Vector control method of induction motor

This invention provides a vector control method for controlling an induction motor that is used to drive an elevator based on secondary time constants, wherein the secondary time constants, which have been corrected for temperature, are determined in a short amount of time and the precision of the determination is improved, by passing a constant current of the same phase as during a similar operation to the excitation side of the induction motor while the elevator is mechanically stopped and then determining the secondary time constants.

TECHNICAL FIELD 
This invention relates to a vector control method whereby an induction 
motor is used to drive an elevator based on secondary time constants. In 
particular, it pertains to a vector control method with which the 
precision of the secondary time constant determination is improved. 
BACKGROUND ART 
Elevators may be driven by induction motors. 
By means of vector control of induction motors, the primary current is 
divided into exciting current and torque current and the vector of the 
torque current and flux are crossed in order to obtain the same speed 
variation capability as with direct-current motors or better. 
FIG. 3 is a diagram of one example of a vector control device. The numeral 
1 in the figure is an induction motor and 2 is a speed detector. A speed 
computing section 3 is connected to the speed detector 2. Speed W.sub.n 
that is computed from the detected values by the speed computing section 3 
and speed command N are balanced in a summer 3a and the difference into a 
torque computing section 4. A torque current command (I.sub.t) is 
determined by a proportional integration (PI) computation. A primary 
current value (I.sub.1) is determined from this torque current (I.sub.t) 
and an exciting current set value (I.sub.o) by primary current computing 
section 5 using I.sub.1 =.sqroot.I.sub.t.sup.2 +I.sub.o.sup.2. On the 
other hand, the phase angle .phi. of the torque current (I.sub.t) and 
exciting current (I.sub.o) is computed by phase computing section 6 using, 
.phi.=tan.sup.-1 (I.sub.t /I.sub.o). Slip frequency computing section 7 
determines the slip frequency (W.sub.s) from torque current (I.sub.t), 
exciting current (I.sub.o), and secondary time constant .tau..sub.2 of the 
motor as shown below; furthermore, time constant .gamma.2=L2/R2 for 
secondary auto-inductance L2 and secondary resistance R2. 
EQU W.sub.2 =I.sub.t (I.sub.o .multidot..tau..sub.2) 
This slip frequency (W.sub.s) is added to the speed detected value 
(W.sub.n) by adding device 8 to obtain the primary frequency (W.sub.o). 
The aforementioned primary current (I.sub.1), phase angle .phi. and angular 
frequency (W.sub.o) are input to a three-phase current computing section 9 
to calculate primary currents I.sub.a, I.sub.b, and I.sub.c of motor 1. 
Using this current as the current command of inverter 10, primary current 
is fed to motor 1 by inverter 10. Computation by aforementioned 
three-phase current computing section 9 is performed by the following 
equations: 
##EQU1## 
By means of this type of vector control device, secondary time constant 
.tau..sub.2 for determining slip frequency W.sub.s is determined by 
constants L2 and R2 of the motor and changes in secondary resistance R2 
with temperature of the motor are regarded as changes in secondary time 
constant .tau..sub.2 and the primary current phase. Therefore, secondary 
time constant .tau..sub.2 is corrected for temperature by computation from 
the primary voltage when a constant current is passing to the motor. FIG. 
4 shows a graph of changes in primary voltage V.sub.1 (t) when constant 
current i.sub.l is flowing. Primary voltage V.sub.1 (t) is approximated by 
Therefore, secondary time constant .tau..sub.2 is found with the following 
equation by determining primary voltages V.sub.1 (t.sub.1) and V.sub.1 
(t.sub.1) at times t.sub.1 and t.sub.2. 
##EQU2## 
The effects of temperature can be eliminated by using this secondary time 
constant .tau..sub.2 in computations of slip frequency. 
In the aforementioned conventional method for correction of the secondary 
time constant, the steady-state values of i.sub.1 and r.sub.1 at primary 
voltage V.sub.1 (t) are needed for determination of the secondary time 
constant and it takes a long time until primary voltage V.sub.1 (t) 
reaches the steady-state level in these determinations. Consequently, 
there is a problem in that it takes a long time to prepare for operation 
of the motor and a control device is obtained whose use is off to a bad 
start. 
