JP2006271072A - Power converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power converter capable of avoiding stall operation or the like, even in a low-frequency operation area, without using a temperature sensor. <P>SOLUTION: This power converter includes: an inverter 1 for converting a direct current into a desired alternating current to drive a motor; a controller 3 for giving a gate signal to a power device; and a current detector for detecting an output current of the inverter. The controller includes a current calculator 42 for calculating currents for a predetermined period, prior to the predetermined time obtained by the current detector and a limit reducer 41 for reducing a limit value of an output current of the inverter or its command value. With such a constitution, the power converter releases the limit reducer for the predetermined time alone or changes the limit value to an emergency limit value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a power conversion device for driving an electric motor.

  A conventional power converter for driving an electric motor employs the following system configuration. That is, this is a system in which an AC output is obtained from a DC power source by an inverter, and a motor is driven by the AC to generate necessary speed and torque. A normal motor is connected to a speed sensor, and detects the magnetic pole position or speed of the motor for feedback control. A deviation between the speed feedback signal and the speed reference is obtained by a speed controller to calculate a desired torque reference, while a magnetic flux reference is obtained from the speed feedback signal, and a torque current reference is obtained from the torque reference and the magnetic flux reference.

  A current sensor is attached to the output of the inverter device, and a torque current feedback signal is obtained based on the current feedback signal. A voltage reference is obtained by inputting a deviation between the torque current reference and the torque current feedback signal to the current controller, and the voltage reference is input to the PWM modulator to drive each power device constituting the inverter.

  In such a system, the function of limiting the torque current reference may be configured by a torque limiter in the speed controller in consideration of heat generation of the power device of the inverter. A current limit value is used as this input, but the current limit value may be set to decrease as the output frequency of the inverter decreases.

  The reason why the limit value is lowered in accordance with the decrease in the output frequency of the inverter device is as follows.

  Now consider heat generation and cooling for each power device of the inverter. When the inverter device passes a current having an effective value Irms = 1, a peak current that is √2 times the effective value Irms flows in each phase. Further, since the power devices to be energized differ depending on whether the current is positive or negative, it can be seen that the current contributing to the heat generation of the element alone is half of that of Idrms = 0.5. When the period of the sine wave output from the inverter is faster than the cooling time constant of the power device, this calculation is performed and current does not concentrate on one power device. However, when the period of the output sine wave is slow, that is, when the frequency becomes low, the cooling time constant of the power device cannot be ignored, and the time for the current to concentrate on one power device becomes longer. When the extremely low frequency that is close to the direct current is output, the most severe condition occurs. However, the current that contributes to the heat generation in this case is higher than that of the above-described Idrms = 0.5 in the power device that supplies the peak current. Therefore, a very large current flows as Idrms = √2. Therefore, the heat generation of the single element in this case becomes very large.

  For the above reasons, the torque limiter in the speed controller described above has a function of reducing the limit value in response to a decrease in the output frequency when the frequency is a certain frequency or less as a function.

  This limit value is a restriction item when considering the rated capacity of the inverter. As an example in which this restriction becomes a problem, the stall phenomenon will be described below. Assume that the load of the electric motor suddenly increases at a frequency slightly higher than the start frequency of the low-frequency current limit. This state is generally referred to as load impact. If the speed does not drop to the low-frequency current limiting region after the load impact, it is possible to perform constant control regardless of the speed. However, when the load impact is large and the speed falls to the low frequency current limit region, the torque current can only be output up to the limit value, and the motor may stop. This phenomenon is generally called a stall phenomenon. When this stall phenomenon occurs, a torque shortage occurs and a speed reference is given, but a torque commensurate with the speed reference cannot be generated due to the torque or current limit, so the speed feedback signal becomes smaller than the speed reference. For this reason, at the time of stall operation, an output corresponding to the capacity of the inverter cannot be output, and the stall state cannot be avoided. Although it is sufficient if the capacity of the inverter is sufficient, this is not economical.

As described above, in order to avoid stall operation at low frequency as much as possible, the temperature of the power device is accurately detected directly or indirectly without limiting the current at the low frequency, and the temperature (junction of the power device) is a predetermined value. When it comes to the above, the proposal which protects an apparatus is made | formed (for example, refer patent document 1).
JP 2000-333469 A (page 3-5, FIGS. 1 and 2)

  According to the technique disclosed in Patent Document 1, it is possible to operate up to the allowable temperature limit of a power device as an inverter device even in a low frequency region, but a temperature sensor is installed in the vicinity where each power device is mounted. It is necessary to provide it, and the apparatus has a complicated configuration.

  The present invention has been made in view of the above, and an object of the present invention is to provide a power converter that can avoid a stall operation or the like even in a low frequency operation region without using a temperature sensor.

