Tag: IGBT module

A. Damage caused by abnormal load
Indeed, the protection circuit of the frequency converter is already quite complete. For the protection of expensive inverter modules, various inverter manufacturers have put a lot of effort into their protection circuits, from output current detection to IGBT voltage drop detection in the drive circuit, and strive to implement the fastest overload protection with the fastest strain rate! From voltage detection to current detection, from module temperature detection to phase loss output detection, there has not been a protection circuit for any particular electrical appliance yet. A frequency converter has been focused and invested in this approach. When salespeople talk about the performance of frequency converters, they must also mention the protection function of the frequency converter. They often unconsciously promise users that with the comprehensive protection function of the frequency converter, your motor will not be easily burned. This salesperson doesn’t know that this promise will bring him great passivity!

Does the motor really not burn when using a frequency converter? My answer is: compared to power supply, using a frequency converter makes it easier for the motor to burn, and the motor is prone to burning, making it easier for the inverter module of the frequency converter to be “reimbursed” together. The sensitive overcurrent protection circuit of the frequency converter is at a loss here and has no effect at all. This is a major external cause of damage to the frequency converter module. Listen to me explain the truth behind it.
A motor can operate at power frequency, although the operating current is slightly higher than the rated current, there is a certain temperature rise during long-term operation. This is a faulty motor that can indeed run before it burns out. But after connecting to the frequency converter, frequent overloads may occur, making it impossible to operate. It doesn’t matter yet.
A motor that can operate in power frequency mode and has been used normally by the user for many years. Please pay attention to the word “many years”. Users may think of saving electricity bills or need to undergo frequency conversion modifications due to process modifications. But after connecting to the frequency converter, there will be frequent OC faults, which is good. The protection has stopped and the module is not damaged. What’s scary is that the frequency converter didn’t immediately trip the OC fault, but was running for no reason – after only three or two days of operation, the module exploded and the motor burned out. The user relied on the salesperson: the quality of the frequency converter you installed is poor, burned my motor, you want to compensate my motor!
Prior to this, the motor seemed to have no problems and was running well. The operating current was measured because the load was relatively light, reaching half of the rated current; Tested three-phase power supply, 380V, balanced and stable. It really seems like the damage to the frequency converter, along with the damage to the motor.
If I were present, I would be fair and impartial: don’t blame the frequency converter, it’s because your motor is already “critically ill” and suddenly malfunctioned, accompanied by damage to the frequency converter!
A motor that has been in operation for many years, due to temperature rise and moisture, the insulation degree of the winding has greatly decreased, and even has obvious insulation defects, which are at the critical point of voltage breakdown. In the case of power supply at power frequency, the input voltage of the motor winding is a three-phase 50Hz sine wave voltage, and the induced voltage generated by the winding is also relatively low. The surge component in the circuit is small, and the decrease in the insulation degree of the motor may only bring about inconspicuous “leakage current”. However, the voltage breakdown phenomenon has not yet occurred between the turns and phases of the winding, and the motor is still “operating normally”. It should be said that as the degree of insulation aging deepens, even under power supply at power frequency, it is believed that in the near future, the motor will eventually burn out due to voltage breakdown between phases or windings caused by insulation aging. But the problem is, it hasn’t burned down yet.

After connecting to the frequency converter, the power supply conditions of the motor become “harsh”: the PWM waveform output by the frequency converter is actually a carrier voltage of several kHz or even more than ten kHz, and various components of harmonic voltage will also be generated in the motor winding power supply circuit. According to the characteristics of the inductor, the faster the change rate of the current flowing through the inductor, the higher the induced voltage of the inductor. The induced voltage of the motor winding has increased compared to the power frequency supply. Insulation defects that cannot be exposed during power supply at power frequency are caused by the inability to withstand the impact of induced voltage under high-frequency carriers, resulting in voltage breakdown between turns or phases of the winding. The sudden short circuit of the motor winding was caused by a phase to turn short circuit in the motor winding. During operation, the module exploded and the motor burned out.
In the initial stage of start-up, due to the low output frequency and voltage of the frequency converter, when there is a fault in the load motor, although it causes a large output current, this current is often within the rated value. The current detection circuit acts in a timely manner, and the frequency converter implements a protective shutdown action, so there is no risk of module damage. But if the three-phase output voltage and frequency reach high amplitudes under full speed (or near full speed) operation, if there is voltage breakdown phenomenon in the motor winding, it will instantly form a huge surge current. Before the current detection circuit acts, the inverter module cannot withstand it and will explode and be damaged.
From this, it can be seen that protective circuits are not omnipotent, and any protective circuit has its own weaknesses. The frequency converter is unable to effectively protect the motor winding from sudden voltage breakdown during full speed operation. Not only the frequency converter protection circuit, but any motor protector cannot effectively protect against such sudden faults. When such sudden faults occur, it can only be declared that the motor has indeed passed away.
This type of fault is a fatal blow to the inverter output module of the frequency converter and cannot be avoided.

