Modern frequency changer technical analysis
Cheng Xi (Heilongjiang longmay mining group co., ltd.)
The general frequency changer by its merits and so on intellectualization, digitization, network receives people's favor more and more, but along with the general frequency changer application scope's expansion, exposes some questions. In view of these questions, this article carried on has analyzed and proposed the countermeasure.
First, overtone question and countermeasure.
The general frequency changer's main circuit form generally by the rectification part, the contra variant part and the filter part is composed. The rectification part for the three-phase bridge-type uncontrolled rectifier, the invertors part is the IGBT three-phase bridge-type invertors, and the output is the PWM profile. Because the general frequency changer's rectification part uses the diode uncontrolled bridge-type leveling circuit, the middle filter part uses the big electric capacity to take the filter, therefore rectifier's input current in fact is capacitor's charging current, assumes a higher more and precipitous pulse wave, its harmonic component is big. For the harmonic cancellation, may use:
1, Increases the frequency changer electric power supply internal resistance. Usually in the situation, power equipment's internal resistance may play the cushion frequency changer concurrent filter electric capacity's reactive power role. This kind of internal resistance is transformers short-circuit impedance. When power source capacity relative frequency changer capacity hour, then the internal resistance value relatively is bigger, the harmonic content is smaller; When the power source capacity relative frequency changer capacity is bigger, then the internal resistance value relatively is bigger, the harmonic content is bigger. Therefore best choice short-circuit impedance big transformer.
2, Installs the reactor. Installs the reactor is actually increases the frequency changer electric power supply's internal resistance from exterior. Installs the direct-current reactor in frequency changer's exchange side installment exchange reactor or in frequency changer's direct-current side, also or installs, suppresses the harmonic current.
3, Transformer heterogeneity movement. The general frequency changer's rectification part is six pulse wave rectifiers, therefore produces the overtone is big. If
using transformer's heterogeneity movement, causes phase angle mutual deviation 30°like Y-△, △-△the combination two transformer constitution to be equal to 12 pulse wave effects, then may reduce the low order harmonic current 28%, plays the harmonic suppression role.
4, Adjusts frequency changer's carrier ratio. So long as carrier compared to enough big, is inferior the overtone to be possible to suppress effectively, specially the reference wave peak-to-peak value and the carrier peak-to-peak value are smaller than 1:00, 13 following odd number overtones no longer appear.
5, Special-purpose filter. The special-purpose filter use in examining the frequency changer harmonic current the peak-to-peak value and the phase, and produces one with the harmonic current peak-to-peak value to be the same, but the phase just right opposite electric current, passes through in the frequency changer, absorption harmonic current.
Second, matching of load question and countermeasure.
Produces machinery's torque characteristics to divide into three types roughly: Permanent torque loading, air blower pump class load and permanent power termination. In view of the different load type, should choose the different type the frequency changer.
1, Permanent torque loading. The permanent torque loading is refers to the load moment to have nothing to do with the rotational speed, under any rotational speed, the torque maintains constant. But the permanent torque loading divides into the friction class load and the potential energy type capacity reactive翻译
two kinds.
2, Air blower pump class load. The air blower pump class load is present industry field application most equipment, although the pump and air blower's characteristic are many and varied, but mainly by the centrifugal pump and the centrifugal fan application, the general frequency changers are primarily most in this kind of load application. This kind of load is not high to frequency changer's performance requirement, only requests the efficiency and the reliability, therefore the choice has the U/f=const control model frequency changer then, like Mitsubishi frequency changer FR-F540 (L) series. Air blower load in actual movement process, because the rotor inertia is quite big, therefore frequency changer's rise time and the deceleration time are a very important question.
The air blower load's system rotor inertia computation is very important. When frequency changer concrete design, according to the computed result, carries on the
suitable revision, when the frequency changer starts does not occur the overflow trip and the frequency changer decelerate when does not have in the overvoltage trip situation, choice shortest time. But the pump class load in the actual movement process, easy to have the buzz, the build the pressure and the water hangs the effect, therefore time frequency changer shaping, must choose is sui
table for the pump class load the frequency changer, and frequency changer in function hypothesis time must aim at the above question to carry on establishes alone.
