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Sep 6 2018

Method of Handling and Detecting Common Fault of Frequency Converter IGBT Module

Summary

    The insulated gate bipolar transistor (hereinafter referred to as IGBT) is a composite device of MOSFET and GTR. Thus it has the advantages of MOSFET, including fast operation speed, high switching frequency, high input impedance, simple drive circuit and good thermal temperature; it also contains the advantages of GTR, like large current-carrying capacity and high blocking voltage. It is an ideal switch device to replace GTR.

Article coreMethod to Handle and Detect the Common Fault of IGBTPurposeShare Method to Handle and Detect the Common Fault of IGBT
Article nameMethod of Handling and Detecting Common Fault of Frequency Converter IGBT Module CategorySemiconductor Information
ApplicationDevice,CircuitKeywordsIGBT,Protection Circuit





Catalogs

Ⅰ  What Is IGBT
Ⅱ  Structure of IGBT
Ⅲ  Principle Circuit Analysis of the IGB Module
Ⅳ  Common Trouble Handling of IGBT Module
Ⅴ  Detecting method of Frequency converter IGBT module
Ⅵ  Static measurement of frequency converter IGBT module


Introduction

What is IGBT

    Insulated Gate Bipolar Transistor (IGBT) is a composite fully-regulated voltage-driven power semiconductor device consisting of BJT (Bipolar Junction Transistor) and MOS (Insulated Gate Field Effect Transistor MOSFET), which combines the high input impedance of MOSFET and the advantages of the GTR's low conduction state and low voltage drop.


Structure of IGBT 

    On the left is an N-channel enhanced type insulated gate bipolar transistor structure, the N+ region is called the source region, and the electrode attached to it is called the source (emitter E). The N base is called the drain area. The control region of the device is the gate region, and electrode attached thereto is called the gate (gate G). The channel formed next to the boundary of gate region and where it formed is called the Subchannel region, a P-type region (including the P+ and P-regions) between the C and E poles. Drain injector, a P+ region on the other side, is a functional area unique to the IGBT that forms a PNP bipolar transistor together with the drain and Subchannel region by acting as an emitter, and reduces the on-state voltage of the device by injecting holes into the drain and conducting conductive modulation. The electrode attached to the Drain injector is called the drain (collector C).

    The switch of the IGBT can form a channel by adding a forward gate voltage and turn on the IGBT by supplying base current to the PNP (formerly NPN) bipolar transistor. Conversely, it can eliminate the channel by adding a reverse gate voltage and turn off the IGBT by cutting off the base current. To drive on the IGBT only needs to control the input pole N-channel MOSFET, which has characteristics of high input impedance and is basically the same method as that of MOSFET. After formed the channel of the MOSFET, the hole injected from the P+ base into the N-layer conducts conductance modulation on the N-layer, reducing the resistance of it and making IGBT have a low on-state voltage at a high voltage.

IGBT.jpg


Principle Circuit Analysis of the IGB Module

    The IGBT module has three terminals, G, D, and S. Internal electrons will transfer after the voltage is applied across G and S.(the characteristic of semiconductor materials, which accounting why semiconductor materials are used as power electronic switches). Originally, the positive ions are corresponding to the negative ions one by one, and semiconductor material is neutral, but the applying of the voltage leads the electrons to accumulate to one side and form a conductive channel. And electrons become conductors with its conductive properties. If the voltage across GS is removed, the conductive channel disappears and becomes an insulator.

    If a positive driving voltage is applied between the gate and the emitter of the IGB module T, the MOSFET will turn on, and transistor also conducts with a low resistance state forming between the collector and the base of the PNP transistor. If there is no voltage between the gate and the emitter of the IGBT, MOSFET turns off, which cuts off the base current supply for the PNP transistor and turns off the transistor.


