Some misunderstandings in the most popular use of

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Several misunderstandings of using frequency converter

key words: frequency converter; Multiple protection; Capacity; Power factor compensation; Energy saving; use; Understanding

at present, many domestic paper mills have adopted variable frequency speed regulation when transforming the paper machine. It is true that AC frequency converter is a high-tech product combining microelectronic technology and factory strong current technology. It has many advantages, such as convenient control, simple installation, high speed regulation accuracy, good stability and easy operation, and is highly praised by users. However, when we visited the paper mills, we found that many paper mills have misunderstandings about the frequency converter, and this lignin/polypropylene (PP) phenomenon is quite common, which even affects the normal use of the frequency converter, mainly in the following aspects

1 set multiple protection

at present, due to the fear of damage to the frequency converter, multiple protection is often set at the three-phase power incoming line of the frequency converter, such as fuse, leakage protector, AC contactor and three-phase air switch. It is considered that the more protection the project is located in Qinghe economic development Zone, Hebei Province, the safer it will be, but in fact, it is not the case. For example, the leakage protector often limits the normal performance of the converter. General frequency converters are allowed to operate for tens of seconds beyond 150% of the rated current to adapt to the sudden increase of motor startup and load. However, this capacity is often limited by the leakage protector, so that the frequency converter cannot work normally when starting the loaded motor

at present, the use of fuses and contactors is easy to cause phase loss. At present, the common frequency converter is in the "AC-DC-AC" mode, that is, the three-phase AC is changed into DC through the rectifier circuit, and then the DC is controlled by the single-chip microcomputer control circuit in the way of pulse width modulation to control the high-power transistor drive circuit, and the DC is changed into quasi sinusoidal three-phase AC to drive the AC motor according to the requirements of frequency and voltage. If one phase of the fuse is fused or a contact of the contactor is damaged due to burning, the three-phase rectifier will become two-phase rectifier, damaging the working conditions of the frequency converter, which is also one of the main reasons for the damage of the frequency converter. In fact, in order to adapt to various industrial applications, a variety of protection measures are set inside the frequency converter, such as overvoltage protection, undervoltage protection, overcurrent protection and overheating protection, which do not need to be set separately. In order to control the power supply, it is generally necessary to connect an AC contactor with more than twice the rated current of the frequency converter in series on the power incoming line of the frequency converter. However, it should be noted that except in case of emergency, the contactor can only be switched on and off when the frequency converter stops running

2 about the capacity of the frequency converter

at present, the paper machine load that can be directly driven by the AC motor will often be overloaded after adding the AC frequency converter with the same power. Therefore, some users believe that the actual power of the frequency converter is lower than the rated power marked on the nameplate, so their capacity should be selected higher, for example, 22KW for 15kw. This is actually a misunderstanding. The failure of the frequency converter to drive normally is caused by its internal overload protection circuit. When the starting speed rises too fast, the acceleration of the system is too large. According to the electromechanical transmission dynamic equation tm-tl=j ε (where TM is the driving torque of the motor, TL is the load torque of the paper machine, j is the moment of inertia of the paper machine system, ε It is known that the motor is mainly driven by loads with large moment of inertia such as drying cylinder, and the moment of inertia J of the paper machine system is very large (the inertia is very large). Therefore, when TL and j are certain, it is necessary to obtain a large acceleration ε, The drive torque TM must be increased to overload the frequency converter. The above situation can be avoided as long as the setting time of speed increase is extended through the frequency converter. If not, add a y/△ start control circuit (and add a surge absorber) at the output end of the frequency converter (the end connected to the motor) without increasing the capacity of the frequency converter

3 power factor compensation

at present, general paper mills have installed power factor compensation devices to dynamically compensate the reactive power caused by the inductive load of the motor. However, after using the frequency converter, the original compensation device is no longer effective due to the isolation of the internal DC part of the frequency converter. The compensation device is only effective when it is added between the frequency converter and the motor

4 about energy saving and power saving

at present, most users think that using frequency converters can save energy significantly, which needs to be treated dialectically. The legendary frequency converter can save 30% or even 50% of electric energy, which has neither theoretical basis nor reliable application examples. Even the manufacturers of frequency converters dare not guarantee that they will save energy. Our company has used various brands and models of frequency converters since 1987. We can say that the use effect is very good, but no obvious energy-saving effect has been found. This illusion caused to users is the result of exaggerated publicity by businesses

at present, the mechanical characteristic curve of general AC motors is certain. Both theory and practice have proved that when the load power is less than the rated power of the motor interface temperature, its efficiency decreases with the reduction of load torque, that is to say, the motor will relatively consume power when it is lightly loaded. The frequency converter will automatically adjust the v/f value according to the size of the load (where V is the voltage of the motor stator winding and F is the voltage change frequency of the stator winding), and change the mechanical characteristic curve of the motor to adapt to the load, so as to improve the efficiency and achieve the purpose of energy saving. However, when designing a set of paper machine transmission device, the capacity of the motor is matched with the load, and the motor generally works in a state close to full load. Therefore, this power-saving characteristic of the frequency converter can not be brought into play in the paper machine. Only when it is used for fan or pump loads, and the flow often changes according to the use requirements, it is necessary to configure corresponding sensors to form a closed loop together with the frequency converter, so as to achieve the purpose of energy saving. Since the power factor compensation device of the frequency converter is generally a special option, especially the nano particles prepared by physical methods, users need to purchase them from the frequency converter manufacturer. At present, most users do not use them. Therefore, the use of frequency converter increases reactive power instead

at present, the frequency converter is not mysterious, but once it is damaged, the maintenance cost is relatively high. Therefore, users should carefully read the relevant technical data and instructions before use. If necessary, they can ask professional technicians to guide the design or consult the relevant technical departments

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