JP2007336756A - Current controller for three-phase ac motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stable torque output without torque ripple by performing current feedback control, when one of the current detectors of different phases of a three-phase AC motor becomes defective to allow noises to be mixed therein. <P>SOLUTION: A current controller for a three-phase AC motor comprises a first rotary coordinate converter for converting current to a first d-axis current and a first q-axis current based on the information of the current detectors of the U-phase, the V-phase, and the W-phase of the three-phase AC motor; a second rotary coordinate converter for converting current to a second d-axis current and a second q-axis current, based on the information of the detectors of the U-phase and the V-phase; a third rotary coordinate converter for converting current to a third d-axis current and a third q-axis current, based on the information of the detectors of the U-phase and the W-phase; and a fourth rotary coordinate converter for converting current to a fourth d-axis current and a fourth q-axis current, based on the information of the detectors of the V-phase and the W-phase. The current controller selects the deviation of the d-axis current wherein the absolute value of the deviation between the d-axis current command and the first to the fourth axis current is minimized to feed it back to the d-axis current controller, and selects the deviation of the q-axis current wherein the absolute value of the deviation between the q-axis current command and the first to the fourth q-axis current is minimized to feed it back to the q-axis current controller. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a current control device that detects a current of a three-phase AC motor with a current detector and feeds back the detected current value to a current controller to control the current.

  The configuration of current control of a conventional three-phase AC motor is shown in FIG. 4, and the configuration will be described based on FIG.

A three-phase balanced AC voltage is applied to the three-phase AC motor 1 by the power converter 2, and a three-phase balanced current flows.

The current detectors 31 to 33 detect a U-phase current, a V-phase current, and a W-phase current flowing through the three-phase AC motor 1, respectively.

The rotary coordinate converter 5 includes a U-phase current value iu, a V-phase current value iv, and a W-phase current value iw detected by the current detectors 31 to 33, and a position from the position detector 4 attached to the three-phase AC motor 1. Based on the information θ, three-phase-dq coordinate conversion is performed based on the equations (1) and (2), and a d-axis current id and a q-axis current iq that can be handled as a DC state on the rotation coordinates are output.

  The d-axis current controller 81 receives the deviation between the d-axis current command 61 and the d-axis current id and outputs a d-axis voltage command vdref via the PI adjuster so that the deviation becomes zero.

The q-axis current controller 82 receives the deviation between the q-axis current command 62 and the q-axis current, and outputs a q-axis voltage command vqref via the PI adjuster so that the deviation becomes zero.

The rotary coordinate converter 9 performs dq-three-phase coordinate conversion based on the d-axis voltage command vdref, the q-axis voltage command vqref, and the position information θ from the position detector 4 attached to the three-phase AC motor 1, and performs balanced three-phase conversion. AC voltage commands vref, vvref, vwref are given to the power converter 2.

This current control configuration is also applied to Patent Documents 1 to 4 and the like.

FIG. 5 shows that in the conventional current control configuration, an abnormality occurs in one of the outputs of the three current detectors 31 to 33 provided in each phase of the three-phase AC motor 1, and a current value different from the true value is fed back. The d-axis current id and the q-axis current iq in the case of

The section A in FIG. 5 represents the operation when the outputs of the three current detectors are normal, the d-axis current id substantially matches the d-axis current command 61, and the q-axis current iq is q It is assumed that it is in a steady state that substantially matches the shaft current command 62.

Section B in FIG. 5 shows a case where only the output of the U-phase current detector 31 among the three current detectors 31 to 33 is abnormal and becomes a fixed value X1, and section C in FIG. 5 shows the V-phase current detection. 5 shows a case where only the output of the detector 32 is abnormal and becomes the fixed value X2, and a section D in FIG. 5 shows a case where only the output of the W-phase current detector 33 is abnormal and becomes the fixed value X3. Due to these current values, an erroneous deviation occurs between the d-axis current id and the d-axis current command 61 that have been subjected to rotational coordinate conversion based on the equations (1) and (2), and similarly q-axis currents iq and q An erroneous deviation also occurs with respect to the shaft current command 62.

This erroneous deviation is input to the d-axis current controller 81 and the q-axis current controller 82, and each current controller 81, 82 tries to compensate for this deviation, so that a ripple occurs in the actual current. A torque ripple is generated in the output of the three-phase AC motor 1.

