ABSTRACT:
Field orientation techniques
without flux measurements depend on the parameters of the motor, particularly
on the rotor resistance or rotor time constant. Since these parameters
change continuously as a function of temperature it is important the value
of rotor resistance be continuously estimated online. A fourth order sliding
mode flux observer is developed in this research. The observer does not
depend on the values of the rotor resistance or rotor time constant.
Two sliding surfaces representing combinations of estimated flux and current errors are used to enforce the flux and current estimates to their real values. Employing the use of Lyapunov
functions, the authors prove that the sliding surfaces can be forced to
zero in finite time by using to switching functions. The equivalent values
of the switching functions (low frequency components) are proven to be
the rotor resistance and the inverse of the rotor time constant. This later
property is used to simultaneously estimate the rotor resistance and inverse
of the time constant without prior knowledge of either the rotor resistance
or the magnetizing inductance. Furthermore, since the algorithm is quite
robust to the variation of leakage inductance, the only offline (or self
commissioned) estimation needed is for the stator resistance.
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