In order to achieve the above object, a power conversion device according to a first aspect of the present invention includes an inverter that converts a direct current to a desired alternating current to drive an electric motor, and a control unit that provides a gate signal to the power device of the inverter. Current detecting means for detecting the output current of the inverter,
The control unit is configured to calculate a current calculation unit that calculates a current of a predetermined period before the predetermined time obtained by the current detection unit, and the output current of the inverter or a limit value of the command value of the output frequency of the inverter. Limit reduction means for reducing according to the reduction, and when the calculated value of the current calculation means is less than a predetermined value and the output current of the inverter or its command value is applied to the limit value, the limit reduction is performed for a predetermined time. The means is canceled or the limit value is changed to an emergency limit value.

  The power conversion device according to the second aspect of the present invention includes an inverter that drives a motor by converting direct current to desired alternating current, a control unit that supplies a gate signal to a power device of the inverter, and an output of the inverter Current detecting means for detecting current; and speed detecting means for directly or indirectly detecting the speed of the electric motor, wherein the controller is configured to obtain a current for a predetermined period before the predetermined time obtained by the current detecting means. Alternatively, current calculation means for calculating torque current, and limit reduction means for reducing the limit value of the output current of the inverter, the torque current or any command value thereof according to the reduction of the output frequency of the inverter, The calculation value of the current calculation means is not more than a predetermined value, and the output current of the inverter, the torque current or any command value thereof is the limit. When suffering from, is characterized in that so as to change the limit reduction means to release, or very limit the limit value for a predetermined time.

  The power conversion device according to the third aspect of the present invention includes an inverter that drives a motor by converting direct current to desired alternating current, a control unit that supplies a gate signal to a power device of the inverter, and an output of the inverter Current detecting means for detecting a current, and the control unit calculates current for a predetermined period before the predetermined time obtained by the current detecting means, and outputs current of the inverter or its command. Limit reduction means for reducing the limit value of the value according to the reduction of the output frequency of the inverter, and when the calculation value of the current calculation means is equal to or less than a predetermined value and receives a limit release command from the outside, a predetermined time Only the limit reducing means is canceled, or the limit value is changed to an emergency limit value.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the power converter device which can avoid a stall driving | running etc. also in a low frequency operation area | region, without using a temperature sensor.

Hereinafter, an embodiment of a power conversion device of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram of a power conversion apparatus according to Embodiment 1 of the present invention.

  The inverter 1 receives a DC power supply, converts it into AC having a desired voltage and frequency, and drives the electric motor 2 with the output. The inverter 1 includes a main circuit in which power devices are bridge-connected, and each power device is ON / OFF controlled by a gate signal from the control unit 3. A speed detector 4 is attached to the electric motor 2, and this output is given to the control unit 3 as a speed feedback signal. Further, a current detector 5 is provided on the output side of the inverter 1, and this output is also given to the control unit 3. Hereinafter, an internal configuration of the control unit 3 will be described.

  The speed reference given from the outside is compared with the speed feedback signal described above, and the speed controller 31 performs PID control on the error signal and gives the output to the vector controller 32 as a current reference. The vector controller 32 decomposes the current reference into a magnetic flux axis current reference and a torque axis current reference from a preset magnetic flux reference signal and speed feedback signal.

  The torque shaft current reference is compared with the torque feedback current obtained by converting the current feedback signal by the three-phase to two-phase converter 34 via the torque limiter 33, and the torque shaft current controller 35 performs PID control. . Further, the magnetic flux axis current reference is compared with the magnetic flux feedback current obtained by converting the current feedback signal by the three-phase to two-phase converter 34, and is subjected to PID control by the magnetic flux current controller 36. The outputs of the torque axis current controller 35 and the magnetic flux current controller 36 are the torque axis voltage reference and the flux axis voltage reference, respectively, which are input to the three-phase voltage reference generator 37, where the voltage reference for each phase is used as the voltage reference for each phase. The converted signal is supplied to the PWM circuit 38. The PWM circuit 38 modulates the voltage reference of each phase with a predetermined carrier signal and outputs a gate signal to be applied to the power device of the inverter 1.

  The slip calculator 39 calculates the slip frequency of the motor 2 from the torque axis current reference and the magnetic flux reference, adds the calculation result and the speed feedback signal to obtain the inverter output frequency, and integrates it with the integrator 40. Thus, the output phase reference of the inverter 1 is obtained. This output phase reference becomes the conversion phase reference of the above-described three-phase to two-phase converter 34 and three-phase voltage reference generator 37.

  The inverter output frequency obtained in the above process is input to the low frequency range limit reduction computing unit 41 so that the torque limit value of the torque limiter 33 is reduced as the frequency decreases.

