Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
1999-09-15
2001-04-10
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C388S815000
Reexamination Certificate
active
06215262
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a speed control method for a Switched Reluctance Motor(SR motor), and more particularly to a speed control for a SR motor when employing a load instance response of which is not large, that is a load having a large mechanical inertia.
2. Description of the Conventional Art
Generally, a pulse width modulation control and a current control are used for an SR motor speed control method. Particularly, the PWM control method controls the operation speed of the SR motor by applying a pulse width modulated signal to a switching transistor on the basis of a pulse corresponding to an output value of a detection sensor which detects a speed, while the current control method prevents an overcurrent from flowing to motor coils by setting a hysteresis band width of a current in a switching interval wherein the switching transistor is switched and switching the switching transistor so that the current applied to the motor coils exists in the hysteresis band width which has been set.
FIG. 1
is a sectional diagram illustrating a three-phase SR motor in general. As shown therein,
10
is a rotor,
20
is a stator, and La,Lb,Lc are coils winding the stator
20
.
FIG. 2
is a circuit diagram illustrating a control circuit for controlling a speed of the SR motor in FIG.
1
. As shown therein, the control circuit includes an inverter
21
consisting of six switching transistors Q
1
-Q
6
, freewheel diodes D
1
-D
6
and a direct current DC condenser C connected with a power supply in parallel, a detection sensor
22
detecting a rotation of the rotor of the motor and outputting a signal in accordance with the detection, a speed detecting unit
23
determining a location of the rotor from the signal outputted from the detection sensor
22
and thereby outputting a detection pulse signal ps, and a speed control unit
24
outputting a plurality of switching signals cs
1
-cs
6
to gates of switching transistors Q
1
-Q
6
, respectively, of the inverter
21
, the three upper switching transistors Q
1
-Q
3
being serially connected with the three lower switching transistors Q
4
-Q
6
through the coils La,Lb,Lc.
FIG. 3
is a diagram illustrating waveforms of the switching signals cs
1
,cs
4
applied to the gates of the switching transistors, respectively, and a current ia accordingly applied to the A-phase coil La when the three-phase SR motor is controlled in the PWM voltage control method. The speed detecting unit
23
detects a present location of the rotor
10
from the signal supplied from the detection sensor
22
and accordingly outputs detection pulse signals ps
1
,ps
2
to the speed control unit
24
whenever the rotor
10
rotates by predetermined degrees, for example, 60°. When receiving the detection pulse signal ps
1
, the speed control unit
24
outputs the fourth switching signal cs
4
at a high level to the switching transistor Q
4
and outputs the first switching signal cs
1
repeating the high and low states, that is the pulse width modulated signal, to the first switching transistor Q
1
. The fourth switching transistor Q
4
maintains an on state in accordance with the fourth switching signal cs
4
, while the first switching transistor Q
1
alternately becomes on and off in accordance with the first switching signal cs
1
. Here, as shown in
FIG. 3
, a pattern of the current ia flowing to the A-phase coil La has a saw-tooth waveform a size of which gradually increases. When the rotor continuously rotates, that is taking an example of the three-phase, when the rotor rotates by 60° after the second pulse signal ps
1
is generated, the speed detecting unit
23
outputs the second detection pulse signal ps
2
to the speed control unit
24
. Accordingly, the speed control unit
24
outputs the first and fourth switching signals cs
1
,cs
4
at the low state, whereby the first and fourth switching transistors Q
1
,Q
4
are turned off and the current is flowing to the A-phase coil La starts decreasing and eventually becomes zero. The speed control unit
24
outputs the second and fifth signals cs
2
,cs
5
at the high state, whereby the current flows to the B-phase coil Lb (not shown).
Here, when increasing a pulse duty of the first switching signal cs
1
, it is possible to increase the speed of the motor. Then, since an average time for which the first switching transistor Q
1
is turned on while the fourth switching transistor Q
4
is turned on increases, an increase interval of the A-phase current ia lengthens and a decrease interval thereof shortens so that the speed of the motor increases.
While, when the load increases, the speed of the motor decreases in proportion to the increased volume of the load, and accordingly an interval T between the first detection pulse signal ps
1
and the second detection pulse signal ps
2
lengthens. That is, the interval T which is a mechanical angle becomes larger than 60°. In this case, the time for which the fourth switching transistor Q
4
is turned on lengthens and the switching time for the first switching transistor Q
1
lengthens, whereby the A-phase current ia increases. Further, when the speed of the motor considerably decreases due to radical increase in the load, the interval T abruptly lengthens, which means the switching interval T of the first switching transistor Q
1
lengthens, so that the size of the A-phase current ia radically increases, thereby possibly exceeding a rated current. Here, the rated current means a current value above a level by which the motor and the inventor can be damaged. In this case, to prevent the break-down of the motor due to the excessive current, it is required to provide a separate current protecting circuit to cut off the excess current flowing to the system. To provide such a current protecting circuit a complicated circuit is needed and accordingly the manufacturing cost of the system increases.
To prevent the excess current, a control method is applied, which previously sets a predetermined current hysteresis band width and controls the current value not to exist out of the hysteresis band width, such method being called a current control.
FIG. 4
is a diagram illustrating waveforms of the switching signals cs
1
,cs
4
applied to the gates of the switching transistors, respectively, and a current ia accordingly applied to the A-phase coil La when the three-phase SR motor is controlled in the current control.
When the first detection pulse signal ps
1
is generated, the speed control unit
24
outputs the first and fourth switching signals cs
1
,cs
4
at a high state and accordingly the A-phase current ia increases. When the value of the A-current ia which has been increasing exceeds the current hysteresis band, the speed control unit
24
turns off the first switching transistor Q
1
and thus the A-current ia starts decreasing. When the value of the A-current ia which has been decreasing becomes lower than the current hysteresis band width, the speed control unit
24
turns on the first switching transistor. Such a process is continuously repeated until the second detection pulse signal ps
2
is generated. Thus, the A-phase current ia does not exceed the hysteresis band width, thereby preventing the overcurrent from flowing to the A-phase coil La.
In the PWM control system and the current control system, a switching frequency is a radio frequency, which is about 15-20 kHz, thereby preventing noises produced when the switching transistor is switched. However, to perform the high-speed switching an expensive power switching device such as an integrated gate bipolar transistor (IGBT) or a field effect transistor (FET) is required and also a switching loss due to the high-speed switching is unavoidable.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a speed control method for an SR motor which obviates the problems and disadvantages in the conventional art.
An object of the present invention is to provide a speed control method for an SR motor that controls the SR motor using a low-priced regular
Duda Rina I.
Fleshner & Kim LLP
LG Electronics Inc.
Nappi Robert E.
LandOfFree
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