3, Permanent power termination. The permanent power termination refers to the load which the torque roughly and the rotational speed is in reverse proportion, like coiler, unwinding coiler and so on. Speaking of certain speed change scope, when actuates the permanent power termination using the frequency changer, usually the consideration selects the permanent torque velocity modulation method below some rotational speed, when is higher than this rotational speed only then selects the permanent power velocity modulation method. Usually is called this rotational speed spots the base frequency, but this point correspondence's voltage outputs the rated voltage for the frequency changer.
Third, gives off heat the question and the countermeasure.
Frequency changer's giving off heat is produces by the internal loss. Carries on the radiation to the frequency changer, usually use:
1, Uses the ventilator radiation. Frequency changer's internal installation ventilator may carry off frequency changer's box body interior radiation, if the ventilator cannot the normal work, immediately stop the frequency changer movement.
2, Reduces installs the ambient temperature. General frequency changer's environment operating temperature general request in - 10℃ to - 50℃, if can take the measure to reduce the frequency changer operating temperature as far as possible, then frequency changer's service life lengthens, the performance is also quite stable. May adopt two methods regarding this question: First, constructs the independent frequency changer low pressure, the interior installs the air conditioning, maintains the low pressure the temperature in +15℃to +20℃between; Second, frequency changer's installment space needs to satisfy the frequency changer instruction for use the request.
The above talks about the frequency changer gives off heat is refers to the frequency changer in the rated range the normal operation loss. When the frequency
changer has the unusual movement (for example overflow, when crosses pressure, the loss which overload and so on) produces, must avoid this kind of phenomenon through the normal shaping the occurrence.
Regarding the air blower pump class load, take Mitsubishi frequency changer FR-F540 as the example, when the frequency changer is opposite in its rated load 120% overloads, its duration is 60 seconds, and does not allow within 300 seconds to present the second overload. When the frequency
changer presents the overload, because the power unit it winds through the excess current elevates temperature, causes the frequency changer overheated, by must cause its temperature decrease to cause frequency changer's overheated protection movement as soon as possible to eliminate, this undercooking is frequency changer's overload cycle. The different frequency changer, its overload multiple, the overload time and the overload cycle are not same, and its overload multiple is bigger, the overload time is shorter.
Electrical machinery which actuates regarding the frequency changer, may divide into two kinds according to its working condition: Long-term working system and redundant short-time working system. The long-term working system's electrical machinery may accord its name brand stipulation data long-term movement. In view of this kind of load, the frequency changer may carry on the shaping according to the electrical machinery data plate data, like continuous running oil pump, if its electrical machinery power is time 22kW, may choose the FR-F540-22k frequency changer then. The redundant short-time working system electrical machinery, its characteristic is the duplication and short-time, namely electrical machinery's operating time and the idle time carries on alternately, moreover is quite short, the two sum, according to the country stipulated that do not surpass for 60 seconds. The redundant short-time working system electrical machinery permits its overload, and has certain elevati
on of temperature. This time, if chooses the frequency changer according to the electrical machinery data plate data, will create frequency changer's damage inevitably. In view of this kind of load, the frequency changer in the reference electrical machinery data plate data's situation must according to the electrical machinery load diagram and frequency changer's overload multiple, the overload time, and the overload cycle comes the shaping. If is redundant the short-time movement the elevator, its electrical machinery power is 18.5kW, may choose the FR-A540-22K frequency changer.
Through has the problem to the general frequency changer movement process in the analysis, proposed solves these question actual countermeasure, along with the
new technology and the new theory unceasingly on frequency changer's application, frequency changer existence's these questions hopefully solves through frequency changer's function and the compensation. Will enhance unceasingly along with the industry scene and the social environment to frequency changer's request, satisfy the actual need the falsehood “the green” the frequency c hanger also soon to appear on the market.

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