Electricity: motive power systems – Induction motor systems
Reexamination Certificate
2000-06-19
2002-01-01
Donels, Jeffrey (Department: 2837)
Electricity: motive power systems
Induction motor systems
Reexamination Certificate
active
06335605
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vector controller for an induction motor and, more particularly, to a vector controller for an induction motor that is capable of automatically adjusting a set value of a secondary resistance of the induction motor, namely, a resistance of a rotor of the induction motor.
2. Description of the Related Art
In general, vector control has been extensively used in industrial fields as a method for quick control of an output torque of an induction motor. The following will briefly describe the vector control.
The vector control is carried out to independently control a torque and a secondary magnetic flux of an induction motor by representing a current or magnetic flux of a three-phase induction motor in terms of a vector of a coordinate system known as a d-q coordinate system. The d-q coordinate system is a rotating coordinate system with two orthogonal axes that rotate in synchronization with a power source, one of the axes being taken in a direction of a secondary magnetic flux.
In the vector control, a torque current command value IQR, a magnetic flux current command value IDR, and a slip angular frequency command value &ohgr;s* are computed according to the following expressions (1) through (3) using a torque command value T*, a secondary magnetic flux command value &PHgr;
2
* and a motor constant. A method for deriving the expressions is well known, and described in, for example, “Vector Control of AC Motor” by Takayoshi Nakano, published by Nikkan Kogyo Shimbunsha; therefore, the description of the method will be omitted herein.
IQR
=
T
*
Φ2
*
×
1
P
×
L2
M
(
1
)
IDR
=
Φ2
*
M
(
2
)
ω
s
*
=
IQR
IDR
×
R2
L2
(
3
)
where
P: Number of pairs of poles of motor
M: Mutual inductance of motor (H)
L
1
: Primary self inductance of motor (H)
L
2
: Secondary self inductance of motor (H)
R
2
: Secondary resistance value of motor (&OHgr;)
Thus, in the vector control, the slip angular frequency command value &ohgr;s* is computed according to expression (3) to conduct the control. Expression (3) includes the secondary resistance value R
2
of the motor. The value of R
2
varies with changes in an ambient temperature or temperature changes caused by heat generated by the induction motor itself. Therefore, for the value of R
2
employed for the computation in accordance with expression (3), a value corrected by taking into account a predicted change of R
2
caused by a temperature change must be used.
As a known vector controller that takes such a secondary resistance correction into account, there is one disclosed in, for example, Japanese Patent Laid-Open No. 6-343282.
FIG. 12
is a block diagram showing a configuration of the known vector controller. The vector controller shown in
FIG. 12
includes a vector control unit
49
for controlling an induction motor
55
to be controlled according to a secondary magnetic flux command &PHgr;
2
* and a torque command TM*, a waveform analyzing unit
50
that receives an induction motor revolution angular velocity &ohgr;r (hereinafter referred to simply as “angular velocity &ohgr;r) and performs a waveform analysis on the angular velocity or, a parameter adjusting unit
51
for adjusting a parameter (a set value of the secondary resistance in this example) according to an output of the waveform analyzing unit
50
, a subtracter
52
that subtracts the angular velocity &ohgr;r from a velocity command &ohgr;r* to compute a velocity deviation, and a velocity controller
53
that outputs the torque command value TM* based on a difference between the velocity command &ohgr;r* and the angular velocity &ohgr;r determined by the subtracter
52
so that the angular velocity &ohgr;r follows the velocity command &ohgr;r*. The vector controller further includes a power converting unit
54
that controls a primary current value I
1
according to a primary current command value I
1
* output from the vector control unit
49
, the induction motor
55
to be controlled that rotates at a predetermined velocity and a predetermined torque according to the primary current value I
1
, a velocity detector
56
that detects the angular velocity &ohgr;r of the induction motor
55
, and coefficient setters
57
and
58
for a secondary resistance R
2
installed in the vector control unit
49
. The following will describe an operation of the related art based mainly on a secondary resistance correction method.
In the related art, a signal that has been step-changed to the velocity command &ohgr;r* is input to perform computation for correcting the secondary resistance. A waveform of the angular velocity or when the velocity command &ohgr;r* has been step-changed is saved in the waveform analyzing unit
50
, and a feature quantity of a response waveform is calculated. The feature quantity calculated by the waveform analyzing unit
50
is supplied to the parameter adjusting unit
51
to calculate a correction amount of the secondary resistance set value R
2
so as to correct a set value of the secondary resistance R
2
set at the coefficient setters
57
and
58
.
FIG. 13
illustrates a configuration example of the waveform analyzing unit
50
. A waveform of the angular velocity &ohgr;r is sampled by a sample holding circuit
501
and saved in a memory
502
, then a feature quantity is calculated by a microprocessor
503
. An example of a specific characteristic value employed as the feature quantity and a method for determining the specific characteristic will be discussed later in detail.
A configuration example of the parameter adjusting unit
51
is constituted by a microprocessor and a memory similarly as in the case of the waveform analyzing unit
50
shown in FIG.
13
. The figure will be omitted because it is identical to
FIG. 13
except for the absence of the sample holding circuit. In this case, an adjustment rule based on a feature quantity is stored in the memory.
Regarding the adjustment of the secondary resistance, the adjustment rule decides a feature quantity to be calculated by the waveform analyzing unit
50
and how a correction amount of the secondary resistance is determined by the parameter adjusting unit
51
by employing the feature quantity. An example of the adjustment rule will be described in conjunction with FIG.
14
.
FIG. 14
shows simulation results illustrating influences exerted by an erroneous setting of the secondary resistance R
2
on a velocity step response waveform. The response waveform is also subjected to influences of a transfer function of the velocity controller
53
. Hence, for the purpose of simplicity, in the response waveform of
FIG. 14
, the transfer function of the velocity controller
53
includes only a proportion factor.
When a true value of the secondary resistance R
2
is denoted as R
2
*,
FIG. 14A
illustrates a case wherein the value of the secondary resistance R
2
set in the vector controller is equal to the true value R
2
* (R
2
=R
2
*). In this case, the torque command TM*, which is an output of the velocity controller
53
, and an actually generated torque TM of the induction motor
55
are equal. Therefore, a transfer function of the velocity &ohgr;r of the induction motor
55
with respect to the torque command TM* will be determined by the following expression (4), where J denotes a moment of inertia of the induction motor
55
, and S denotes a Laplacean.
(&ohgr;
r/TM*
)=(1/
J·S
) (4)
Therefore, when a proportion gain of the velocity controller
53
is denoted as GP, a closed loop transfer function G&ohgr; of the velocity &ohgr;r in relation to the velocity command &ohgr;r* will be determined by the following expression:
G
&ohgr;=(&ohgr;
r/&ohgr;r*
)=[
GP/J·S]/[
1
+GP/J·S]=
1/[1+(
J/GP
)
S]
(5)
The above expression (5) represents a transfer function of a primary delay factor having a time constant expressed as J/GP, and a step response waveform thereof will be represented by th
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