其它由供电或负载方面引起的原因，如过、欠压、负载重、甚至堵转引起的过流等故障，在变频器的保护电路正常的前提下，是能有效保护模块安全的，模块的损坏机率将大为减小。在此不多讨论。
B、由变频器本身电路不良造成的模块损坏
1、 由驱动电路不良对模块会造成一级危害
由驱动电路的供电方式可知，一般由正、负两个电源供电。+15V电压提供IGBT管子的激励电压，使其开通。-5V提供IGBT管子的截止电压，使其可靠和快速的截止。当+15V电压不足或丢失时，相应的IGBT管子不能开通，若驱动电路的模块故障检测电路也能检测IGBT管子时，则变频器一投入运行信号，即可由模块故障检测电路报出OC信号，变频器实施保护停机动作，对模块几乎无危害性。
而万一-5V截止负压不足或丢失时（如同三相整流桥一样，我们可先把逆变输出电路看成一个逆变桥，则由IGBT管子组成了三个上桥臂和三个下桥臂，如U相上桥臂和U相下桥臂的IGBT管子。）， 当任一相的上（下）桥臂受激励而开通时，相应的下（上）桥臂IGBT管子则因截止负压的丢失，形成由IGBT管子的集-栅结电容对栅-射结电容的充电，导致管子的误导通，两管共通对直流电源形成了短路！其后果是：模块都炸飞了！
截止负压的丢失，一个是驱动IC损坏所造成；还有可能是驱动IC后级的功率推动级（通常由两级互补式电压跟随功率放大器组成）的下管损坏所造成；触发端子引线连接不良；再就是驱动电路的负供电支路不良或电源滤波电容失效。而一旦出现上述现象之一，必将对模块形成致命的打击！是无可挽回的。
2、脉冲传递通路不良，也将对模块形成威胁
由CPU输出的6路PWM逆变脉冲，常经六反相（同相）缓冲器，再送入驱动IC的输入脚，由CPU到驱动IC，再到逆变模块的触发端子，6路信号中只要有一路中断——
a、变频器有可能报出OC故障。逆变桥的下三桥臂IGBT管子，导通时的管压降是经模块故障检测电路检测处理的，而上三桥臂的IGBT管子，在小部分变频器中，有管压降检测，大部分变频器中，是省去了管压降检测电路的。当丢失激励脉冲的IGBT管子，恰好是有管压降检测电路的，则丢失激励脉冲后，检测电路会报出OC故障，变频器停机保护；
b、变频器有可能出现偏相运行。丢失激励脉冲的该路IGBT管子，正是没有管压降检测电路的管子，只有截止负压存在，能使其可靠截止。该相桥臂只有半波输出，导致变频器偏相运行，其后果是电机绕组中产生了直流成分，也形成较大的浪涌电流，从而造成模块的受冲击而损坏！但损坏机率较第一种原因为低。
若此路脉冲传递通路一直是断的，即使模块故障电路不能起到作用，但互感器等电流检测电路能起到作用，也是能起到保护作用的，但就怕这种传递通路因接触不良等故障原因，时通时断，甚至有随机性开断现象，电流检测电路莫名所以，来不及反应，而使变频器造成“断续偏相”输出，形成较大冲击电流而损坏模块。
而电机在此输出状态下会“跳动着”运行，发出“咯楞咯楞”的声音，发热量与损耗大幅度上升，也很容易损坏。
3、电流检测电路和模块温度检测电路失效或故障，对模块起不到有效地过流和过热保护作用，因而造成了模块的损坏。
4、主直流回路的储能电容容量容量下降或失容后，直流回路电压的脉动成分增加，在变频器启动后，在空载和空载时尚不明显，但在带载起动过程中，回路电压浪起涛涌，逆变模块炸裂损坏，保护电路对此也表现得无所适从。
对已经多年运行的变频器，在模块损坏后，不能忽略对直流回路的储能电容容量的检查。电容的完全失容很少碰到，但一旦碰上，在带载启动过程中，将造成逆变模块的损坏，那也是确定无疑的！
C、质量低劣、偷工减料的少部分国产变频器，模块极易损坏
这是国民劣根性的一种体现，民族之痒啊。不错，近几年变频器市场的竞争日趋激烈，变频器的利润空间也是越来越狭窄，但可以通过技术进步，提高生产力等方式来提高自身产品的竞争力。而采用以旧充新、以次充好、并用减小模块容量偷工减料的方式，来增加自己的市场占有率，实是不明智之举呀，纯属一个目光短浅的短期行为呀。
1、质量低劣、精制滥造，使得变频器故障保护电路的故障率上升，逆变模块因得不到保护电路的有效保护，从而使模块损坏的机率上升。
2、逆变模块的容量选取，一般应达到额定电流的2.5倍以上，才有长期安全运行的保障。如30kW变频器，额定电流为60A，模块应选用150A至200A的。用100A的则偏小。但部分生产厂商，竟敢用100A模块安装！更有甚者，还有用旧模块和次品模块的。此类变频器不但在运行中容易损坏模块，而且在启动过程中，模块常常炸裂！现场安装此类变频器的工作人员都害了怕，远远地用一支木棍来按压操作面板的启动按键。
容量偏小的模块，又要能勉强运行，模块超负荷工作，保护电路形成同虚设（按变频器的标注功率容量来保护而不是按模块的实际容量值来保护），模块不出现频繁炸毁，才真是不正常了。
这类机器，因价格低廉，初上市好像很“火”，但用不了多长时间，厂家也只有倒闭一途了。

The reason for the third type of module damage should not have been a single cause. Hopefully, in the near future, the only reasons for module damage will be the first two.
For domestic frequency converters, sometimes it’s just a piece of mouse manure that spoils a pot of soup. Many frequency converters are also quite good, not inferior to foreign products, and they are of good quality and affordable.