JPH0755055B2 - Output current limiting method for voltage source PWM inverter - Google Patents

Description

The present invention relates to an output current limiting method for a voltage type PWM (pulse width modulation) inverter.

[Conventional technology]

As a conventional technique, a case where a voltage type PWM inverter is used to drive an induction motor at a variable speed will be described here. FIG. 4 is a block diagram showing such a control system.

The main circuit is composed of six switching elements (in this example, the transistors T _{r1 to} T _r6 are shown, but not limited to this) and a reactive element for processing ineffective components as shown in FIG. It is composed of a normal voltage type three-phase inverter 1 composed of _six diodes D _{1 to} D ₆ and, for example, a three-phase induction motor 2 serving as a load. Further, although current control is not particularly performed in such a system, a current detector 3 is usually provided for device protection.

By the way, a V / F (voltage / frequency) constant control method is known as a general example of driving an induction motor at a variable speed in such a system. Since this method is already well known, the details are omitted here, but the basic operation is
This will be described with reference to FIG.

In FIG. 2B, when the frequency command is first set by the frequency setting device 10, the output voltage command values v _U ^* , v _V that give the magnitude and frequency of the three-phase voltage to be output from the inverter 1 in accordance therewith. ^* , V _W ^* is calculated by the calculation circuit 11, and U, V,
It is input to the W-phase PWM pattern generation circuits 20, 30, and 40, respectively. The PWM pattern generation circuits 20, 30, and 40 are adders 2
1, a voltage command value V _U ^* , V _V that is provided from the voltage command value calculation circuit 11 and is composed of a comparator 22, an inverter 23, etc.
^* , V _W ^* is compared with the carrier signal from the carrier signal generator 12, and according to the result, the output signals of the upper and lower arm transistors of the corresponding phases are controlled so as to be turned on and off. It is given to the corresponding base drive circuit.

Here, the cause of the so-called overcurrent in which the output current of the PWM inverter exceeds a certain limit value will be described.

Overcurrent is generated in a system as shown in FIG. 4 in the following two cases. One is the case where a fault current such as an output short circuit flows, in which case it is necessary to immediately turn off the transistor to stop the operation of the inverter and reduce the current. The other is a case where an overcurrent occurs in the control, such as a case where the load of the electric motor changes abruptly. In this case, it is desirable to control so as not to exceed the overcurrent level without reducing the current or stopping the inverter. As a control method, for a sudden change in the set value, for example, when an overcurrent occurs, the speed set value is once returned to the value before the abrupt change inside the arithmetic circuit 11 and the overcurrent is eliminated. It is conceivable to gradually change this set value to. In addition, in order to reduce the slip S of the induction machine 2 inside the arithmetic circuit 11 when the load suddenly changes, the speed setting value is once lowered when S> 0.
Then, as the overcurrent is eliminated, the current is gradually raised, and when the slip S is S <0, a method of performing the opposite control can be considered.

[Problems to be solved by the invention]

By the way, all of the methods described above are required to be executed at a speed sufficiently commensurate with a change in current. However, when the arithmetic circuit 11 is composed of, for example, a microprocessor or the like, such control is required. This requires a considerable amount of time, and in many cases it is not possible to actually suppress the overcurrent.Therefore, even in such cases, in the past, all transistors were pulsed off to cut off or cut off the current as in the case of a failure. It is usually done. In other words, the inverter current is stopped even if an overcurrent occurs due to control or load changes, so the motor current immediately becomes zero, and the current intermittently repeats that the current rises again when the inverter starts operating. become. Therefore, in a system in which the occurrence of such current interruption or the frequent repetition of such current interruption is involved, there is a problem that a large capacity inverter must be replaced.

Therefore, the present invention limits at least the overcurrent level generated by the control such as the sudden change of the set value and the sudden change of the load so that the overcurrent level is not exceeded. It is an object of the present invention to provide a method for limiting the output current of a voltage-type PWM inverter that can perform continuous control without interrupting the current.

[Means for solving problems]

A detecting means for detecting a deviation of the inverter output current exceeding a predetermined current limit value as its polarity and a correcting means for correcting the output voltage command value according to the deviation and its polarity are provided.

[Action]

When the output current of the PWM inverter exceeds the specified current limit value, the deviation amount that exceeds the current limit value and the polarity direction when it exceeds
For each phase of the PWM inverter output, detected by the detecting means, a voltage signal corresponding to the detected deviation is added to the output voltage command signal of the phase in which the deviation is detected, in the opposite polarity direction to the detected polarity direction. The output voltage command signal of the phase is corrected by adding in the polarity direction by the correction means, and the output voltage is controlled thereby, so that the output current of the phase is limited to the predetermined current limit value or less.

〔Example〕

FIG. 1 is a voltage type PWM for explaining an embodiment of the present invention.
It is a block diagram which shows an inverter. As is clear from the figure, in this embodiment, the coefficient multipliers 25, 35, 45 and the outputs of the coefficient multipliers 25, 35, 45 are compared with the voltage commands v _U ^* , v _V ^* , v _W ^* from the voltage command calculation circuit 11, respectively. The feature is that an adder 24 for adding polarities as shown in the figure is provided, and the other points are the same as in FIG. In addition,
Although only the U phase is specifically shown in the PWM pattern generation circuit (see reference numeral 20), it goes without saying that the V pattern and W phase are similarly configured.

