1 Mga sanhi at panganib ng pagbuo ng kasalukuyang baras ng motor
Ang boltahe ng baras sa umiikot na mga de-koryenteng makina ay karaniwang nabubuo ng shaft alternating magnetic flux. Ang stretching line variable frequency motor ay pinapagana ng PWM variable frequency power supply. Sa oras na ito, ang boltahe ng baras ng motor ay pangunahing nabuo ng vector sum ng three-phase output boltahe ng power supply at ang non-zero zero-sequence component. Ang frequency converter PWM pulse width modulation ay humahantong sa pagtaas ng mga high-frequency harmonic na bahagi sa sistema ng speed control drive. Ang mga harmonic na sangkap na ito ay bumubuo ng electromagnetic induction sa mga bahagi tulad ng rotating shaft, stator windings at cables. Ang epekto ng pagkakabit ng boltahe ng ibinahagi na kapasidad sa motor ay bumubuo ng sistema ng karaniwang mode loop. Ang boltahe ng common-mode na ito ay nag-o-oscillate sa mataas na frequency at capacitively na pinagsama sa rotor upang makabuo ng pulsed boltahe mula sa shaft patungo sa lupa, na bubuo ng zero-sequence current sa system, at ang motor bearing ay bahagi ng zero-sequence na ito. loop. Tingnan ang Larawan 1
Ang shaft voltage ay bumubuo ng closed loop sa pamamagitan ng motor shaft, bearing, stator frame o auxiliary device upang makabuo ng shaft current, tulad ng ipinapakita sa Figure 2. Ang post-processing AC motor ay gumagamit ng rolling bearings. Sa panahon ng operasyon, sa ilalim ng pagkilos ng boltahe ng baras, ang rotor ng motor ay bumubuo ng isang metal na kontak sa motor stator sa pamamagitan ng tindig, na bumubuo ng isang malaking kasalukuyang baras. Kapag ang tumatakbong baras ay nakipag-ugnayan sa anumang bahagi maliban sa umiikot na katawan, isang discharge ang nangyayari sa bahaging iyon. Kung ang dielectric strength ng bearing oil film ay lumampas, ang shaft voltage ay magiging sanhi ng discharge breakdown ng lubricating oil film upang bumuo ng loop upang makabuo ng shaft current. Ang mataas na temperatura na nabuo sa pamamagitan ng bahagyang discharge energy release ng shaft current ay maaaring matunaw ang maraming maliliit na lugar sa inner ring, panlabas na singsing o bola ng tindig upang bumuo ng mga uka, na nagreresulta sa ingay at panginginig ng boses. Kung hindi ito matukoy at mahawakan sa oras, mabibigo ang tindig.
In addition, when the frequency conversion motor is running, in the stator coil, the DC-AC voltage inverter applies a square wave voltage whose frequency and amplitude can be changed. The 3-fold high-frequency harmonics contained in it generate high voltage peaks, and the peaks have a fast rise time. The generated magnetic field generates capacitive electromagnetic resonance in the grounding circuit of the rotor and bearing, stator and motor casing, and the super-high voltage charge is accumulated and released. It will break down the grease film formed between the bearing balls and the inner casing, causing an instant impact on the bearing.
2 Measures and countermeasures
Since the shaft voltage is generated along with the design, manufacture, installation and operation of the equipment, it is generally unavoidable for users. However, the damage caused by the shaft voltage must meet two conditions: one is the existence of the shaft voltage; the other is the destruction of the insulation (lubricating oil film) of the bearing, which provides a path for the shaft current, and the two are indispensable. Since the shaft voltage cannot be avoided, the emphasis should be placed on the prevention and control of the shaft current, that is, the shaft current should be reduced to a harmless level. Generally, the surface density of the shaft current should be guaranteed to be 0.4A/mm2 or the shaft voltage should be below 0.35 mV.
2.1 Suppress power harmonics
When the inverter is used for power supply, the system should be properly grounded, the smoothing reactor and d V/d t filter should be selected reasonably, and the standing wave effect of the cable between the inverter and the motor should be considered, and the length of the cable between the inverter and the motor should be shortened as much as possible. Reduce harmonics, reduce shaft current, noise, vibration and motor temperature rise. To reduce or suppress the amplitude of overvoltage spikes, two methods can be used: one is to add a reactor (choke coil) or filter on both sides of the cable, which can effectively slow down the rising speed of the output voltage pulse at the power supply end; the other is to add a reactor (choke coil) or filter on both sides of the cable In the design, the cable length between the inverter and the motor should be minimized, and the oscillation period of the transient wave process between the two should be reduced by shortening the cable length, thereby reducing the overvoltage at the motor end.
2.2 Grounding of the motor load side bearing and insulation of the non-load side bearing
For the shaft voltage caused by static charge, a carbon brush grounding mechanism is installed on the drive shaft of the motor drive system near the motor side, so that the grounding brush can be reliably grounded and contacted with the rotating shaft, and the motor shaft current is introduced into the ground to ensure the rotating shaft. The zero potential effectively avoids the occurrence of harmonic electromagnetic resonance and the accumulation of charges on the inner and outer walls of the bearing, and avoids the instantaneous shaft current formed by the charge discharge. Its schematic is shown in Figure 3.
Since the shaft current is the current generated by the shaft voltage through the motor shaft, bearing, stator frame or auxiliary device to form a closed loop, the motor system may include 4 grounding points, one of which is the auxiliary device such as a tachometer installed at the shaft end of the motor on the non-load side. , encoder, etc.; the second is the motor 2-end bearing; the third is the motor coupling to the mechanical equipment. Therefore, at present, when the shaft voltage is high, the frequency conversion motor with rolling bearing structure should use insulating bearings, or add a sleeve in the bearing chamber of the end cover, add an insulating layer between the sleeve and the end cover, and fasten the bolts of the inner and outer covers with insulating sleeves and sleeves. Insulating pads and other means to ensure bearing insulation.
3 Conclusion
With the development of frequency conversion speed regulation technology, the shaft current problem of frequency conversion motor cannot be ignored. In order to effectively maintain the running state of the variable frequency motor and prolong the service life, corresponding measures must be taken to prevent the shaft current. Under the premise that it is impossible to eliminate the shaft current fundamentally, taking insulation measures for the motor is the simplest and most effective way to prevent the shaft current. In addition, regular vibration inspection of motor bearings, combined with bearing failure analysis, can help detect early bearing damage caused by shaft currents.
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2022-09-09