Common Trouble Handling of IGBT Module


circuit

    The frequency converter is composed of main circuit, power supply circuit, IGBT drive, protection circuit, cooling fan and so on. Its structure is mostly unit form or modular form. Due to incorrect using and unreasonable environment setting, the converter may fails work properly to meet the expected operation results. To prevent this, it is necessary to carefully analyze the cause of the failure in advance.

1. Common fault analysis of the main circuit

    The main circuit is mainly composed of three-phase or single-phase rectifier bridge, smoothing capacitor, filter capacitor, IGBT converter bridge, current limiting resistor, contactor and other components. Many of the common faults are caused by electrolytic capacitors, whose life is mainly determined by the DC voltage and internal temperature at the both ends of the capacitor. And its life usually depends on the internal temperature,as the capacitor type has already been selected in the circuit design. Therefore, the appropriate ambient temperature should be considered during installation and measures can be taken to reduce the pulsating current. To extend the life of the capacitor, an AC or DC reactor with improved power factor can be used to reduce ripple current. 

    Electrostatic capacity, which is relatively easy to measure, is often used to judge the deterioration of electrolytic capacitor when maintaining a capacitor. Replacing a electrolytic capacitor should take to consider if electrostatic capacity is less than 80% of the rated value and insulated resistance is below 5 MΩ.

2. Typical fault analysis of the main circuit

    Symptom: The frequency converter trips during acceleration, deceleration or normal operation.

    It should be distinguished first whether the fault is due to the load or the frequency converter. If it was caused by the frequency converter, the current at tripping can be queried through history. And if the current exceeds the rated current of the frequency converter or the set value of the electronic thermal relay while the three-phase voltage and current are balanced, there may be overload or sudden changes, such as motor stalling. Acceleration time can be extended appropriately when the load inertia is large, which will not damage the frequency converter. If the current is within the rated current of the frequency converter and the set range of the electronic thermal relay, it can judge that the IGBT module or related part has failed.

    First, whether the IGBT module is damaged can be determined by measuring the positive and negative resistances between the main circuit output terminals U, V, W of the frequency converter and the P, N terminals of the DC side. If the module is not damaged, then the drive circuit may be failed. If the IGBT module is over-current or the frequency converter trips to ground short-circuit during deceleration, the converter module of the upper half or its drive circuit is faulty. When the IGBT module is over-current during acceleration, the module of the lower half or its drive circuit is faulty. These faults are mostly caused by external dust entering the frequency converter or the moist environment.

3. Failure Analysis of Control Loop 

    In the control loop, it is the power source, mainly the smoothing capacitor and snubber capacitor in the IGBT circuit board, that affects the life of the frequency converter, and the principle is the same as above. Since the capacitors are soldered on the circuit board, it is difficult to judge the deterioration by measuring the electrostatic capacity. And the pulsating current passing through the capacitor is a constant value that will not be affected by the main circuit load. So, its life is determined by temperature and power-on time.

    The power circuit board supplies power to the control circuit, the IGBT drive circuit, the surface operation display panel and the fan. Rectified through switching power supply, these power sources are generally the DC voltage output from the main circuit. Therefore, if a certain power supply is short-circuited, not only will rectifier circuit of this circuit damage, but other parts of the power supply may be affected. If the control power supply is short-circuited with the common ground due to incorrect operation and switch on the power supply circuit board is partially damaged, this will cause other power supply to be powered off, etc. And such condition is easy to find out by observing the power supply circuit board.

    The core of the converter is the logic control circuit board. It gathers large-scale integrated circuits such as CPU, MPU, RAM, EEPROM, etc. And it has high reliability, and has a low probability of failure. Sometimes starting up may lead to all control terminals closed at the same time which causes the EEPROM to conk out. And resetting the EEPROM would handle this situation.

    The IGBT circuit board contains driver circuits, buffer circuits, and protection circuits of  over-voltage and phase loss. The PWM signal from the logic control board will input the voltage drive signal into the IGBT module through photocoupler. Therefore, the photocoupler on the IGBT module should also be measured while detecting the mode.

