correcting the rotor time constant estimate used in the slip frequency calculator of indirect field oriented controllers is presented. The controller possess near digital deadbeat performance, and simultaneously maintains its correctness down to zero speed. The robust controller dynamics are based on an operating point form of digital deadbeat control. In this controller, an operating point approximate inverse model relates changes in the machine parameters to deviation in the rotor flux components. This allows nearly exact, one step (deadbeat) prediction of the correction necessary for the slip calculator gain.<<ETX>>

Low cost efficiency maximizer for an induction motor drive

A method of efficiency maximization is proposed which utilizes sensing of the third-harmonic component of the air gap flux. This signal is used to determine the resulting instantaneous position of the fundamental component of the air gap flux and, consequently, the torque and flux producing components of the stator current. In addition, the third-harmonic signal is also used to determine the rotor speed. Hence, the output of the machine can be calculated with only a single sensor wire attached to the neural point of the machine. The flux producing component can now be readily adjusted to produce the minimum input power for a fixed amount of output power (fixed speed).<<ETX>>

A new field oriented controller utilizing spatial position measurement of rotor end ring current

An approach to induction motor field-oriented control is proposed which utilizes measurement of the spatial position but not amplitude of the rotor end ring currents. Since drift problems caused significant errors in the amplitude measurement inherent in previous schemes, the method is considerably more robust than previously reported controllers. The motor torque can be accurately controlled even down to zero-frequency operation. The controller is completely independent of rotor-time-constant variations.<<ETX>>

A new direct field oriented controller employing rotor end ring current detection

A novel direct field-oriented control scheme for induction motor drive applications that employs rotor end ring current detection and removes the dependence of the controller accuracy on temperature is presented. This field orientation scheme does not require an incremental encoder for rotor position sensing. The motor torque can be accurately controlled even down to zero frequency operation. The controller is completely independent of rotor time constant variations. The rotor end ring current is measured via Hall-effect sensors. An adaptive controller for the rotor current gains eliminates the error introduced in the measurement due to temperature changes of the Hall sensors. Consequently, the field-oriented controller remains robust, even for conditions where the machine internal air temperature reaches 100 degrees C. Experimental results showing the practical feasibility of the proposed direct field-oriented controller are presented.<<ETX>>