The example shown in FIG. 5 shows a case where the output of the current detector becomes a fixed value over one period of the current waveform, but noise is mixed into the outputs of the current detectors 31 to 33 and an abnormality is instantaneously generated. Even if it occurs, torque ripple occurs in the output of the three-phase AC motor 1 during the period when noise is mixed.
Patent 3719910 Japanese Patent No. 3719426 Japanese Patent No. 3707528 Japanese Patent No. 3395815

  The problem to be solved is that when the current detector fails or noise is mixed in the current detector, an error occurs in the current detection value that is fed back to the current controller. The torque ripple is generated from the motor by performing the current control.

According to claim 1 of the present invention, a power converter for applying a voltage to a three-phase AC motor based on a three-phase balanced voltage command and a U-phase current detection for detecting a current flowing in the U-phase of the three-phase AC motor And three current detectors, a V-phase current detector for detecting the current flowing in the V-phase and a W-phase current detector for detecting the current flowing in the W-phase, and the three-phase detected by the current detector A rotary coordinate converter for converting into a d-axis current and a q-axis current that can be handled as a DC state on the rotary coordinate based on the current value of the AC motor and information on a position detector attached to the three-phase AC motor, and a d-axis current A d-axis current that outputs the d-axis voltage command for zeroing the deviation between the d-axis current command and the d-axis current by using the calculation result of the d-axis deviation calculator for calculating the deviation between the command and the d-axis current. Deviation between controller, q-axis current command and q-axis current A q-axis current controller for outputting a q-axis current command and a q-axis voltage command for zeroing the deviation of the q-axis current by using the calculation result of the q-axis deviation calculator to be calculated, and the d-axis voltage command and the a rotating coordinate converter for converting the three-phase AC voltage command into a balanced three-phase AC voltage command based on a q-axis voltage command and information on the position detector attached to the three-phase AC motor, and a current flowing through the three-phase AC motor In the current control device for controlling
Of the three current detectors, a second d-axis current and a second q-axis current that can be treated as a DC state on the rotational coordinates based on information of the U-phase current detector, the V-phase current detector, and the position detector. A second rotating coordinate converter for converting to a DC state on the rotating coordinates based on information of the U-phase current detector, the W-phase current detector and the position detector among the three current detectors. A third rotation coordinate converter that converts the current into a third d-axis current and a third q-axis current, the V-phase current detector, the W-phase current detector, and the position detector among the three current detectors And a fourth rotational coordinate converter for converting the fourth d-axis current and the fourth q-axis current to be treated as a direct current state on the rotational coordinates, and calculating a deviation between the d-axis current command and the second d-axis current. Second d-axis deviation performance A second q-axis deviation calculator for calculating a deviation between the q-axis current command and the second q-axis current, and a third d-axis deviation calculation for calculating a deviation between the d-axis current command and the third d-axis current. A third q-axis deviation calculator for calculating a deviation between the q-axis current command and the third q-axis current, and a fourth d-axis deviation calculation for calculating a deviation between the d-axis current command and the fourth d-axis current. A fourth q-axis deviation calculator for calculating a deviation between the q-axis current command and the fourth q-axis current, the d-axis deviation calculator, the second d-axis deviation calculator, and the third d-axis deviation calculator A d-axis current selector for selecting an output having the smallest absolute value from outputs of the fourth d-axis deviation calculator, the q-axis deviation calculator, the second q-axis deviation calculator, and the third q-axis deviation calculation A q-axis current selector for selecting an output having a minimum absolute value among outputs of the fourth q-axis deviation calculator, and d The output of the current selector as an input of the d-axis current controller, characterized in that the output of the q-axis current selector as an input of the q-axis current controller.