4. Cooling system   

    The cooling system mainly includes a heat sink and a cooling fan. The cooling fan has a shorter life. When the service life is near, the fan generates vibration and the noise increases, finally, the fan stops, and frequency converter trips while IGBT overheating. The life of the cooling fan is limited by the bearing, which is about 10000~35000h. When the converter runs continuously, it should replace the fan or bearing every 2 to 3 years. To extend the life of the fan, fans of some products only operate when the frequency converter runs.

5. Interference of external electromagnetic induction

    If there is a source of interference around the converter, they invade the converter through the radiation or power line, causing the control loop to malfunction and operate abnormally or shutdown, and even damage the converter severely. The specific methods to reduce noise interference are: Installed absorption device, such as RC surge absorber, which does not exceed 20cm, on the control coils of all relays and contactors around the converter, to prevent surge voltage; Try to shorten the control loop by not more than 5mm and keep it a distance of 10cm or more with the main circuit; When the converter is far away from the motor (more than 100m), the cross-sectional area of the conductor can be increased to ensure that the line voltage drop is within 2%. At the same time, the converter output reactor should be installed to compensate the distribution of the charging current of the capacitor due to long-distance.

    Grounding terminal of frequency converter should be grounded according to rules and reliably placed at special grounding point, and never mixed with welding, power grounding. A radio noise filter installed at the input terminal of the converter can reduce the input higher harmonic and thus reduce the noise effects from the power line to the electronic equipment. Meanwhile, a radio noise filter installed at the output terminal can reduce the line noise at the output end.


Detecting method of Frequency converter IGBT module

1. Judging polarity

    Choosing R*1KΩ, if the resistance of a pole and other two poles is infinite and after the pen is exchanged that the resistance are still infinite, then the pole is G. The other two poles measured again and if the resistance is infinite but become lower after exchanged the pen, then jueged that the red pen connect to C and the black pen connect to E.

2. Judging multimeter

    Choosing R*10KΩ, the black pen connect to C, the red pen connect to E, the resistance is zero. The IGBT is triggered to turn on when G and C are touched by finger simultaneously and multimeter turns to the direction of smaller resistance and indicates a certain value. If the IGBT  stops when G and E were touched again and multimeter points to zero, then judged IGBT is normal.

3. Testing matters needing attention

    Any pointer multimeter can be used to detect IGBT. R * 10KΩ must be chose when judging the IGBT, because when below R * 1KΩthe internal battery voltage is too low, and the IGBT can not be turned on when testing, thus the IGBT can not be judged. This method can also be used to detect the quality of power field effect transistor (P-MOSFET).


 Static measurement of frequency converter IGBT module

    The IGBT module used in the frequency converter is a seven-unit integrated module (FP15R12KE3G), namely three-unit rectifier, three-unit inverter and one-unit brake. The module has self checking unit of temperature., measuring by diode file of multimeter.

IGBT.jpg

1. Static measurement of rectifier bridge

    The electrical schematic diagram of the three phase bridge rectifier is as follows, its measure method is the same as the ordinary diode.

electrical schematic diagram.png

Electrical schematic diagram of three phase bridge rectifier and IGBT

2. Static measurement of the frequency converter freewheeling diode 


converter unit.jpg

 converter unit electrical schematic

    The measuring method is the same as the ordinary diode. In general, the damage can be judged by measuring the freewheeling diode of the IGBT.

3. Static measurement of the brake unit

    In the figure, BRK is the brake trigger end. According to the use environment, the user can connect the braking resistor between the terminals P and PB. The selection of the resistance specification refers to the KVFC+ series inverter user manual.

brake unit.png

Electrical diagram of brake unit

Seven-unit IGBT measurement reference.png

Seven-unit IGBT measurement reference

    Quick measurement module tips: the test pen placed on the P terminal does not move, another test pen measures R, S, T, PB, U, V, W, N respectively, and then compare the reference values in the above table. The actual measured value is normal if it not far from the range of the table.

 IGBT measuring process and results .png

 IGBT measuring process and results 


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