06-12-2012, 11:59 AM
Adaptive Speed Identification for Vector Control of Induction Motors without Rotational Transducers
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Abstract
The paper describes a model-reference adaptive
system (MRAS) for the estimation of induction motor speed
from measured terminal voltages and currents. The estimated
speed is used as feedback in a vector control system, thus
achieving moderate bandwidth speed control without the use of
shaft-mounted transducers. This technique is less complex and
more stable than previous MRAS tacholess drives. It has been
implemented on a 30-hp laboratory drive, where its effectiveness
has been verified.
INTRODUCTION
MO DERN CONTROL techniques for ac motor drives were developed largely as a result of the search for
low-cost alternatives to high-performance four-quadrant
dc servo drives. In these applications, the use of shaftmounted
tacho generators and resolvers is established
practice, and the digital shaft-position encoder used in the
most effective vector-control schemes is considered acceptable.
Nevertheless, the shaft encoder does present
problems. Delicate optical encoders with internal signalconditioning
electronics are widely used. These lower the
system reliability, especially in hostile environments, and
require careful cabling arrangements with special attention
to electrical noise. There are also situations where
the positional feedback is extremely difficult to obtain.
This is particularly true for the case of linear-motor drives
such as for transportation vehicles. Finally, the encoder is
a cost factor since the provision of special motor-shaft
extensions and encoder-mounting surfaces leads to more
expensive machines.
Dynamic Response of MRAS Speed Identification
In general, the quantities Gr and or are time varying,
and each may be regarded as an input to a system described
by (2). In order to investigate the dynamic response
of the MRAS speed identifier, it is necessary to
linearize these equations for small deviations about a
particular steady-state solution. If this is done in a stationary
reference frame, the resulting linear equations will
still be time varying; therefore, it is useful to first transform
the equations to a reference frame rotating synchronously
with the stator current vector.
PRAC~ICACOL NSIDERATIONS
Use of Auxiliary Variables
In practice, the rotor flux observer based on (1) is
difficult to implement because of the pure integration of
sensed variables that is required. This leads to problems
with initial conditions and drift. The MRAS structure has
advantages in this regard because the model outputs need
not be the actual motor flux components but can be
auxiliary variables related to them. Although it is desirable
to eliminate the pure integrals, it is also desirable to
retain some measure of low-pass filtering in their place.
This helps to normalize the model outputs and attenuates
the high-frequency components normally found in the
motor terminal voltages. Fig. 10 shows two possible ways
of modifying the system of Fig. 5 to achieve this objective.
In each case, an identical linear transfer matrix is inserted
into both the reference and the adjustable model. This
block must appear in the output of the reference model
since the input cannot be altered, but in the case of the
adjustable model.