JPS5911278B2 - Induction motor control device using current source inverter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control device for an induction motor that continues operating an induction motor idling at a given rotational frequency by adjusting the operating frequency of a current source inverter.

In recent years, as a device to control the rotational speed of an induction motor,
Variable voltage variable frequency inverters have become popular25.

One type of inverter is a current source inverter.
Since a current source inverter has a large-capacity reactor inserted in its DC circuit, the output AC current supplied to the induction motor connected to the current source inverter has a rectangular wave and a
The output AC voltage is characterized by a substantially sinusoidal wave due to the induced electromotive force of the induction motor. One method of operating an induction motor using a combination of a current source inverter and an induction motor is to connect an induction motor idling at a given rotational frequency to a current source inverter, and to There is a method in which an induction motor is brought into operation by a current source inverter from an idling state by supplying the motor with a rectangular wave current having an operating frequency slightly lower than a given rotational frequency. When starting such operation, the current source inverter starts ignition control at an operating frequency slightly lower than the arbitrary rotational frequency of the induction motor, so the operation of the inverter is prevented from commutation failure. It is necessary to start without fail. Figure 1 shows a configuration diagram of a current source inverter.

1 is a forward conversion rectifier with phase control for converting AC power of an AC power source into an arbitrary frequency; 2 is a self-excited inverse converter for converting the above DC power into AC power of an arbitrary frequency; 3 is an intermediate A DC reactor inserted to smooth the current in a DC circuit.

Further, 4 is a switch for connecting a three-phase induction motor load to the inverter device, and 5 is a three-phase induction motor load.
In a current source inverter with this type of configuration, the leakage inductance of the three-phase induction motor, which is the load, is used to perform commutation between each phase, so when starting the operation of the three-phase induction motor with the current source inverter, , the switch 4 is closed, and the three-phase induction motor 5 is connected to the current source inverter while idling at an arbitrary rotation frequency. Figure 2 is an explanatory diagram of the commutation phenomenon of an inverter with a three-phase induction motor load connected, and the parts of the AC power supply, forward conversion rectifier, and DC reactor in Figure 1 are omitted. There is. The commutation phenomenon of a current source inverter will be explained below with reference to FIG. In Fig. 2, UT, VT, and WT are U-phase and W-phase thyristors, UO, D, and WD are U-phase and W-phase diodes, respectively, Cl, C2, and C3 are commutating capacitors, and Lu, Lv , Lw are equivalent representations of the leakage inductance of the three-phase induction motor load.

Now consider the commutation from U phase to phase. When the U phase is conducting, commutating capacitors C1 and C3 are charged to EcO with the polarity shown, and C2 is in a state where there is no charge. Here, when a gate pulse is applied to the V-phase thyristor (T), VT becomes conductive, and therefore UT becomes C
It is reverse biased for a certain period of time with a voltage EcO of 1, and immediately becomes non-conductive. At this time, the v-phase diode (D) cannot conduct because it is reverse biased by the voltage of the commutating capacitor. Therefore, the current (VD) is A,
The current flows along the path b, and the commutating capacitor C1 is charged so that the sign is opposite to that shown in the figure. When C1 is charged and the terminal voltage of C becomes zero, diode VD becomes conductive, and current begins to flow to phase V as well, and both UD and D become energized.
The commutation overlap period of UD and D begins. This commutation overlap period continues until the current 1D flowing in the U phase becomes zero and completely shifts to the phase, and during this period, the commutation capacitor continues to be charged.
When this commutation overlap period ends, the U-phase diode (UD) becomes non-conductive, and commutation is completed. At this time,
The commutating capacitor C1 has EcO in the opposite direction to the polarity shown.
C3 is charged to the point where it has no charge and is ready for the next commutation. Figure 3 shows the commutation capacitor C1 during the commutation period.
This is a diagram illustrating the voltage change of . TO is the time when the gate signal is applied to the thyristor (T), T is the time when the diode (D) starts conducting, and T2 is the time when the diode (UD) is applied.
), the period from TO to t1 is the reverse bias period of the thyristor (UT), and the period from T to T2 is the commutation overlap period. The commutation overlap period is a constant value determined only by the capacitance of the commutation capacitor and the capacitance of the load inductance. On the other hand, the charging voltage EcO is the load current (I
D) increases in proportion to the magnitude of The reverse bias period (t1-TO) is proportional to the initial charging voltage EcO and inversely proportional to the load current (ID). As is clear from this relationship, under the condition that the load current 1D is constant, the charging voltage of the commutating capacitor is also sufficiently high, and therefore the reverse bias time becomes a constant value determined only by the circuit constants. However, if this inverter is fired at a certain frequency when the load current 1D is zero, the voltage of the commutating capacitor will decrease, so if the load current 1D is passed in this state, the inverter side It may not be possible to obtain sufficient reverse bias time. FIG. 3b is an example of this, and shows the capacitor voltage change when the load current 1D is increased while the commutating capacitor voltage is decreased. - At the time of commutation before the step, the load current was very small, so the initial value of the capacitor voltage EcO is small, but the load current has increased by the time of the current commutation, so the capacitor As the charging speed increases, the reverse bias time (t/-TO) becomes shorter. If this time (t/-TO) becomes shorter than the minimum time required to make the thyristor non-conductive,
The thyristor becomes conductive again at T,' and a commutation failure between the phases occurs. Once this occurs, the application of a ignition signal from the ignition control device to the inverter will not restore the inverter to normal operation. When the current increases in this way, sufficient reverse bias time for the thyristor cannot be obtained, which may result in commutation failure.

