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You should pay attention to these shortcomings of frequency converter

2020-07-04

The wiring distance between the motor and the frequency converter is too long, causing the new motor to burn


Burning two new motors together! The output current and voltage of the inverter are balanced, the input voltage is balanced, the current is unstable, and the fluctuation is 20% - 50%. The frequency converter is burnt out in a 35Hz turn to turn short circuit. The grid voltage is very stable, 720V. The distance between the motor and the frequency converter is 120m. The input and output terminals of the frequency converter are all equipped with reactors. The motor current is very small, and the ambient temperature is about - 10 degrees, which can eliminate the cause of motor overheating. What is the cause of this situation that may cause motor failure?


Analysis reasons: the current of the load motor is not large, which is caused by the frequency converter. The inverter is too far away from the motor. The harmonic of the output current of the inverter breaks through the insulation between the two parts, resulting in a short circuit. The quality of the current and voltage at the motor side can be measured with an oscilloscope.


The cable is long and there is a certain distributed capacitance between the cable and the earth. This capacitance will affect the transmission of electricity, resulting in a certain phase shift between the current and the voltage, which will make the voltage on the motor (peak can be seen by oscilloscope, but can not be seen by ordinary multimeter), and the high voltage will break down the motor winding.


Solution: if the withstand voltage is not enough to burn the motor, we can use the frequency converter with international certification (CE, UL) and suitable reactor to avoid this problem。


Does the frequency converter affect the motor


Ordinary asynchronous motors are designed according to constant frequency and constant voltage, which can not fully meet the requirements of variable frequency speed regulation. The following is the influence of frequency converter on motor:


1。 Efficiency and temperature rise of motor


No matter what kind of frequency converter, harmonic voltage and current will be produced in different degrees during operation, which makes the motor run under non sinusoidal voltage and current. According to the information, at present


For example, the commonly used sine wave PWM frequency converter has zero low order harmonics, and the remaining high-order harmonic components which are about twice the carrier frequency are 2U + 1 (U is the modulation ratio).


Higher harmonic will cause the increase of stator copper loss, rotor copper (aluminum) loss, iron loss and additional loss, especially rotor copper (aluminum) loss。 Because the asynchronous motor is rotating at the synchronous speed corresponding to the fundamental frequency, the rotor loss will be very large after the high harmonic voltage cuts the rotor bar with large slip。 In addition, additional copper consumption due to skin effect should be considered。 These losses will make the motor generate extra heat, reduce efficiency and reduce output power。 If the ordinary three-phase asynchronous motor is operated under the condition of non sinusoidal power supply output by frequency converter, its temperature rise will generally increase by 10% - 20%。


2. The problem of motor insulation strength


At present, many small and medium frequency converters adopt PWM control mode. Its carrier frequency is about several thousand to more than ten kilohertz rise rate, which is equivalent to the steep impulse voltage applied to the motor, so that the turn to turn insulation of the motor can withstand 4-6 times of the voltage. Superimposed on the running voltage of the motor, it will pose a threat to the insulation of the motor to the ground. The stator winding of the motor should bear a very high voltage in addition to the repeated impact of high voltage Accelerated aging of rectangular chopper generated by frequency converter


3. Harmonic electromagnetic noise and vibration


The vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complex when the ordinary asynchronous motor is powered by frequency converter. The time harmonics contained in the frequency conversion power supply interfere with the inherent space harmonics of the electromagnetic part of the motor, forming various electromagnetic exciting forces. When the frequency of electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, resonance will occur, which will increase the noise. Due to the wide range of operating frequency and wide range of rotating speed, the frequency of various electromagnetic force waves is difficult to avoid the natural vibration frequency of each component of the motor.


4. Adaptability of motor to frequent starting and braking


Since the motor can be started without impact current at a very low frequency and voltage, and can be quickly braked by various braking methods provided by frequency converter, which creates conditions for frequent starting and braking. Therefore, the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating force, which brings about the mechanical structure and insulation structure Fatigue and accelerated aging problems.


Frequency converter will produce high odd harmonic


The 5th and 7th times have great influence on the frequency converter and motor。 Usually, in order to reduce the influence of harmonics, reactors and absorption capacitors will be added。 Filter can also be added at the output end of frequency converter。


How to calculate the harmonic power of inverter fed motor?


Method 1


The amplitude and phase of each harmonic of voltage and current are obtained by Fourier transform. The active power of each harmonic is calculated according to P = √ 3uicos φ. The harmonic power is obtained by adding the active power of all harmonics.


Method 2


The total active power is measured, and the fundamental amplitude and phase of voltage and current are obtained by Fourier transform. The fundamental active power is calculated according to P = √ 3uicos φ. The harmonic power is obtained by subtracting the fundamental active power from the total active power.


OPE电竞竞猜娱乐Generally, the higher the harmonic frequency is, the lower the accuracy is. The second method is recommended.


Influence of inverter harmonics on motor and improvement measures


OPE电竞竞猜娱乐1、 The reason of motor damage is that the frequency converter produces a high peak voltage on the stator winding of the motor. The amplitude of the peak voltage exceeds the insulation strength of the winding, resulting in winding damage. The magnitude of the peak voltage will be more than three times the rated operating voltage of the frequency converter, for example, for rated voltage 3


General purpose frequency converter


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