According to a second aspect of the present invention, in the current control device according to the first aspect, the first determination unit that outputs 1 when the output of the d-axis deviation calculator is selected and outputs 0 when the output is not selected; When the output of the second d-axis deviation calculator is selected, it outputs 1 and when it is not selected, the second determiner that outputs -1 and the output of the second determiner are integrated and the lower limit is 0. A first counter, a third determiner that outputs 1 when the output of the third d-axis deviation calculator is selected and outputs -1 when the output is not selected, and an output of the third determiner. A second counter that integrates and has a lower limit value of 0, a fourth determiner that outputs 1 when the output of the fourth d-axis deviation calculator is selected and outputs -1 when it is not selected, When the output of the fourth discriminator is integrated and the third counter whose lower limit is 0 and the output of the first discriminator is 1, Includes a reset signal for clearing the count values of the first counter, the second counter, and the third counter to 0, and the second counter indicates that the W-phase current detector has failed when the first counter exceeds a predetermined value. If the third counter exceeds a predetermined value, the display indicates that the U-phase current detector has failed.

  Even if one of the three current detectors that detect the current of each phase of the three-phase AC motor fails or noise is mixed in the current detector, torque ripple is not generated and stable torque can be output. Become.

  The purpose of providing a stable torque output without generating torque ripple even if one of the three current detectors detecting the current of each phase of the three-phase AC motor fails or noise is mixed in the current detector, This was realized without causing a delay due to filtering to the current detection value and without multiplexing the current detector for one phase of the three-phase AC motor.

  FIG. 1 shows an embodiment of claim 1 of the present invention, and the present invention will be described in detail based on FIG.

  A three-phase balanced AC voltage is applied to the three-phase AC motor 1 by the power converter 2, and a three-phase balanced current flows.

The current detectors 31 to 33 detect a U-phase current, a V-phase current, and a W-phase current flowing through the three-phase AC motor 1, respectively.

Position information θ from the position detector 4 attached to the three-phase AC motor 1 is input to the rotary coordinate converters 51 to 54.

The rotary coordinate converter 51 uses the U-phase current value iu, the V-phase current value iv, the W-phase current value iw detected by the current detectors 31 to 33 and the position information θ from the position detector 4 to (3) and (4 ) To perform three-phase-dq coordinate conversion and output a d-axis current id1 and a q-axis current iq1.

The rotary coordinate converter 52 is based on the U-phase current value iu and V-phase current value iv detected by the current detectors 31 and 32 and the position information θ from the position detector 4 based on the equations (5) and (6). Phase-dq coordinate conversion is performed and d-axis current id2 and q-axis current iq2 are output.

The rotary coordinate converter 53 is based on the U-phase current value iu and the W-phase current value iw detected by the current detectors 31 and 33 and the position information θ from the position detector 4 based on the equations (7) and (8). Phase-dq coordinate conversion is performed and d-axis current id3 and q-axis current iq3 are output.

The rotary coordinate converter 54 is based on the V-phase current value iv and the W-phase current value iw detected by the current detectors 32 and 33 and the position information θ from the position detector 4 based on the equations (9) and (10). Phase-dq coordinate conversion is performed and d-axis current id4 and q-axis current iq4 are output.

The d-axis current selector 101 is a d-axis deviation calculator whose absolute value is minimized based on the calculation results of the d-axis deviation calculators 711 to 714 that calculate the deviation between the d-axis current command 61 and the d-axis currents id1 to id4, respectively. Select the output.

The q-axis current selector 102 is a q-axis deviation calculator whose absolute value is minimized based on the calculation results of the q-axis deviation calculators 721 to 724 that calculate the deviation between the q-axis current command 62 and the q-axis currents iq1 to iq4, respectively. Select the output.

  The d-axis current controller 81 receives the deviation between the d-axis current command 61 selected by the d-axis current selector 101 and the d-axis current and inputs the d-axis voltage command vdref via the PI adjuster so that the deviation becomes zero. Is output.

The q-axis current controller 82 receives the deviation between the q-axis current command 62 selected by the q-axis current selector 102 and the q-axis current and inputs the q-axis voltage command vqref via the PI adjuster so that the deviation becomes zero. Is output.

The rotary coordinate converter 9 is based on dq− based on the equations (11) and (12) based on the d-axis voltage command vdref, the q-axis voltage command vqref, and the position information θ from the position detector 4 attached to the three-phase AC motor 1. Three-phase coordinate conversion is performed, and balanced three-phase AC voltage commands vref, vvref, vwref are given to the power converter 2.

FIG. 3 shows the true value of any of the outputs of the three current detectors 31 to 33 for detecting the current of each phase of the three-phase AC motor 1 in the current control configuration to which claim 1 of the present invention is applied. Represents the d-axis current and the q-axis current when different current values are fed back.

