Method and Apparatus for Estimating Rotor Position in a Sensorless Synchronous Motor

Abstract

The present invention provides a simple, robust, and universal position observer for use with sensorless synchronous machines. The observer may be implemented using an equivalent EMF model of a synchronous machine or, alternately, using a sliding mode controller based on the equivalent EMF model of the synchronous machine. The observer may be used on any type of synchronous machine, including salient or non-salient pole machines such as a permanent magnet, interior permanent magnet, wound rotor, or reluctance synchronous machine. The observer provides low sensitivity to parameter variations and disturbances or transient conditions in the machine. In addition, no knowledge of speed is required as an input to the observer and an estimated position may be calculated using a subset of the machine parameters.

Description

BACKGROUND OF THE INVENTION[0001]The subject matter disclosed herein relates to control of electric motors and, in particular, to estimating the position of a rotor in a synchronous motor without a position feedback device.[0002]Synchronous motors are electric motors having a magnetic field established in both the rotor and the stator of the motor. The magnetic field in the rotor is typically generated by passing a DC current through slip rings to the winding of the rotor. Alternately, permanent magnets may be mounted on or in the rotor to provide a continuous magnetic field in the rotor. The magnetic field in the stator is typically generated by applying an AC voltage to the winding of the stator at a frequency that matches the frequency of rotation of the magnetic field in the rotor. As a result, knowledge of the position of the rotor is important for high performance control of the synchronous motor.[0003]It is known that high performance control of synchronous motors can be achi...

AI Technical Summary

Benefits of technology

[0008]The subject matter disclosed herein describes a simple, robust, and universal position observer for sensorless control of both salient and non-salient pole synchronous machines. The observer may be implemented using an equivalent EMF model of a synchronous machine or, alternately, using a sliding mode controller based on the equivalent EMF model of the synchronous machine. The observer may be used on any type of synchronous machine, including salient or non-salient pole machines such as a permanent magnet (PM), interior permanent magnet (IPM), wound rotor, or reluctance synchronous machine. The observer provides fast convergence and low sensitivity to parameter variations, disturbances, or transient conditions in the machine. In addition, no knowledge of speed is required as an input to the observer and the estimated position may be calculated using a subset of the motor parameters, which reduces sensitivity to variations or inaccuracies in stored motor parameters.

[0013]The q axis inductance may be obtained from a table of q axis inductance values that are a function of the current drawn by the synchronous machine. It is still another feature of the present invention that the model compensates for variations in the q-axis inductance that are dependent on the amount of current drawn by the machine, reducing the sensitivity of the equivalent EMF model to parameter variations.

[0017]It is, therefore, another feature of the invention to provide a robust, bang-bang type controller wherein the bang-bang controller toggles between an upper and a lower maximum value in response to the sign of the input to the controller. The controller provides quick convergence and simple implementation.

Problems solved by technology

The mechanical coupling to the shaft of the rotor and the electrical connection to the motor controller result in additional material and installation expense.

The encoder further creates an additional source of failure and subsequent maintenance expense within the system.

The wide variety of synchronous machines, therefore, presents a significant challenge for developing a universal position observer capable of providing an accurate rotor position estimate for each type of synchronous machine.

However, back EMF models for many types of synchronous motors are subject to a fundamental limitation: these motor models are speed dependent.

The drive may erroneously conclude that the motor is operating at zero speed when, in fact, the motor is rotating at a low speed.

Consequently, back EMF observers can perform well at high speed but are not suited for operation at low or zero speed.

These observers may not be suitable for all types of synchronous motors.

Other models make assumptions that are not valid for all operating conditions.

While such assumptions may be valid under constant operating conditions, the assumptions are not always valid during transient conditions, such as start-up or when the load on the motor changes.

Consequently, these other position observers have not provided acceptable performance over a broad range of motors and operating conditions.

Signal injection techniques typically have poor dynamic response.

As a result, signal injection predicts rotor position best at low or zero speed and is not suited for operation at high speed.

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