Chapter 7

Observation of the Load Torque of an Electrical Machine 1

7.1. Introduction

The requirements on speed and position control for electro-mechanical actuators have a strong impact on the control loops of variable speed drives. The electromagnetic torque must be perfectly controlled to obtain the most satisfying evolutions in speed or position. Regardless of the care we take in the construction of the electrical machine and the power quality of this machine, the resulting electromagnetic torque very often contains a ripple. It is increased in so far as electromotive forces are not sinusoidal and/or the relation between decoupling frequency and fundamental frequency is too low. This torque ripple is visible in the speed and position behavior of the machine, notably at low speed.

In addition, the resulting mechanical loads often have undefined characteristics in terms of the low-speed neighborhood or even zero or quasi-zero speeds. The variable speed drive’s control in these specific operation zones can turn out to be problematic and its traditional control laws are often wrong. The study proposed in this chapter contributes to this theme, by developing a solution for the improvement of the global actuator behavior.

The major solution considered is based on the association of a powerful control method based on speed and position state feedback presumed measurable here and a method of reconstruction of the total load torque. The main contribution involves the torque observer ...

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