# Methods for improving the low frequency characteristics of Schneider inverters

Shanghai Kuan Ying Automation Equipment Co., Ltd. will talk to you about improving **施De inverters**Low-frequency characteristics

I. Overview

The common problem with AC speed control systems consisting of frequency converters is that they do not perform well when the system is operating in low frequency regions. Ideally, the main performance is that the starting torque is small when the low frequency starts, which makes the system difficult to start or even unable to start. Due to the higher harmonics generated by the nonlinearity of the inverter, the torque ripple of the motor and the motor heat are generated, and the motor running noise is also increased. During low-frequency steady-state operation, the motor voltage is shaken due to fluctuations in the grid voltage or changes in system load and the odd amplitude of the inverter output voltage waveform. When the distance between the inverter and the motor is large and the harmonics interfere with the control circuit, it is easy to cause the motor to crawl. Due to the above various phenomena, the speed regulation characteristics and dynamic quality indicators of the speed control system composed of frequency converter are seriously reduced. This paper analyzes the low frequency mechanical characteristics of the system and the low frequency characteristics of the frequency converter, and proposes corresponding measures to make the system The low frequency operation characteristics are improved. Second, the low frequency mechanical characteristics of the inverter

1, low frequency start characteristics

Asynchronous motor changes the stator frequency F1, you can smoothly adjust the synchronous speed of the motor, but with the change of F1, the motor The mechanical properties will also change, especially in the low frequency region, according to the zij large torque formula of the asynchronous motor:

Temax=3/2{np(U1/W1)2}/{R1/W1+/(R2/ W1) 2+(LL1+LL2)2} where np the number of poles of the motor;

R1 the resistance of each phase of the stator;

R2 the resistance of each phase of the rotor that is folded to the stator side;

LL1 the leakage inductance of each phase of the stator; LL2 per leakage inductance of the rotor folded to the stator side; U1 the voltage per phase of the motor stator; W1 the angular frequency of the power supply

It can be seen that Temax decreases as W1 decreases, and at low frequencies, R1 is not negligible. Temax will decrease as W1 decreases, and the starting torque will also decrease, even failing to drive the load. 2. Low-frequency steady-state characteristics The formula of the torque at the steady-state operation of the motor is as follows: TL=3np(U1/W1)2SW1R2/{(SR1+R2)2+S2W2(LL1 +LL2)2}

When the angular frequency W1 is rated, R1 can be ignored. At low frequencies, R1 can not be ignored, so in the low frequency region, due to the increase in the proportion of the voltage drop on R1, it will not be able to maintain M constant, especially in the case of grid voltage changes and load changes, the system will appear jitter and crawl. III. Low-frequency characteristics of the inverter speed control system 1. Harmonic analysis

The speed control system consisting of frequency converters, except for the nonlinearity of the frequency converter, in addition to the fundamental current in the stator of the motor There are also harmonic currents. Due to the presence of higher harmonics, the motor loss and inductance are increased, cosφ is reduced, and the output torque is affected, and a pulsating torque of 6 times the fundamental frequency is generated. According to the 5th and 7th harmonics in the current waveform, the 5th harmonic frequency in the stator current of the three-phase motor is F5=5F1 (F1 is the fundamental current frequency), which is in the air gap of the motor. The magnetic potential and the magnetic field in which the spatial negative sequence is generated. The rotational speed n51 of the magnetic field is 5 times the rotational speed n11 of the magnetic field generated by the fundamental current, and rotates in the opposite direction to the fundamental magnetic field, since the motor speed is constant, and is assumed to be close. N11, such that the rotor current of 6 times the fundamental frequency is induced in the rotor by the 5th harmonic magnetic potential, and the combination of the current and the fundamental magnetic potential of the air gap produces a pulsating torque 6 times the fundamental frequency. The magnetic field generated by the 7th harmonic is in phase with the fundamental wave, but the rotational magnetic field generated by it is 7 times the fundamental magnetic field; the harmonic; the rotational speed, so the corresponding rotor current harmonic and air gap main The relative rotational speed of the magnetic field is also six times the fundamental frequency, which also produces a pulsating torque that is six times the fundamental frequency. The above two pulsating torques 6 times the fundamental frequency oscillate the electromagnetic torque of the motor, although the average value is zero, but the pulsating torque makes the motor speed uneven, which affects the low frequency operation. Big. 2. Generation of pulsating torque in quasi-square wave mode ψ1, ψ2 are respectively the space vectors of the stator flux linkage and the rotor flux linkage, in the steady-state quasi-square wave (QSW) operation mode ( The thyristor in the bridge is triggered by a 1800 electrical angle pulse. ψ1 moves along the perimeter of the regular hexagon during the output period. Ψ2 moves along a circular motion concentric with the hexagon. In the quasi-square wave mode, the ψ1 and ψ2 motions are continuous, but they are significantly different. When the vector ψ2 is rotated at a constant stator voltage angular velocity W1, the vector ψ1 is The constant linear velocity runs along the perimeter of the regular hexagon, and the constant velocity of the vector ψ1 causes the angular velocity to change, which in turn causes the angle δ of ψ1 and ψ2 to change. In addition, the amplitude of ψ1 is constant when moving along the hexagonal trajectory. The extent has also changed. When the motor is unloaded, the angle between the steady state ψ1 and ψ2 and the torque T are zero at W1t=0, π/6, π/3, and when W1T≠0, π/6, π/3 , δ is not zero, it causes the low-frequency torque pulsation together with the above-mentioned ψ1 amplitude change, the frequency is 6 times of the stator voltage fundamental wave, corresponding to a constant δ mean when the motor is loaded, low-frequency torque The pulsation will be superimposed above the constant torque mean. Fourth, the system low-frequency characteristics improvement measures

