Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1990-04-24
1994-02-08
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318809, 318802, H02P 540
Patent
active
052851451
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a current-limit system for a voltage-type inverter, and more particularly to a current-limit system that limits the output current of the inverter which increases in response to any changes in the frequency set value or any unexpected or sudden changes in the load of the inverter.
BACKGROUND OF THE INVENTION
The inventors of the present application proposed, in their earlier Japanese Patent Application Laying-Open No. 62-123965, a current-limit system that can be used in voltage-type PWM (pulse-width modulation) control inverters. An example of the principal circuit of a PWM control inverter to which the current-limit system may be applied is shown in FIG. 22. In FIG. 22, E.sub.1 designates a dc (direct current) power supply, INV, an inverter, Tr.sub.1 to Tr.sub.6, transistors, D.sub.1 to D.sub.6, feedback diodes that have inverse-parallel connections across the transistors Tr.sub.1 to Tr.sub.6, and IM denotes a three-phase induction motor (which will be simply referred to "motor") that serves as a load.
The inverter INV provides the constant V/f (voltage/frequency) control in the known manner: the appropriate command value for the output voltage of the inverter INV is generated based on the frequency set value for the output voltage; this command value is compared with a carrier waveform so that a PWM signal may be produced; and this PWM signal enables the transistors Tr.sub.1 to Tr.sub.6 to provide the output voltage of the inverter INV that meets the requirements specified by the output voltage command value mentioned above.
The output current limit system for the inverter INV is provided with a current transformer or the like to measure the magnitude of currents iu, iv, and iw of respective phases (u, v, w), in order to determined that any of the currents exceed their specific limit value. If one of the phase currents iu, iv, and iw exceeds its positive limit value, the transistor in the upper arm for that phase is turned off, whereas if one of the currents exceeds its negative limit value, the transistor in the lower arm for the phase is turned off. This current-limit system will be described more specifically below.
First, it is assumed that respective phase currents are positive when they flow in the direction shown in FIG. 22, in the case where the motor IM is driven by the inverter INV. Then, it is also assumed that each pair of transistors Tr.sub.1 and Tr.sub.4, transistors Tr.sub.2 and Tr.sub.5, and transistors Tr.sub.3 and Tr.sub.6 in the upper and lower arms that forms a phase u, a phase v, and a phase w, respectively, has a switching pattern represented by (Su, Sv, Sw). It is noted that the corresponding transistors in the upper arm are in the ON state when Su, Sv and Sw assume the value of "1", respectively, whereas the corresponding transistors in the lower arm are in the ON state when Su, Sv and Sw assume the value of "0" respectively.
If the current of the phase u exceeds its positive limit value when the switching pattern is (1 0 0) (which means that transistors Tr.sub.1, Tr.sub.5 and Tr.sub.6 are in the ON state), the switching pattern for the phase u will be forced to change from "1" to "0". Changing the switching pattern to (0 0 0) causes the transistor Tr.sub.1 in the upper arm for the phase u to turn off, decreasing the u-phase current flow. Changing the switching pattern from "1" to "0" with regard to u-phase means to turn off the transistor Tr.sub.1 in the upper arm while turning on the transistor Tr.sub.4 in the lower arm. Turning the transistor Tr.sub.1 off when the current flows in the positive direction makes the current to flow through the transistors in the lower arms of the other phases and through the diode D.sub.4 in the lower arm of the unphase. Therefore, the fact that the transistor Tr.sub.4 is turned on has no significant meaning. Rather, turning off the transistor Tr.sub.1 of the upper arm makes the current flow decrease.
If the u-phase current flow exceeds its negative limit value, the transistor Tr.sub.4 i
REFERENCES:
patent: 4275343 (1981-06-01), Fulton et al.
patent: 4722042 (1988-01-01), Asano et al.
Ishii Shin'ichi
Minowa Hirofumi
Nagao Yoshinobu
Yamazoe Masaru
Yanase Takao
Cabeca John W.
Fuji Electric & Co., Ltd.
Shoop Jr. William M.
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