Electricity: motive power systems – Induction motor systems
Reexamination Certificate
2001-08-17
2003-03-04
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
C318S798000, C318S799000, C318S609000, C318S610000, C318S801000, C318S805000, C363S013000, C363S037000
Reexamination Certificate
active
06528966
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vector control apparatus, and more particularly, to a sensorless vector control apparatus and method for controlling a variable speed operation and speed of an induction motor.
2. Description of the Background Art
In general, an induction motor has been primarily used for a constant speed operation as it is more difficult to control in comparison to a DC motor. However , as a vector control theory is introduced and a high performance central processing unit (CPU) or digital signal processor (DSP) is developed, the induction motor is now capable of being controlled for a variable speed operation.
The vector control theory is a method in which three phase AC powers (‘a’ phase, ‘b’ phase and ‘c’ phase) inputted at 120° intervals are disassembled (converted) by a direct axis and a quadrature axis of 90° intervals, and their size are controlled to a desired value and restored (inverted) to the three phase powers, to thereby control the three AC powers. This method is mainly used to control the induction motor.
In order to vector-control the induction motor, the speed or magnetic flux information of the induction motor is required. In order to measure the speed or the magnetic flux information, a speed sensor or a magnetic flux sensor such as a Tacho generator, resolver or a pulse encoder is required.
However, since the sensors include an electronic circuit, the induction motor having the sensors must be used within the operable temperature range of the electronic circuit, and signal wiring between the speed sensor and the inverter incurs much expense.
Also, the use of sensors is avoided as the connections between the sensors and the induction motor are susceptible to damage upon impact of the motor.
Accordingly, recently, various speed estimation methods of the induction motor has been proposed with respect to the sensorless vector control without the speed sensor. Among them, as a high speed algorithm method, a method based on a model reference adaptive system, an adaptive observer, that is, a method for estimating a speed or a slip frequency independently from a main control system, is used for consideration of a stability of a speed estimation, and as a low speed algorithm method, a high frequency injection method is used.
FIG. 1
is a schematic block diagram of a speed control apparatus which supplies a synchronous speed according to a voltage to frequency method in accordance with the prior art.
As shown in
FIG. 1
, the conventional speed control apparatus includes an angular velocity generator
1
receiving a command frequency (F) by a user's input, converting it to an electric angular velocity (We) to be applied to a motor, and outputting it; a voltage generator
2
receiving the command frequency (F), generating a voltage (V) according to a Voltage to Frequency ratio(V/F ratio), and outputting it; and an inverter
3
controlling a speed of an induction motor (IM) by using the electric angular velocity (We) outputted from the angular velocity generator
1
and the voltage outputted from the voltage generator
2
.
The operation of the conventional speed control apparatus constructed as described above will now be explained.
Generally, in an industrial site, the speed detecting unit is not required, and instead, a common inverter of a variable voltage variable frequency (VVVF) method which is simply controlled is widely used.
In order to constantly maintain a flux of the induction motor, the common inverter constantly controls a ratio between an output voltage of the inverter and an output frequency (V/F=constant), and a synchronous speed (rpm(rotation per minute)) of a rotational magnetic field is controlled by varying the output frequency.
Synchronous speed (rpm)=120*F/P (1)
wherein ‘P’ indicates the number of poles of a stator winding and ‘F’ indicates a command frequency of a current flowing at the stator winding.
Input voltage (Vs) is determined as follows:
Vs=Rs*Is+(Lls+Lm)*dls/dt (2)
wherein ‘Rs’ indicates a stator resistance, ‘Is’ indicates an input current of the induction motor’, ‘Lls’ indicates stator leakage reactance, and ‘Lm’ indicates a magnetized reactance.
In the case where the stator resistance (Rs) of the inductor motor is not used in equation (2), the equation becomes:
Vs=(Lls+Lm)*dls/dt (3)
Generally, the stator leakage reactance (Lls) is relatively small compared to the magnetized reactance (Lm) in equation (3). Accordingly, equation (3) is computed by equation (
4):
Vs=Lm*dls/dt=We*Lm*ls=2&pgr;F*&phgr; (4)
In equation (4), since Vs/F=2&pgr;F*&phgr;, by constantly providing the ratio of Vs/F, the motor can be controlled while constantly maintaining the flux.
Accordingly, when the command frequency (F) is determined, it is converted to a synchronous speed (We=2&pgr;F) and applied to the induction motor. At this time, in order to constantly maintain the flux of the induction motor, a voltage is generated corresponding to the command frequency (F) so that the V/F ratio is constant, and outputted to the inverter.
Then, the inverter generates three phase voltages by using the synchronous speed (We) and a voltage and supplies them to the induction motor (IM). That is, if the ratio of the V/F is constantly provided, since the flux is constantly maintained, the induction motor can be controlled.
In this respect, since the induction motor is rotated at a slower speed than the synchronous speed, a slip is obtained by the following equation (5):
Slip=(We−Wr)We
Wherein ‘We’ indicates a synchronous speed and ‘Wr’ indicates a speed of the induction motor.
FIG. 2
is a graph showing a slip-torque curve wave form of a load and a motor according to the V/F method of the conventional art.
As shown in
FIG. 2
, the induction motor is operated at an intersection point of the load and the slip-torque curve of the induction motor, and a corresponding current flows.
FIG. 3
is a schematic block diagram of the vector control apparatus in accordance with the conventional art.
As shown in
FIG. 3
, a vector control apparatus having an inverter for receiving a speed command value (wr*) from a user and supplying three phase currents required for an induction motor, including: a first proportional integrator
5
for receiving an error between the speed command value (wr*) inputted from a user and a speed (wr) actually detected from an induction motor and generating a current command value of ‘q’ axis component (iqse*); a second proportional controller
8
for receiving an error, that is difference between the current command value of ‘q’ axis (iqse*) according to a rating of a motor and an actual current of ‘q’ axis (iqse) flowing through the motor, and generating and outputting a voltage (vqse) for operating the motor at the speed command value (wr*); a third proportional integrator
9
for receiving an error between a current command value of ‘d’ axis component (idse*) according to a rating of the motor and an actual current of ‘d’ axis (idse) flowing at the motor, and generating and outputting a voltage (vdse) for operating the motor at the speed command; a static coordinate system converter
10
for receiving the two phase voltages (vqse and vdse) and outputting three phase voltages Va, Vb and Vc; a synchronous coordinate system converter
12
for measuring three phase currents (ias, ibs and ics) inputted to the induction motor, changing the actual current (idse) of ‘d’ axis flowing at the motor and the actual current (iqse) of ‘q’ axis flowing at the motor, and outputting it; a slip frequency generator
13
for receiving a current command value of ‘q’ axis component (iqse*) and a current command value of ‘d’ axis component (idse*) and generating a slip frequency; an arithmetic control signal generator
14
for receiving a slip frequency (Wslip) of the slip frequency generator
13
and the actually detected speed (Wr) of the induction motor and ge
Birch & Stewart Kolasch & Birch, LLP
Duda Rina I.
LG Industrial Systems Co. Ltd.
Nappi Robert E.
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