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Analysis of the mechanism of the shaft voltage and shaft current of variable frequency motor (2)
Source: Bearing network time: 2013-04-29
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First assume that the cable length is zero; according to the existence of bearing current, the dv/dt current is mainly caused by the input trip voltage; therefore, the dv/dt current size is related to the inverter carrier frequency and voltage rise time. Inverter carrier The higher the frequency; the more dv/dt currents occur in a sine wave period; but the current amplitude does not change at this moment. The pulse voltage rise time is a resolution factor that affects the dv/dt current amplitude; other distributed capacitance The size of the dv/dt current also affects the magnitude of the dv/dt current. The direct cause of the EDM current is the presence of the shaft voltage; therefore, the magnitude of the shaft voltage determines the EDM current amplitude; the magnitude of the shaft voltage is determined by the size of the input voltage and the distribution within the motor. The size of the capacitor. Although the inverter carrier frequency and pulse voltage rise time will affect the shape of the shaft voltage; however, the peak value of the shaft voltage is not connected to both; therefore, the EDM current has no substantial contact with the two; this is the EDM current. The biggest difference from the dv/dt current. Of course; the EDM current is also related to the breakdown voltage of the bearing oil layer; the higher the breakdown voltage; the greater the EDM current of the attack. Convenient for comments; Cheng breakdown voltage greater than or equal to the voltage axis.
4.1 change the rise time tr
The simulation results show that the shaft voltage and bearing current waveforms with different rise times are shown in Figure 5. Graphs a) and b) are the shaft voltage waveforms; Figures c) and d) are the bearing current waveforms; The current is the EDM current; the other is the dv/dt current. It can be seen from the analysis; 1) tr increases the bearing current reduction; includes the dv/dt current and the EDM current. Especially the dv/dt current amplitude decreases very significantly; The effect on the EDM current is not significant; this is primarily because the EDM current is determined by the shaft voltage and the bearing impedance, 2) when tr is less than a certain value (about 200ns); the dv/dt current is even higher than the EDM current, and 3) the rise is changed. Time has little effect on the shaft voltage, 4) Special appearance: the shaft voltage exhibits two vibrations during voltage breakdown; tr does not affect the first vibration; but affects the second vibration; and the second vibration follows the rise of tr And the reason is that the coupling path of the stator winding to the rotor still exists after the bearing is short-circuited; so a dv/dt current vibration is presented.
4.2 Changing the coupling parameters and bearing parameters The larger the coupling capacitance of the stator winding to the rotor; the higher the shaft voltage; the dv/dt current and the EDM current are added, the bearing capacitance is reduced. The dv/dt current is reduced. The bad EDM current is unchanged. At this moment, the shaft voltage rises. The reason is: in the common mode circuit; the shaft voltage is formed by the voltage coupling of the stator windings to the rotor core; the existence of this voltage is maintained by the bearing capacitance and the rotor-to-case coupling capacitance. The two are connected in parallel; and then connected in series with the former; therefore, the shaft voltage is distributed according to the capacitance value; the larger the capacitance is, the smaller the voltage drop is. Generally, the bearing capacitance and the rotor-to-case coupling capacitance are much larger than the stator winding-to-rotor coupling capacitance. In the case of only changing the bearing capacitance; the bearing capacitance is more. 霾 17. 缛莸 е е 迪 迪 æ‹… æ‹… æ‹… æ‹… ç‚Š ç‚Š æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ æŽè°¥å²¢ d d d d d d d d d d d d d d d d d d d d d When /dt is constant; the capacitance resistance decreases. dv/dt current decreases. The simulation results are shown in Figure 6.
5 According to the theoretical research and simulation analysis of the early years, one of the primary reasons for the onset of motor bearing current is that the high frequency pulse output by the inverter has too high dv/dt front and rear edges; thus it can be seen that the bearing current is A useful method is to reduce the dv/dt of the inverter output voltage. However, the pulse voltage rise time of the inverter itself is determined by the switching characteristics of the power equipment; therefore, the device can only be modified at the output of the inverter. The dv/dt. of the voltage is one of the most straightforward ways to reduce the output voltage of the inverter. The most straightforward way is to have a large reactor on the inverter output; it can form a so-called "sine wave filter"; The pulse voltage output from the transformer becomes a complete sine wave voltage after passing through the large reactor; this eliminates the shaft voltage and the bearing current.
However, the value of this method is that the power loss of the reactor is large; the volume is large; the cost is high; it is not very suitable for use in the general frequency conversion speed regulation system. This paper chooses the compromise method; the inductance value of the inverter is not connected at the output end of the inverter. The large inductance changes with the speed of the æº current; together, the RC reactance is set between the output lines to absorb the higher harmonics of the output voltage; this can properly reduce the dv/dt value of the rising edge of the output pulse voltage; the intention of reaching the bearing current of the æº.
The inverter output filter reduces the voltage rise rate of the input pulse voltage of the motor; thus, the voltage coupling effect of the distributed capacitance in the motor is greatly weakened; the shaft voltage and the resulting EDM current are both reduced; together because of the voltage change rate The resulting dv/dt current is also significantly reduced; thus the filter can be usefully pressed for the onset of the bearing current. Figure 8 shows the motor bearing current simulation waveform before and after participating in the filter (ungrounded); during this; the inverter carrier The frequency is 5KHZ; the pulse voltage rise time is 200ns; the cable length is 100m. It can be seen from the figure; whether the EDM current is still dv/dt current is significantly reduced. It is also found in the simulation; the filter is grounded; regardless of dv/dt The current is still significantly reduced when the EDM current is relatively ungrounded; the reason is that the RC branch has a stronger effect of absorbing higher harmonics; the voltage waveform can be better improved.
6 Conclusion Under the high-frequency PWM pulse input; the voltage coupling effect of the distributed capacitance in the motor constitutes the common mode path; then the shaft voltage and the bearing current are difficult. The bearing current mainly exists in three ways: dv/dt current, EDM current and ring The current of the shaft current is not only related to the coupling capacitance parameters of each part in the motor; it is related to the rise time and amplitude of the pulse voltage. This paper focuses on the bearing current of the first two methods. The dv/dt current is the first and the rise time of the PWM. Tr related; the smaller the tr, the greater the magnitude of the dv/dt current.
The higher the carrier frequency of the inverter; the more dv/dt current components in the bearing current. The EDM current presents a certain contingency; only when the smooth oil layer of the bearing is broken down or the inner touch of the bearing can be presented; its amplitude depends on the top priority. The thickness of the shaft voltage is based on the falling pulse voltage rise rate; a kind of inductive inductance at the output of the inverter is supplemented by the RC absorption network to reach the voltage of the yoke and the bearing current; the simulation results verify the method. Usefulness.
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