  The torque shaft feedback current obtained by the three-phase to two-phase converter 34 is calculated as an effective value for a predetermined past period by an effective value calculation circuit 42 having a signal delay function. The effective value of the torque shaft feedback current is compared with a predetermined upper limit value by the comparison circuit 43, and 1 is given to one input of the AND circuit 44 when the difference is not more than the predetermined value. The other input of the AND circuit 44 is given a signal of 1 when the torque limiter 33 reaches the limit. When the AND condition is satisfied, the AND circuit 44 gives a short-term limit release command to the low frequency range limit reduction calculator 41 via the one-shot circuit 45.

  The effects of the above configuration will be described below. First, if the effective value calculation circuit 42 calculates an effective value for a period dating back to the past that is equal to or greater than the cooling time constant of the power device used in the inverter 1, this current is the power device at that time. It is approximately proportional to the temperature of Further, the upper limit value serving as a comparison reference for the comparison circuit 43 represents the limit temperature of the power device. Accordingly, the comparison difference in the comparison circuit 43 is substantially proportional to the temperature margin of the power device. Therefore, if this difference is equal to or greater than a predetermined value, the limit operation of the low frequency reduction limit 41 is canceled for a short time and the pulse current is allowed. It becomes possible to do.

  In this embodiment, the control unit 3 has been described as performing so-called vector control. However, the control unit 3 does not necessarily need to be vector control. Further, although the torque limiter 33 is provided with a limit on the torque current reference, it may be provided on the current reference of the output part of the speed controller 31. Moreover, although the electric motor 2 was demonstrated as an induction motor, this may be a synchronous motor. Further, the effective value calculation circuit 42 has been described as calculating the effective value of the past current. However, this calculation does not necessarily have to be an effective value. For example, the average value does not change the basic effect.

  FIG. 2 is a block diagram of the power conversion apparatus according to the second embodiment of the present invention. About each part of this Example 2, the same part as the block block diagram of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The second embodiment differs from the first embodiment in that the current detected by the current detector 5 is supplied to the effective value calculation circuit 42A instead of the torque shaft feedback current.

  As described above, by using the feedback current including the magnetic flux axis feedback current in addition to the torque axis feedback current as an input for estimating the temperature of the power device, more accurate temperature estimation is possible.

  FIG. 3 is a block diagram of a power conversion apparatus according to Embodiment 3 of the present invention. About each part of this Example 3, the same part as the block block diagram of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The third embodiment is different from the first embodiment in that the ON time of the limit release signal output from the one-shot circuit 45 is changed according to the comparison difference of the comparison circuit 43A.

  As described above, the comparison difference in the comparison circuit 43A is substantially proportional to the temperature margin of the power device. Therefore, if the temperature margin is small, the on-time is shortened, and conversely, if the temperature margin is large, the on-time is increased, so that more rational limit release control can be performed.

  FIG. 4 is a block diagram of a power conversion apparatus according to Embodiment 4 of the present invention. About each part of this Example 4, the same part as the block block diagram of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The fourth embodiment differs from the first embodiment in that a torque change rate detector 46 that outputs 1 when the change rate of the torque axis current reference exceeds a predetermined value is provided, and this output is input to the other input of the AND circuit 44. It is the point comprised so that.

  Focusing on the rate of change rather than the torque itself, it is possible to detect the current increase state at the time of startup or impact load more quickly. Therefore, when the change rate of the torque shaft current reference exceeds a predetermined value, if limit reduction in the low frequency range is canceled, faster limit release control can be performed.

  FIG. 5 is a block diagram of a power conversion apparatus according to Embodiment 5 of the present invention. About each part of this Example 5, the same part as the block block diagram of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The fifth embodiment is different from the first embodiment in that the other input signal of the AND circuit 44 is not a limit signal from the torque limiter 33 but an external limit release command.

  For example, if limit commands from the outside are obtained from detection signals such as start conditions, acceleration conditions, load engagement, or torque shortage conditions, faster limit release control can be performed according to the operating state of the system. It becomes possible.

  FIG. 6 is a block diagram of a power conversion apparatus according to Embodiment 6 of the present invention. About each part of this Example 6, the same part as the block block diagram of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The difference between the sixth embodiment and the first embodiment is that the limit is not released by the output of the one-shot circuit 45, but this output is given to the emergency limit setting circuit 47, and the output of the emergency limit setting circuit 47 is used for a short time. Only the limit value of the torque limiter 33 is changed to a predetermined emergency limit value.

  When avoiding the stall operation as described above or securing the starting torque, the instantaneous torque may be insufficient by simply releasing the reduction limit in the low frequency range and returning it to the normal limit value in the high frequency range. Therefore, in this embodiment, if the emergency limit value is set larger than the normal limit value in the high frequency range, for example, the instantaneous torque can be increased only for a short time. Further, the emergency limit value may be set to be equal to or lower than the normal limit value in the high frequency range according to the load characteristics of the electric motor.