The coefficient units 25, 35, 45 are composed of amplifiers having a dead zone corresponding to the current limit value, or those having a function equivalent thereto, each of which is connected to the current detector 3 for detecting the output current of each phase of the inverter. ing. Therefore, the coefficient unit 25,3
5,45 detects the deviation of the inverter output current that exceeds the current limit value as its polarity, and outputs the output as PWM of each phase.
It is applied to the adder 24 in the pattern generation circuits 20, 30, 40 with the polarity as shown. The polarity of the current detected by each coefficient unit is made to coincide with the polarity of the output voltage signal. In this case, for example, the current flowing in the motor is positive.

By doing so, for example, when a positive current flows in the U phase and its value exceeds the current limit value, the coefficient unit 25 causes a positive value proportional to the deviation exceeding the current limit value from the state of zero output until then. Output a signal. This output is the voltage command value v _U
^* Provided to adder 24 as a negative offset, regardless of its own polarity. On the other hand, when the current in the negative direction exceeds the limit value, it is given as a positive offset amount with respect to the voltage command value v _U ^* . The same applies to the V and W phases. In this way, the voltage command value is corrected in accordance with the current deviation and the polarity thereof, and the output voltage is suppressed by performing the same voltage control as in the related art.

Here, the reason why overcurrent can be suppressed by the above control will be described.

In general, PWM inverter main circuit can be considered in that one phase in the steady state, the equivalent circuit thereof as FIG. 2 (b), the AC voltage source v _U (= v _U ^*) and reactor L and the counter electromotive It can be considered as a model of the induction motor represented by the voltage e _U. In addition, in this equivalent circuit, a current flows due to the difference voltage between v _U and e _U , so the equivalent circuit in (a) of the figure further differs from the difference voltage source v _U −e _{U as} shown in (b) of the figure. It can be converted into a circuit including the reactor L. Note that the polarity of the current is the same as in FIG.
The direction of the arrow in the figure is positive.

Now consider the case where the forward current exceeds the limit value. In this case, as described above, the negative offset component is superposed on the voltage command value v _U ^* via the coefficient unit 25, so that the output voltage can be controlled at the speed of substantially the carrier frequency. The equivalent circuit of (b) is shown in Fig. 3 (a).
You can think of it as something like. That is, FIG. 3 (a)
As described above, the negative offset voltage .DELTA.v works in the direction (-) of decreasing the current with respect to the positive current, so that the increase of the current can be suppressed. Similarly, when the current in the negative direction exceeds the limit value, the positive offset amount Δv is superimposed on the voltage command value v _U ^* , so that the equivalent circuit is as shown in FIG. In this case as well, the positive offset voltage Δv works in the direction of decreasing the negative current, so that the current can be limited with this.

Thus, regardless of the magnitude and polarity of the difference voltage v _U −e _U ,
It can be seen that the current can be limited only by the polarity of the output current.
The fact that it is irrelevant to the differential voltage means that it is irrelevant to the output voltage v _U ^* of the inverter, the magnitude of the back electromotive force e _U of the load, and the phase as well. It means that the current can be limited regardless of driving or braking.

〔The invention's effect〕

As described above, according to the present invention, when the current of any output phase of the voltage type PWM inverter exceeds the limit value,
Immediately at the speed of the carrier frequency, only the current increase above the limit value of the phase in which the overcurrent is detected is suppressed, and the current value is given during that time so that the arithmetic circuit does not exceed the limit value. Since the voltage command value is calculated, the control of the phase in which the overcurrent is detected can be quickly returned to the normal control without interrupting the current, and the output current limiting method according to the present invention can be applied to the electric vehicle. When applied to electric motor control, it is possible to obtain the effect that the current can be limited regardless of driving or braking. When performing the above calculation, it is necessary for the induction motor to determine the polarity of the slip S. However, this can be done at the same speed as the calculation of the voltage command value, and it goes without saying that it can be sufficiently realized. Nor.

[Brief description of drawings]

FIG. 1 is a voltage type PWM for explaining an embodiment of the present invention.
FIG. 2 is a configuration diagram showing an inverter, FIG. 2 is a circuit diagram showing an equivalent circuit for one phase of the inverter in a steady state, FIG. 3 is a circuit diagram showing an equivalent circuit for one phase of the inverter when current is limited, and FIG. It is a block diagram which shows the conventional example of a form PWM inverter control system. Explanation of code 1 …… Voltage type PWM inverter, 2 …… Induction motor, 3 ・ ・ ・
… Current detector, 10 …… Frequency setter, 11 …… Voltage command value calculation circuit, 12 …… Carrier signal generator, 20,30,40 …… PW
M pattern generator, 21,24 ... Adder, 22 ... Comparator, 23 ... Inverter, 25, 35, 45 ... Coefficient multiplier.

Claims (1)

[Claims] 1. A method of limiting the output current of a voltage-type PWM inverter, the voltage of which is controlled based on a pulse width modulation (PWM) signal obtained by comparing an output voltage command signal with a carrier signal which determines a modulation frequency, When the output current of the PWM inverter exceeds the predetermined current limit value, the deviation amount that exceeds the current limit value and the polarity direction when it exceeds the current limit value are detected for each phase of the PWM inverter output, and the voltage corresponding to the detected deviation amount is detected. The output voltage command signal of the phase is corrected by adding the signal to the output voltage command signal of the phase in which the deviation is detected in the polarity direction opposite to the detected polarity direction, and the output voltage is thereby corrected. An output current limiting method for a voltage-type PWM inverter, which is controlled so that the output current of the phase is limited to the predetermined current limit value or less.