DISCLOSURE OF INVENTION 
This invention was devised in light of the problems previously described 
and its purpose is thus to present a vector control method for an 
induction motor with which the secondary time constant corrected for 
temperature is determined in a short amount of time and the precision of 
the determination is thereby improved. 
According to the present invention, the precision of the determination of 
the secondary time constant .tau..sub.2 is improved by passing a current 
which stabilizes the excitation state of the induction motor. 
Fluctuations in phase can be avoided and the precision of the determination 
can be improved by passing a constant current of the same phase as when 
operation is initiated for a specific amount of time while the axle of the 
motor is mechanically stationary, either immediately after the elevator 
has stopped or immediately before it is started by the operation command. 
The theory behind induction motors is dislocation based on phase, but as 
long as the axle is mechanically stationary, errors will not occur, even 
when the phase is different from the stop phase. 
Our solution for solving the aforementioned problems is a vector control 
method and apparatus for an induction motor whereby by means of vector 
control of an induction motor used to drive an elevator based on its 
secondary time constant, wherein the secondary time constant is determined 
by passing constant current of the same phase as during the following 
operation to the excitation side of the induction motor while the elevator 
is mechanically stopped. It is preferred that the controls be used 
immediately after the elevator has been stopped or immediately before it 
has been started as the timing at which the constant current flows. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the following detailed description 
of a best mode embodiment thereof, as illustrated in the accompanying 
drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Therefore, according to the present invention, a device shown in FIG. 5 has 
been presented for vector control with correction for temperature by 
determination of the secondary time constants in a short amount of time. 
In FIG. 5, control circuit 11 corresponds to computing sections 4 through 
9 in aforementioned FIG. 3 and inverter body 12 and rectifier 13 
correspond to inverter 10. Primary voltage detecting circuit 14, according 
to our invention, is connected to the primary side of induction motor 1 
and secondary time constant computing circuit 15 is connected to this 
primary voltage detecting circuit 14. Primary voltages V.sub.1 (t.sub.1), 
V.sub.1 (t.sub.2), and V.sub.1 (t.sub.3) and determination interval 
.DELTA.t (=t.sub.2 -t.sub.1, t.sub.3 -t.sub.2) from primary voltage 
detecting circuit 14 are input to secondary time constant computing 
circuit 15. Secondary time constant .tau..sub.2 is calculated and output 
to control circuit 11. During computation of the secondary time constants, 
control circuit 11 feeds back the detected values of a current detector 16 
and controls the on-off ratio of the switch transistors of inverter body 
12 in order to apply constant voltage to motor 1. When this constant 
voltage is applied, primary voltage detecting circuit 14 detects primary 
voltages V.sub.1 (t.sub.1), V.sub.1 (t.sub.2) and V.sub.1 (t.sub.3) at 
times t.sub.1, t.sub.2, and t.sub.3 of constant time interval .DELTA.t, as 
shown in FIG. 6, and secondary time constant .tau..sub.2 is calculated by 
secondary time constant computing circuit 15 using the following equation: 
EQU .tau..sub.2 =.DELTA.t/(ln.multidot.V.sub.1 ((t.sub.1 -t.sub.2)/(t.sub.2 
-t.sub.3))) 
Moreover, when the aforementioned constant voltage is applied, primary 
currents i.sub.1 (t.sub.1), i.sub.1 (t.sub.2) i.sub.1 (t.sub.3) at times 
t.sub.1, t.sub.2, and t.sub.3 of constant time interval .DELTA.t can be 
detected, as shown in FIG. 9, and secondary time constant .tau..sub.2 can 
be calculated from the following equation: 
EQU .tau..sub.2 =.DELTA.t/(ln{i.sub.1 ((t.sub.2 -t.sub.1)/(t.sub.3 -t.sub.2))} 
In either case, calculation of secondary time constant .tau..sub.2 can be 
determined in a relatively short amount of time of 2.DELTA.t and it does 
not take a long time to determine steady-state values i.sub.1 and r.sub.1. 