The section A in FIG. 3 represents the operation when the outputs of the three current detectors 31 to 33 are normal, and the d-axis current is id1, id2, id3 under the condition that the three-phase currents are balanced. , Id4 all have the same value, and iq1, iq2, iq3, iq4 all have the same value for the q-axis current. At this time, it is assumed that the d-axis current substantially matches the d-axis current command 61, and the q-axis current is in a steady state that substantially matches the q-axis current command 62.

A section B in FIG. 3 represents a case where an abnormality occurs only in the output of the U-phase current detector 31 among the three current detectors 31 to 33 and becomes a fixed value X1.

Since the output of the U-phase current detector 31 becomes abnormal, the d-axis currents id1, id2, and id3 that have been subjected to rotational coordinate conversion using the current value iu of the U-phase current detector 31 are the values in section B of FIG. As shown in the d-axis current, the current value is different from the true value, and the d-axis deviation calculators 711 to 713 have a large deviation between the d-axis current id1, id2, id3 and the d-axis current command 61.

On the other hand, the d-axis current id4 that is subjected to rotational coordinate conversion without using the current value iu of the U-phase current detector 31 becomes a true value under the condition that the three-phase current is balanced, and the U-phase current Compared to the d-axis currents id1, id2, and id3 that are subjected to rotational coordinate conversion using the information of the detector 31, the d-axis deviation calculator 714 is unlikely to have a large deviation.

In the d-axis current selector 101, the output of the d-axis deviation calculator 741 whose absolute value is minimum is selected from the calculation results of the d-axis deviation calculators 711 to 714 and is input to the d-axis current controller 81.

The same applies to the q-axis current. The q-axis current iq4, which has been subjected to rotational coordinate conversion without using the current value iu of the U-phase current detector 31, is true under the condition that the three-phase current is balanced. The q-axis current selector 102 selects the output of the q-axis deviation calculator 724 having the minimum absolute value from the calculation results of the q-axis deviation calculators 721 to 724, and inputs it to the q-axis current controller 82. .

Section C in FIG. 3 shows a case where only the output of the V-phase current detector 32 is abnormal and becomes a fixed value X2, and the current value iv of the V-phase current detector 32 is used for the d-axis current. The d-axis current id3 that has undergone rotational coordinate conversion becomes a true value under the condition that the three-phase current is balanced, and the d-axis current selector 101 uses the calculation results of the d-axis deviation calculators 711 to 714. The output of the d-axis deviation calculator 713 that minimizes the absolute value is selected and input to the d-axis current controller 81.

The same applies to the q-axis current. The q-axis current iq3, which has been subjected to rotational coordinate conversion without using the current value iv of the V-phase current detector 32, is true under the condition that the three-phase current is balanced. The q-axis current selector 102 selects the output of the q-axis deviation calculator 723 that has the minimum absolute value from the calculation results of the q-axis deviation calculators 721 to 724, and inputs it to the q-axis current controller 82. .

A section D in FIG. 3 represents a case where an abnormality occurs only in the output of the W-phase current detector 33 and becomes a fixed value X3. For the d-axis current, the current value iw of the W-phase current detector 33 is used. The d-axis current id2 subjected to rotational coordinate conversion is true under the condition that the three-phase current is balanced, and the d-axis current selector 101 uses the calculation results of the d-axis deviation calculators 711 to 714. The output of the d-axis deviation calculator 712 having the minimum absolute value is selected and input to the d-axis current controller 81.

The same applies to the q-axis current. The q-axis current iq2, which is the rotational coordinate conversion without using the current value iw of the W-phase current detector 33, is true under the condition that the three-phase current is balanced. The q-axis current selector 102 selects the output of the q-axis deviation calculator 722 that has the minimum absolute value from the calculation results of the q-axis deviation calculators 721 to 724 and inputs the output to the q-axis current controller 82. .

When abnormality is generated in one of the outputs of the three current detectors 31 to 33 that detect the current of each phase of the three-phase AC motor 1 by configuring the current control to which claim 1 of the present invention is applied. However, the three-phase AC motor 1 can output a stable torque without generating a torque ripple without inputting an erroneous deviation to the d-axis current controller 81 and the q-axis current controller 82.

