Extending the Service Life of Electric Motors
How screened cables can reduce harmful bearing currents
When operating electric motors with frequency converters, undesirable bearing currents often occur which can significantly impact the service life of the rolling contact bearings. Scientists at TU Darmstadt have investigated these bearing currents - and found out that they can be reduced in some cases with the help of screened motor supply cables.
Bearing currents are undesirable currents that occur during the operation of electric motors. They flow through the roller bearings that connect the movable rotor to the fixed motor stator. Bearing currents occur mainly in drives where the speed is changed with the help of a frequency converter and can cause considerable damage to the bearings. Particularly in cases that have an inappropriate earthing concept, bearing currents with high current densities can occur, which melt or even steam the metal in the bearing ring. The result is craters and so-called riffles in the roller bearing, which become noticeable with increased vibrations and even faster wear. As a result, the bearing often fails only a few months after installation.
There are four kinds of bearing currents, each of which is influenced by different factors and has varying degrees of impact:
- Capacitive bearing currents are caused by the voltage across the bearing changing and compensating the currents flowing. These are typically too small to be harmful.
- Discharge bearing currents (EDM) are pulsed discharge currents. They can occur if the voltage increases in the bearing across the lubricant film until it is electrically broken.
- Circular bearing currents are circular currents in the electrical machine. This is caused by a high-frequency magnetic ring flux around the shaft, which induces a voltage along the shaft.
- Rotor-to-earth bearing currents can occur if the rotor of the electric drive is additionally earthed through a gearbox or another device and its earth impedance is lower than that of the stator. The current flows proportionally from the motor housing through the bearings into the shaft and across the earth connection of the rotor.
However, by designing the bearing appropriately, using hybrid or ceramic bearings as well as brushes, filters and earthing rings, design engineers have some options to reduce and avoid these unwanted currents.
Screening Makes the Difference
A decisive criterion is the selection of the right motor cable - this was the result of a study by the Institute for Electrical Energy Conversion at the Technical University of Darmstadt, carried out with the cooperation of HELUKABEL. The scientists compared the occurring bearing currents for cable types with and without screening in several test setups. In the process, they found out that rotor-to-earth bearing currents in particular can be significantly reduced by using screened cables.
Screened cables are a simple and effective method to protect rolling-contact bearings from premature wear and failure. An additional advantage: their screening also prevents unwanted electrical radiation to other devices and thus contributes to the electromagnetic compatibility (EMC) of the entire system.
In its tests, the team from the TU Darmstadt used HELUKABEL motor cables of the type TOPFLEX-EMV-UV-3 PLUS 2YSLCYK-J. This double-screened cable is designed for interference-free operation of frequency converters. Furthermore, it is UV and weather resistant and therefore suitable for outdoor use. HELUKABEL offers its TOPFLEX series in many versions according to different requirements - and has an extensive range of servo and feedback cables for industry and mechanical engineering.
Kuongeza maisha ya huduma ya motors umeme Jinsi cables screened inaweza kupunguza hatari kuongoza mikondo wakati wa kuendesha motors umeme na frequency converters, zisizohitajika kuongoza mikondo mara nyingi kutokea ambayo inaweza kwa kiasi kikubwa kuathiri maisha ya huduma ya rolling kuwasiliana kuongoza. Wanasayansi katika TU Darmstadt wamechunguza hizi bearings mkondo - na kupatikana kwamba wanaweza kupunguzwa katika baadhi ya kesi kwa msaada wa screened motor usambazaji cables. Mtiririko wa kubeba ni mtiririko usiofaa ambao hutokea wakati wa uendeshaji wa motors za umeme. Wao mtiririko kupitia rollers kuunganisha rotor kusonga kwa stator motor fasta. Bearing mikondo kutokea hasa katika drives ambapo kasi ni iliyopita kwa msaada wa frequency converter na inaweza kusababisha uharibifu mkubwa kwa bearing. Hasa katika kesi ambazo zina dhana isiyofaa ya ardhi, mikondo ya kubeba na wiani wa juu wa sasa inaweza kutokea, ambayo huyeyuka au hata mvuke chuma katika pete ya kubeba. Matokeo ni craters na hivyo aitwaye riffles katika roller bearing, ambayo kuwa noticeable na kuongezeka vibrations na hata haraka kuvaa. Matokeo ni kwamba mara nyingi bearing hiyo hushindwa kufanya kazi miezi michache tu baada ya kuwekwa. Kuna aina nne za mishipa ya kuunganisha, kila moja ambayo huathiriwa na mambo tofauti na ina viwango tofauti vya athari: Mishipa ya kuunganisha ya capacitive husababishwa na voltage katika mishipa ya kuunganisha na kusawazisha mtiririko wa sasa. Kwa kawaida hizo ni ndogo sana hivi kwamba haziwezi kusababisha madhara. - Discharge bearing currents (EDM) ni pulsed discharge currents. Wanaweza kutokea kama voltage huongezeka katika bearing katika filamu lubricant mpaka ni umeme kuvunjwa. - Circular bearing mikondo ni mikondo mviringo katika mashine ya umeme. Hii husababishwa na high-frequency magnetic pete flux kuzunguka shaft, ambayo husababisha voltage kando ya shaft. - Rotor-kwa-ardhi bearings mkondo inaweza kutokea kama rotor ya umeme drive ni ziada grounded kupitia gearbox au kifaa kingine na ground impedance yake ni chini ya ile ya stator. Mtiririko sasa mtiririko uwiano kutoka nyumba motor kupitia bearings katika shaft na katika uhusiano wa dunia ya rotor. Hata hivyo, kwa kubuni bearing ipasavyo, kwa kutumia hybrid au makaa ya mawe kama vile brushes, filters na pete grounding, wahandisi kubuni kuwa na baadhi ya chaguzi kupunguza na kuepuka mikondo hii zisizohitajika. Uchunguzi wa nguvu za umeme ni muhimu kwa ajili ya kuamua ubora wa umeme wa umeme, kulingana na utafiti uliofanywa na Taasisi ya Uongozi wa Nishati ya Darmstadt, ambayo inashirikiana na HELUKABEL. Wanasayansi walilinganisha mikondo ya kubeba inayotokea kwa aina za kebo na bila kuchunguza katika miundo kadhaa ya majaribio. Katika mchakato huo, waligundua kwamba rotor-kwa-ardhi bearings mikondo hasa inaweza kupunguzwa kwa kiasi kikubwa kwa kutumia kebo screened. Screened cables ni njia rahisi na ufanisi wa kulinda rolling-mawasiliano lagging kutoka kuvaa kabla ya wakati na kushindwa. Faida ya ziada: screening yao pia huzuia mionzi isiyo ya lazima ya umeme kwa vifaa vingine na hivyo inachangia uwiano wa umeme wa umeme (EMC) wa mfumo mzima. Katika majaribio yake, timu kutoka TU Darmstadt kutumika HELUKABEL motor cables ya aina TOPFLEX-EMV-UV-3 PLUS 2LSYK-J. Hii double-screened cable ni iliyoundwa kwa ajili ya uendeshaji wa kuingiliana-free ya frequency converters. Kwa kuongezea, ni UV na hali ya hewa sugu na kwa hiyo yanafaa kwa matumizi ya nje. HELUKABEL inatoa mfululizo wake wa TOPFLEX katika matoleo mengi kulingana na mahitaji tofauti - na ina aina kubwa ya servo na kebo za kurudisha kwa viwanda na uhandisi wa mashine.