Nevertheless, although the phase of the current when operation of the 
induction motor starts is usually constantly controlled, the phase when 
the induction motor used to drive elevators stops is not constant. That 
is, FIG. 8 is a waveform diagram showing an example of the current phase 
in elevator control and, as shown in this figure, when the elevator brakes 
are operated, the elevator stops at desired time T.sub.1 and control 
"cutoff" occurs after a short amount of time, but there may be 
fluctuations in the stopping phase at this time, as shown by the broken 
curve in the figure. When controls are initiated during the subsequent 
operation, preexcitation at desired time T.sub.2 occurs and then the 
elevator is operated once again. 
Consequently, when constant current i.sub.1 is passed to the induction 
motor when the following operation is started, the primary voltage 
EQU V.sub.1 (t)=i.sub.1 (r.sub.1 +r.sub.2 e-t/.tau..sub.2) 
is determined, and secondary time constant .tau..sub.2 is determined, 
fluctuations in phase produce fluctuations in the waveform of voltage 
V.sub.1 (t) and there is no precision of detection of secondary time 
constant .tau..sub.2. 
Examples of the method of this invention will be described in detail while 
referring to the figures below. 
By means of this invention, a current that stabilizes the excitation state 
is passed in order to improve the detection frequency of secondary time 
constant .tau..sub.2 for repetitive starting operations of an induction 
motor for driving an elevator with the aforementioned type of control 
system. 
FIG. 1 is a waveform diagram of an example of a method according to this 
invention. By means of this example, the control device for determination 
of secondary time constant .tau..sub.2 when the following operation begins 
is operated as shown below after the elevator has stopped and before 
control "cut" occurs (e.g., "end control" of FIGS. 1 and 8) occurs. 
(1) The axle of the motor becomes stationary by operation of mechanical 
brakes when the elevator is stopped. 
(2) Constant current i.sub.1 of the same phase as when operation is started 
is passed for a selected amount of time. This time as shown after motion 
has stopped, in FIG. 1(a) may be determined from the time constant of the 
motor. Even if the phase of the current i.sub.1 ' varies from the stop 
phase, the axle is mechanically stationary, and therefore, there are no 
errors. However, fluctuations in voltage V.sub.1 (t) at this time will 
occur with differences in phase, as shown by dashed lines in FIG. 1(b). 
(3) The next operating cycle begins after a period of time has passed. 
Voltage V.sub.1 (t) at constant current i.sub.1 is determined and 
secondary time constant .tau..sub.2 is computed, as previously described 
in connection with FIGS. 4, 6, 7 and 8. The specific time may be as long 
as several seconds and should not pose problems in terms of the control 
sequence, such as the opening/closing sequence of elevator doors, etc. 
(4) The effects of temperature are thereafter eliminated using the 
calculated secondary time constant .tau..sub.2 in computations of vector 
control. 
FIG. 2 shows the waveform diagram of another example of this invention. By 
means of this example, the control device is operated immediately before 
the elevator is started in order to prevent errors in detection of 
secondary time constant .tau..sub.2 that are produced due to differences 
in current phase when the previous operation stopped. 
(1) Elevator control-initiating signals are applied. 
(2) Constant current i.sub.1 ' of the same phase as when operation is 
started is passed for a selected amount of time with the brakes being 
mechanically applied. This time may be determined from the motor time 
constant, as in the other example. Furthermore, there may be variations in 
voltage V.sub.1 '(t) at this time due to the phase difference when the 
elevator is stopped. 
(3) The desired current cut off time is determined from the time constant 
of the elevator. 
The next operation cycle begins and .tau..sub.2 is determined, as before. 
As previously explained, by means of this invention, a vector control 
method for an induction motor is presented with which the secondary time 
constant that has been corrected for temperature is determined in a short 
amount of time and the precision of determination is improved. 
Although the invention has been shown and described with respect to an 
exemplary embodiment thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the invention.