Rotor Time Constant Determination
of
Induction Motor Drives
Via Steady-State Torque Analysis
E. Cermto,
M.
Coco,
A.
Consoli,
A.
Raciti,
A.
Testa
Dipartimento Elettrico Elettronico e Sistemistico
Universita' di Catania
Viale Andrea Doria.
6
-
95
125
-
Catania, Italy
Abstract
-
The paper presents
a
simple and
straightforward way to calculate the rotor time constant
of an Induction Motor by analysing the steady-state
electromagnetic torque. The main advantage of the
proposed approach consists in the simple
implementation, moreover it can
be
performed either off-
line, as
a
tool
to
immediately calculate the rotor time
constant of an asynchronous motor, either on line, to tune
a F.O.C. drive. The only constraint is that a constant
load is required during the calculation procedure that
takes few seconds. This is not a problem in laboratory
measurements nor generally in a large number of
applications in which the load is strictly related to the
speed, nor in servodrives where constant load operation
are usually scheduled during the working cycles. The
paper starts with a theoretical development of the new
rotor time constant calculation procedure, followed by
a
validation by mean of simulation and experimental tests.
INTRODUCTION
The Field Oriented Control (FOC) allows to
independently control the torque and the flux of an
asynchronous motor, by acting on the d,q components of
the stator current in a rotating reference frame aligned
with the rotor flux vector. Consequently FOC requires
knowledge of the actual angular position of the rotor
flux. In fact only by correctly aligning the stator current
vector with respect to the rotor flux, it is possible to
independently control the torque and the flux of an
induction motor, by acting on two separate components
of the stator current. Two different techniques has been
developed in order to calculate the actual position of the
rotor flux:
the Direct Field Oriented Control (DFOC) approach,
according to which the position of the rotor flux vector
is computed from stator current and voltage;
the Indirect Field Oriented Control (IFOC) technique
that predicts the position of the rotor flux by mean of a
simple expression derived from the model of the
induction motor, under the hypotheses of a perfect field
orientation and of
a
constant rotor flux.
The, use of DFOC drives is practically restricted to
those applications that do not require low speed
operations, such
as
pumps and spindles.
At the contrary
the IFOC is today
a
well established technique of
regulation that, performing
a
fast and fully decoupled
control of torque and flux even at zero speed, is widely
applied to all kinds of high performance induction motor
drives suitable for machine tools, robotics and servo
systems.
The IFOC technique is essentially an opcn loop
predictive approach that computes an expected position
of the rotor flux exploiting
a
suitable model of the
machine. This made such
a
control technique very
sensitive to variations in motor parameters
[l].
In fact
although
a
properly tuned IFOC drive can provide
dynamic performances similar to those of equivalent DC
or
brushless motor drives, an incorrect rotor time
constant causes
a
misalignment between the stator
current and rotor flux, noticeably reducing the overall
system performances. In fact
a
sluggish torque response
is obtained during transients while at steady state a
mistuned drive shows its flux amplitude different from
the reference one. This results in an increased
saturation effect if the flux amplitude is increased,
or
in
a
lower available torque if the flux amplitude is reduced.
An accurate tuning procedure is therefore usually
required before to run any IFOC drive. Moreover an on
line adaption of the system to the variations of the rotor
time constant with the temperature, is often required in
order to maintain constant the performances of the
system. As the IFOC approach is the only that allows to
easily realize high performance drives using reliable and
low cost induction motors, in the last fifteen years
several research efforts have been spent to overcome the
problems related either to the initial tuning of the
system, either to the compensation of rotor time
constant changes during run time. Different approaches
[2][3] have been proposed
to
initially tune an
IFOC
drives considering for example the effects of suitable test
signals or observing the dynamical behaviour of the
drive in open loop speed tests. Several techniques
[4][5][6][7] have been also developed to identify the
rotor time constant during run time using respectively:
suitable test signals, state observers, Kalman filters,
adaptive control, reactive power
or
third harmonic
voltages. All of these approaches need normally
additional sensors that are not strictly required in the
original IFOC scheme thus increasing costs and
complexity. Moreover some of them introduce very
complex methodologies that cannot be practically used,
or
properly work only in limited ranges
of
the load. the
operating frequency
or
the saturation level.
0-7803-1772-6/94/$3.00
@
1994 IEEE
1318