The example shown in FIG. 3 shows a case where the output of the current detector becomes a fixed value over one period of the current waveform, but when noise is mixed in the output of the current detector and an abnormality occurs instantaneously However, the deviation calculated by the d-axis current and the q-axis current closer to the true value obtained by rotating coordinate conversion without using the detected current value of the phase in which noise is mixed becomes the d-axis current controller 81 and the q-axis current controller 82. Since the feedback is provided, the three-phase AC motor 1 can generate a stable torque output without generating torque ripple.

FIG. 2 shows an embodiment of claim 2 of the present invention, and a detailed description of claim 2 of the present invention will be given based on FIGS.

FIG. 2 shows a judgment unit 11 for judging a faulty part and a faulty part when any of the three current detectors 31 to 33 for detecting the current of the three-phase AC motor 1 is faulty. The display 15 to be added is added to the d-axis current selector 101.

When the output of the d-axis deviation calculator 714 that does not use the current value iu of the U-phase current detector 31 is selected in the d-axis current selector 101, the output of the U-phase current detector 31 is abnormal. As a result, the determiner 114 outputs 1, the determiner 112 and the determiner 113 output −1, and the determiner 111 outputs 0.

When the output of the d-axis deviation computing unit 713 that does not use the current value iv of the V-phase current detector 32 is selected, it is determined that the output of the V-phase current detector 32 is abnormal and the determiner 113 is 1 , The determiner 112 and the determiner 114 output −1, and the determiner 111 outputs 0.

When the output of the d-axis deviation calculator 712 that does not use the current value iw of the W-phase current detector 33 is selected, the determiner 112 assumes that the output of the W-phase current detector 32 is abnormal. , The determiner 113 and the determiner 114 output −1, and the determiner 111 outputs 0.

When the output of the d-axis deviation calculator 711 using the current values of all the three-phase current detectors 31 to 33 is selected, the determiners 112, 113, and 114 output -1, and the determiner 111 1 is output.

The counter 121 counts up when the output of the determiner 114 is 1, and counts down when it is -1. Similarly, the counter 122 counts according to the output of the determiner 113, and the counter 123 counts according to the output of the determiner 112. However, the counters 121 to 123 are not negative values and are limited to 0 even if -1 is continuously input.

When the output of the determination device 111 becomes 1, the counters 121 to 123 are cleared to 0.

The outputs of the counters 121 to 123 are compared with a predetermined value 14 set in advance, and when the outputs of the counters 121 to 123 exceed the predetermined value 14, the location where the output of the current detector is abnormal is displayed on the display 15. The

When the output of the counter 121 exceeds the predetermined value 14, it is displayed that the U-phase current detector 31 is abnormal, and when the output of the counter 122 exceeds the predetermined value 14, the V-phase current detector 31 32 is displayed as abnormal, and if the output of the counter 123 exceeds the predetermined value 14, the display 15 displays that the W-phase current detector 33 is abnormal.

In order to compare the counters 121 to 1123 with the predetermined value 14 to determine a faulty current detector, a current that is steadily abnormal without misdetecting the fault location due to noise mixed in the output of the current detector The detector can be accurately determined as a failure.

It should be noted that the determination unit 11 that determines the current detector that has failed and the display unit 15 that displays the location of the failure can also be added to the q-axis current selector 102.

  According to the present invention, even when an abnormality occurs in one of the outputs of the three current detectors that detect the current of each phase of the three-phase AC motor, current control using current feedback is continuously performed, and the three-phase AC motor Since it can provide a stable torque output, there is great industrial applicability.

FIG. 2 is a block diagram showing the configuration of claim 1. Example 1 It is the block diagram which showed the structure of Claim 2. (Example 2) The operation | movement waveform at the time of applying Claim 1 is shown. Example 1 It is the block diagram which showed the structure of background art. The operation | movement waveform at the time of applying background art is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Three-phase AC motor 2 Power converter 31, 32, 33 Current detector 4 Position detector 5, 51, 52, 53, 54 Three-phase-dq rotation coordinate converter
61 d-axis current command 62 q-axis current commands 711, 712, 713, 714 d-axis deviation calculators 721, 722, 723, 724 q-axis deviation calculator 81 d-axis current controller 82 q-axis current controller 9 dq-3 Phase rotation coordinate converter 101 d-axis current selector 102 q-axis current selector 11, 111, 112, 113, 114 Judgment device 121, 122, 123 Counter 131, 132, 133 Comparator 14 Predetermined value 15 Display