The block block diagram of the power converter device which concerns on Example 1 of this invention. The block block diagram of the power converter device which concerns on Example 2 of this invention. The block block diagram of the power converter device which concerns on Example 3 of this invention. The block block diagram of the power converter device which concerns on Example 4 of this invention. The block block diagram of the power converter device which concerns on Example 5 of this invention. The block block diagram of the power converter device which concerns on Example 6 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Inverter 2 Electric motor 3 Control part 4 Speed detector 5 Current detector 31 Speed controller 32 Vector calculator 33 Torque limiter 34 Three-phase-2 phase current converter 35 Torque current controller 36 Magnetic flux current controller 37 Three-phase voltage reference Converter 38 PWM control circuit 39 Slip calculator 40 Integrator 41 Low frequency range limit reducer 42, 42A RMS value calculator 43 Comparison circuit 44 AND circuit 45 One-shot circuit 46 Torque change rate detector 47 Emergency limit setting circuit

Claims (7)

An inverter that drives a motor by converting direct current to desired alternating current;
A control unit for providing a gate signal to the power device of the inverter;
Current detection means for detecting the output current of the inverter,
The controller is
Current calculation means for calculating current of a predetermined period before the predetermined time obtained by the current detection means;
Limit reduction means for reducing the limit value of the output current of the inverter or its command value according to the reduction of the output frequency of the inverter;
When the calculated value of the current calculating means is less than a predetermined value and the output current of the inverter or its command value is applied to the limit value, the limit reducing means is canceled for a predetermined time, or the limit value is set to an emergency limit value. The power converter characterized by changing to. An inverter that drives a motor by converting direct current to desired alternating current;
A control unit for providing a gate signal to the power device of the inverter;
Current detection means for detecting an output current of the inverter;
Speed detecting means for directly or indirectly detecting the speed of the electric motor,
The controller is
Current calculation means for calculating current or torque current for a predetermined period before the predetermined time obtained by the current detection means;
Limit reduction means for reducing the limit value of the output current of the inverter, torque current or any command value thereof according to the reduction of the output frequency of the inverter;
When the calculated value of the current calculating means is equal to or less than a predetermined value and the output current of the inverter, the torque current or any one of the command values is applied to the limit value, the limit reducing means is canceled for a predetermined time, or A power converter characterized by changing a limit value to an emergency limit value. An inverter that drives a motor by converting direct current to desired alternating current;
A control unit for providing a gate signal to the power device of the inverter;
Current detection means for detecting the output current of the inverter,
The controller is
Current calculation means for calculating current of a predetermined period before the predetermined time obtained by the current detection means;
Limit reduction means for reducing the limit value of the output current of the inverter or its command value according to the reduction of the output frequency of the inverter;
When the calculated value of the current calculating means is equal to or less than a predetermined value and an external limit release command is received, the limit reducing means is canceled for a predetermined time, or the limit value is changed to an emergency limit value. A power conversion device. An inverter that drives a motor by converting direct current to desired alternating current;
A control unit for providing a gate signal to the power device of the inverter;
Current detection means for detecting an output current of the inverter;
Speed detecting means for directly or indirectly detecting the speed of the electric motor,
The controller is
Current calculation means for calculating current or torque current for a predetermined period before the predetermined time obtained by the current detection means;
Limit reduction means for reducing the limit value of the output current of the inverter, torque current or any command value thereof according to the reduction of the output frequency of the inverter;
When the calculated value of the current calculating means is equal to or less than a predetermined value and an external limit release command is received, the limit reducing means is canceled for a predetermined time, or the limit value is changed to an emergency limit value. A power conversion device. An inverter that drives a motor by converting direct current to desired alternating current;
A control unit for providing a gate signal to the power device of the inverter;
Current detection means for detecting an output current of the inverter;
Speed detecting means for directly or indirectly detecting the speed of the electric motor,
The controller is
Current calculation means for calculating current or torque current for a predetermined period before the predetermined time obtained by the current detection means;
Limit reduction means for reducing the limit value of the output current of the inverter, torque current or any command value thereof according to the reduction of the output frequency of the inverter;
When the calculated value of the current calculating means is less than a predetermined value and the rate of change of the output current of the inverter, the rate of change of the torque current, or the rate of change of any command value thereof exceeds a predetermined value, the limit is set for a predetermined time. A power conversion device, wherein a reduction means is canceled or the limit value is changed to an emergency limit value. The current calculation means includes
The power conversion device according to any one of claims 1 to 5, wherein an effective value of current is calculated. The power converter according to any one of claims 1 to 6, wherein a predetermined time for releasing the limit reducing unit is changed in accordance with a calculation value of the current calculating unit.

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