The present invention provides a current source inverter that can prevent commutation failure due to the above-mentioned causes and can be started instantaneously.

FIG. 4 is a configuration diagram of a control circuit showing an embodiment of the present invention, in which 1 indicates a forward conversion rectifier, 2 a self-excited inverse conversion device, and 3 a DC reactor.

In the control circuit, 6 to 15 have the same configuration as the control circuit of a conventional general variable frequency inverter, 6 is a pattern generation circuit that sets target values of output voltage and output frequency, and 7 is a command for pattern generation circuit 6. 8 is a ring counter that takes one of six states and advances its state every time a pulse from oscillator 7 is input; 9 is a ring counter that changes the frequency of the output pulse in response to the oscillator 8; This is a gate circuit that selects two of the six thyristors of the inverter in response to and sequentially applies gate signals to the thyristors; 10 is a voltage sensor for feeding back the output voltage; 11 is a voltage sensor for feeding back the output voltage; 12 is a current sensor for detecting the main circuit current value and providing feedback; 13 is a current sensor that compares this current value with the current reference signal to determine the firing phase of the forward converter. A current controller for setting, 14 a phase shifter, and 15 a forward converter gate circuit. Further, 16 to 18 indicate circuit configurations for realizing the present invention, 16 is a reference signal generator that sets a reference signal, 17 is a comparator that compares this reference signal with the output signal of the current sensor 12, and 17 is a comparator that compares this reference signal with the output signal of the current sensor 12. When the output signal of sensor 12 becomes larger than the reference signal, a constant voltage is output and 181 is excited. 181 indicates an excitation coil of the relay 18.

182 indicates a contact point of the relay 18.

First, when the induction motor 5, which is idling at a given rotation speed, is connected to a current source inverter to start operation, the operation pattern generator 6 sends the frequency pattern to the oscillator 7 and the voltage pattern to the voltage controller 11. Given.

However, since the contacts 182 are separated because no current is flowing yet, no pulse is input to the ring counter 8, and therefore the output of the ring counter 8 remains in the same state. In this state, the gate circuit 9 continues to output a gate signal to the same thyristor of the inverter 2, the inverter 2 does not perform an inverse conversion operation, and the firing frequency is zero. At this time, direct current can flow through the induction motor through the two thyristors of the inverter 2. Next, as the direct current 1D increases by the forward converter 1, the output signal of the current sensor 12 increases, and when this exceeds the reference signal generated by the reference signal generator 16, the relay 18 is energized. , the excitation coil 181 operates and the contacts 182 come into contact, the ring counter 8 operates according to the pulse of the oscillator 7, and the self-excited inverter 2 starts operating. As described above, according to the present invention, since the self-commutated inverter always starts operating when the current in the main circuit reaches a certain level, the commutation operation is ensured. This has a great effect on improving the stability of current source inverter operation.

[Brief explanation of the drawing]

Fig. 1 is a main circuit configuration diagram of a current source inverter, Fig. 2 is an explanatory diagram of commutation operation of a self-excited inverter, Fig. 3 is a commutation capacitor voltage waveform during commutation, and Fig. 4 is: FIG. 1 is a configuration diagram of a control circuit showing an embodiment of the present invention. In the figure, 1 is a forward conversion rectifier, 2 is a self-excited inverse converter, 3 is a DC reactor, 5 is an induction motor, 16 is a reference signal generator, 17 is a comparator, and 18 is a relay.

Claims (1)

[Scope of Claims] 1. A forward conversion rectifier that converts AC power from an AC power source into DC power of an arbitrary voltage, and a DC power connected to the above forward conversion rectifier via an intermediate DC circuit including a DC reactor. An induction motor idling at a given rotational frequency is connected to the output terminal of a current-source inverter, which is composed of a self-excited inverter converter that converts AC power into alternating current power at a given frequency, and
The magnitude of the current flowing through the induction motor is controlled by controlling the phase shift of the forward converting rectifier by the current control device provided in the forward converting rectifier, and the ignition frequency provided in the self-excited inverse converter is controlled. An induction motor using a current source inverter, in which a control device operates the self-excited inverter to control the frequency of the current flowing through the induction motor, thereby bringing the rotational frequency of the induction motor into the operating frequency of a current source inverter. In the control device, when the current supplied to the induction motor via the forward conversion rectifier and the self-excited inverter is below a predetermined value, the firing frequency of the self-excited inverter is set to approximately zero, and the A means for supplying direct current to the induction motor and controlling the ignition frequency control device so that the ignition frequency of the self-excited inverter becomes a predetermined value after the current reaches a predetermined value. A control device for an induction motor using a current source inverter, characterized by: 2. The means for controlling the ignition frequency control device of the self-excited inverter includes a current detector that detects the current of the induction motor, and a device that determines when the current detected by the current detector exceeds a predetermined value. A current source inverter according to claim 1, comprising a comparator and means for switching the operation of the ignition frequency control device from a stopped state to an operating state according to the output signal of the comparator. Induction motor control device.