Claims (2)

A power converter for applying a voltage to a three-phase AC motor based on a three-phase balanced voltage command, a U-phase current detector for detecting a current flowing in the U-phase of the three-phase AC motor, and a current flowing in the V-phase Three current detectors, a V-phase current detector to detect and a W-phase current detector to detect current flowing in the W-phase, and the current value of the three-phase AC motor detected by the current detector and the three current detectors A rotary coordinate converter that converts d-axis current and q-axis current that can be handled as a DC state on the rotary coordinate based on information of a position detector attached to the phase AC motor, a d-axis current command, and the d-axis current A d-axis current controller that outputs a d-axis voltage command for zeroing a deviation between the d-axis current command and the d-axis current by using a calculation result of a d-axis deviation calculator that calculates a deviation; and a q-axis current command Q-axis deviation calculation that calculates the deviation between the q-axis current and the q-axis current And a q-axis current controller that outputs a q-axis current command and a q-axis voltage command for zeroing the deviation between the q-axis current, the d-axis voltage command, the q-axis voltage command, and the three In a current controller for controlling a current flowing in the three-phase AC motor, comprising a rotary coordinate converter for converting into a balanced three-phase AC voltage command based on information of the position detector attached to the phase AC motor ,
Of the three current detectors, a second d-axis current and a second q-axis current that can be treated as a DC state on the rotational coordinates based on information of the U-phase current detector, the V-phase current detector, and the position detector. A second rotating coordinate converter for converting to a DC state on the rotating coordinates based on information of the U-phase current detector, the W-phase current detector and the position detector among the three current detectors. A third rotation coordinate converter that converts the current into a third d-axis current and a third q-axis current, the V-phase current detector, the W-phase current detector, and the position detector among the three current detectors And a fourth rotational coordinate converter for converting the fourth d-axis current and the fourth q-axis current to be treated as a direct current state on the rotational coordinates, and calculating a deviation between the d-axis current command and the second d-axis current. Second d-axis deviation performance A second q-axis deviation calculator for calculating a deviation between the q-axis current command and the second q-axis current, and a third d-axis deviation calculation for calculating a deviation between the d-axis current command and the third d-axis current. A third q-axis deviation calculator for calculating a deviation between the q-axis current command and the third q-axis current, and a fourth d-axis deviation calculation for calculating a deviation between the d-axis current command and the fourth d-axis current. A fourth q-axis deviation calculator for calculating a deviation between the q-axis current command and the fourth q-axis current, the d-axis deviation calculator, the second d-axis deviation calculator, and the third d-axis deviation calculator A d-axis current selector for selecting an output having the smallest absolute value from outputs of the fourth d-axis deviation calculator, the q-axis deviation calculator, the second q-axis deviation calculator, and the third q-axis deviation calculation A q-axis current selector for selecting an output having a minimum absolute value among outputs of the fourth q-axis deviation calculator, and d The output of the current selector as an input of the d-axis current controller, the current controller of the three-phase AC motor the output of the q-axis current selector, characterized in that an input of the q-axis current controller. The current control device according to claim 1,
1 is output when the output of the d-axis deviation calculator is selected and 1 is output when the output of the d-axis deviation calculator is not selected, and 1 when the output of the second d-axis deviation calculator is selected. And a first counter that outputs -1 when not selected, a first counter that integrates the outputs of the second determiner and has a lower limit value of 0, and an output of the third d-axis deviation calculator Is output when it is selected and outputs -1 when it is not selected, a second counter that integrates the outputs of the third determiner and has a lower limit value of 0, When the output of the 4d-axis deviation calculator is selected, 1 is output, and when it is not selected, -1 is output, and the output of the fourth determiner is integrated, and the lower limit is 0. When the output of the third counter and the first determiner is 1, the counters of the first counter, the second counter, and the third counter A reset signal for clearing the default value to 0, and if the first counter exceeds a predetermined value, the W-phase current detector has failed. If the second counter exceeds the predetermined value, the V-phase current detector. 2. The current control device according to claim 1, wherein when the third counter exceeds a predetermined value, the display indicates that the U-phase current detector has failed.

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