Timothy J. E. Miller

Microcomputer Control of Switched Reluctance Motor

A microcomputer-based four-quadrant control system of a switched reluctance motor is described. The control was implemented with a speed feedback loop, a torque feedback loop, and both the torque and speed feedback loops combined. In addition the controller incorporates a startup operation, sequencing, and synchronized angle steering control. The angle controller was designed using dedicated digital hardware, whereas the other functions were implemented using an Intel 8751 single-chip microcomputer. The complete control system was tested in the laboratory with a 5-hp drive, and the test results were found to be excellent.

Converter Volt-Ampere Requirements of the Switched Reluctance Motor Drive

An algebraic nonlinear analysis of the switched reluctance drive system is described. The analysis is intended to provide an understanding of the factors that determine the kVA requirements of the electronic power converter and to determine the fundamental nature of the torque/speed characteristics. The effect of saturation is given special attention. It is shown that saturation has the two main effects of increasing the motor size required for a given torque, and at the same time decreasing the kVA per horsepower (i.e., increasing the effective power factor by analogy with an ac machine). The kVA per horsepower is lower than predicted by simple linear analysis that neglects saturation. Necessary conditions are also developed for a flat-topped current waveform by correctly determining the motor back-EMF. The reason why it is desirable to allow the phase current to continue (though with much reduced magnitude) even after the poles have passed the aligned position is explained. The theory provides a formula for determining the required commutation angle for the phase current. The basis is provided for an estimation of the kVA requirements of the switched reluctance (SR) drive. These requirements have been measured and also calculated by a computer simulation program.

Instantaneous Torque Control of Electric Motor Drives

In the control of adjustable speed drives, the performance of inexpensive digital integrated circuits is approaching the stage where traditional control algorithms may be displaced by newer algorithms that better exploit their speed and the functional capabilities of their software. This paper introduces the concept of “instantaneous torque control” as an objective worth pursuing in the application of such digital ICs to drive systems. Instantaneous torque control would in principle permit the fastest possible response and the elimination of torque ripple, along with many other advantages not possible with conventional control algorithms, most of which are set up to control a time-averaged torque. This paper develops some of the fundamental principles of instantaneous torque control for the switched reluctance motor, which is used as an example because, like the brushless dc permanent-magnet motor with concentrated windings, it has the potential for rapid response but it can have appreciable torque ripple with unfavorable firing angles. Neither of these machines satisfies the conditions for the existence of a reference-frame transformation that will eliminate the rotor position from the voltage and torque equations, and this opens up a number of interesting questions as to the generality of instantaneous torque control algorithms, and whether they can be incorporated into the general or unified theory of electrical machines.

Single-Phase Permanent-Magnet Motor Analysis

A unified theoretical approach is presented for the calculation of the performance of the single-phase capacitor-start permanent-magnet (PM) ac line-start motor. The earlier work of Chang and of Finch and Lawrenson on capacitor reluctance motors is extended. As far as possible, a common set of reference frame transformations for steady-state, transient, and asynchronous performance is also provided, permitting the use of all the theory developed for the analysis of the balanced polyphase PM motor. A new approximate technique is given for estimating the average asynchronous torque/speed curve from computed acceleration curves.

A Review of the Integral-Horsepower Switched Reluctance Drive

An evaluation of the capabilities of the switched reluctance (SR) motor drive, particularly in small integral-horsepower sizes, is presented, and some of its special features are discussed. The simplicity of the construction of the rotor together with certain advantages in the power circuit, such as unipolar operation and the independence of the phases, are described along with some of the important performance parameters, which are compared with those of typical induction motor drives. It is shown that the ruggedness and simplicity of the SR drive are accompanied by a performance profile that matches that of modern induction motor drives in torque per unit volume, efficiency, converter voltampere requirements, and other parameters. A comparison of three SR motors, including one low-inertia design and one with two stator teeth per pole, shows torque/inertia ratios several times greater than those for induction motor drives.

Robust Torque Control of Switched-Reluctance Motors Without a Shaft-Position Sensor

This paper describes a technique for stabilizing the operation of variable-reluctance stepping-motor drives operating without a shaft-position sensor. In such systems there is a trade-off between the system efficiency and the torque margin (or pull-out torque) which depends on the width of the phase conduction pulse width. The scheme described in the paper permits the motor to run in the steady state with both narrow conduction pulse widths and high efficiency. Under transient or overload conditions the conduction pulse width is increased in response to a change in the dc link current, providing an increase in available torque. Tests on a small motor drive have produced a steady-state torque margin of over 300 percent and of 200 percent under step-change conditions.

The Ferrite Permanent Magnet AC Motor---A Technical and Economical Assessment

The design and analysis study for the conceptual design of an economical high-efficiency ac motor based on permanent magnets is summarized. Material considerations, design trade-off options as well as transient and steady-state performance under normal and abnormal conditions have been considered. The baseline comparison is the high-efficiency induction motor. The permanent magnet (PM) motor must fit into the same frame size and surpass the induction motor on a life cost basis that includes 2.5 years of operation at a 50 percent duty cycle. The study results indicate that a motor with ferrite magnets meets the objectives of the program in ratings of up to 25 hp. A 7.5-hp motor design is carried through the conceptual design stage.

Simplified Electronics for Torque Control of Sensorless Switched-Reluctance Motor

Sensorless control of the switched-reluctance (SR) stepping motor using pulsewidth modulation was introduced in an earlier paper by the authors. This scheme senses the dc link current and permits the motor to operate efficiently with a narrow pulsewidth under normal conditions. Changes in load are sensed by the dc link current, and the pulsewidth is adjusted appropriately. This paper describes the electric implementation of the controller.