1, the starting torque increase

due to the system's pressure drop on R1 at low frequency, so that the system's starting torque decreases with W1 For this purpose, the frequency converter has a torque boost function, which can adjust the torque of the motor in the low frequency region to match the load and increase the starting torque. The automatic torque increase and manual torque lift modes can be selected. The principle is that increasing the stator voltage will increase the starting torque accordingly, but raising the voltage setting too high will cause the motor to be saturated, overheated or overcurrent tripped due to excessive current. For example, the torque adjustment function of the 1336PLUS series inverter can automatically adjust the boost voltage to generate the required voltage. The boost voltage can be selected according to the current required by the predetermined torque. The torque is increased to control the current while making the motor suitable. In the running state, when selecting the manual torque boost, the torque boost value should be set in combination with the actual situation. 2. Improve the low-frequency torque pulsation. The low-frequency torque pulsation of the AC speed control system formed by the inverter directly affects the dynamic characteristics of the system, whether it is the inverter manufacturer or the system integration engineering technician. Trying to improve the technical problem of low frequency pulsation. If the flux control mode or sine wave PWM control mode is adopted, it does not control the on and off of the GTR according to the intersection of the modulated sine wave and the carrier, but always makes the magnetic flux of the asynchronous motor close to the sine wave, and the trajectory of the rotating magnetic field. It is a circle to determine the conduction law of GTR. At very low frequencies, the asynchronous motor is guaranteed to rotate evenly at low speeds, thereby expanding the frequency conversion speed range and suppressing the vibration and noise of the asynchronous motor. The realization of the circular rotating magnetic field is to change the working mode of the GTR by detecting the magnetic flux so that the control link can judge whether the actual magnetic flux exceeds the error range at any time, thereby ensuring that the trajectory of the rotating magnetic field is circular to reduce the torque pulsation. 3. The circumferential PWM method reduces the torque ripple. The meaning of "circumference" is to specify the sub-magnetic link ψ1 space vector in a Gaussian plane along a polygon that is very close to the circumference, which reduces the motor pulsation The purpose is to determine the width and position of the voltage pulse. The three-phase inverter is a full-wave bridge structure. If it is operated in such a way, when one of the AC outputs (a, b, c) is switched on at any time, the DC bus should be connected (should be connected to another DC bus at the same time). Above), this principle can be clearly seen from Figure 1 (a). Obviously there are six ways in which the AC output is connected to the DC bus, which results in six positions of the space vector of the stator voltage U1. These six positions are shown in Figure 1(b), and the six openings in Figure 1(b) are The off state corresponds to the six positions of U1. The thick line position in the figure indicates that the switches 1, 3, and 6 are in the open position, and the instantaneous phase voltage generated by the projection is as follows:

Va=Vb=1/3VdcVc=-2 /3Vdc

Other analogy, the symbols Va, Vb, Vc represent the instantaneous phase voltage values of the three-phase output voltage, if Ia+Ib+Ic=0 is the vertical projection of the space vector on the A, B, and C axes. Obtaining Va, Vb, and Vc, in addition to the above six on/off states, there are two states in which the switches 1, 3, 5, 2, 4, and 6 are simultaneously turned off, in which case the AC output terminal a, b, c is connected to the same potential, U1 and Ua, Ub, Uc are sequentially changed to zero. Applying this operation mode to a three-level PWM inverter can be compared with the two-level PWM. Low harmonic content. The PWM form is a chopping quasi-square wave modulation. The phase voltage on the load consists of a rectangular segment and a zero voltage segment (U1 = 0). At each voltage pulse, the vector ψ1 moves at a constant linear velocity. While still standing at zero voltage, however, since the vector ψ2 is rotated at a constant angular velocity W1, the angle δ between ψ1 and ψ2 appears, so voltage chopping is the main cause of high-frequency torque ripple, frequency and output voltage. The moment pulse frequency is the same. This is due to the inherent PWM, in fact, high frequency torque ripple is difficult to eliminate and superimposed on the low frequency torque ripple. In order to eliminate the low frequency torque ripple of the system, it can work in the following two ways. 1.1 At the moment of the midpoint of the voltage pulse, the angle δ between the vectors ψ1 and ψ2 should be constant for all pulses during steady-state operation, eliminating the low-frequency torque (frequency 6F1) caused by the δ change. The effect of δ=0 under no-load conditions, although the amplitude of ψ1 changes, the low-frequency torque ripple will still be completely eliminated. 1.2 At a constant load (δ-cost≠0), only a change in the amplitude of ψ1 causes low-frequency torque ripple, and the load causes the change in the amplitude of ψ2 to be negligible, so it is necessary to obtain a ψ1 that is closer to the circumference. Vector track. The circumferential PWM uses the spatial position of the no-load vector ψ1 to determine the intermediate point of the voltage pulse, that is, the reasonable combination of the thyristor conduction segment and the zero voltage segment, which can produce a 幅1 whose amplitude variation is negligible. This principle is shown in FIG. , ψ 1 stop time (ie zero voltage segment) marked with dots, determine the voltage pulse position to make them symmetrical, as shown in the middle point of each abscissa in the figure, the pulse width (ie duration) corresponds to the length of the abscissa, required The output voltage is determined. The natural voltage waveform period is determined by the time required for the ψ1 vector to travel one turn along the polygon. By adopting this method, the low frequency torque ripple can be effectively eliminated while keeping the output voltage variable from zero to large value of zui.