Asenkron motörlerde kafes kırılmasının etkileri
Başlık çevirisi mevcut değil.
- Tez No: 19441
- Danışmanlar: PROF.DR. İLHAMİ ÇETİN
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Asenkron motorlar, Kafes kırılması, Induction motors, Cage break
- Yıl: 1992
- Dil: Türkçe
- Üniversite: Marmara Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
Özet yok.
Özet (Çeviri)
Cage induction motors are the most commonly used elec trical machines, they represent a great part o-F the electrical machines produced by the manu-Factur i ng industry all over the world. The main reasons o-F general application are reliability and simple technology. Cage rotors -For high power machines are manufactured with copper rotor bars and end rings, these being brazed or welded together, while machines of lower power are being manufactured with die-casting, technologies. For some years now, it has been a common practice to die-cast the cages. The die-cast rotor has many advantages of simpl icity, ruggedness and durability as well as low cost. One feature of die-casting is the tendency for all castings to have a perfect outside sur face even though the interior may contain voids, blow holes or porosity. Proper die-casting methods assume properly gated moulds, holding proper pressure on the die-casting material and proper heating processes. However, in an induction motor, the die-casting presents a disadvantage as no visual inspection methods will disclose a faulty interior which may affect the electrical performance. Manufacturing die-casted rotors raises several technological problems, as rotors must be. free from im purities. Thus, tests of the rotors of cage induction motors have shown asymmetries in the rotor circuit of these machines, which in case of die-casted rotors are due to technological difficulties. In recent studies three kinds of fault have been identi fied as constituting the bulk of failures to be expected in large induction motors and which occur more often than the next most frequent group. These are, in order of importance, bearing related <41 %), stator related (37“/.) and rotor related (10 %) with half of 'these allocated to cage faults. The remaining failures (12 */.) are scattered amongst a variety of effects. There are, of course, many ways in which a rotor can be defective such as wrong laminations, core stack, skew. General ly, such faults can be detected before installation and approp riate remedial action taken. Rotor-cage faults may arise during manufacture through defective casting in the case of die-cast rotors, or poor jointing in the case of brazed or welded end rings. In service, an undetected fault produces a higher-resis tance bar or joint, which may lead to eventual failure. During operation, the copper rotor bars and end rings of cage motors may brgak^as a consequence of- the -improper technology or very heavy operat ion~”cohd-it ions. The bars are subject to braking and accelerating forces on. the end ring when the motor changes speed. If the motor speed fluctuates because of changing load, or as part of the normal duty cycle, then fatigue failures can occur at the joints between bars and ring.As known, the application o-F a positive sequence voltage system to the stator windings o-F the symmetrical induction mo tor produces a distributed magnetic -Field in its air-gap which is essentially sinusoidal and rotates -Forwards relative to the stator at synchronous speed tuj_. When the rotor is at stand still, the field will induce voltages in the rotor o-F the same -Frequency as applied to the stator. When the rotor is rotating, the -Frequency o-F the induced rotor voltages and currents will depend on the relative velocity between the stator field and the speed of the rotor. The rotor quantities will be of a fre quency equal to the product of the slip and the applied stator frequency. The field rotates forwards relative to the rotor with a velocity s.oi^ and induces in its windings a positive sequence voltage system of voltage s.U^q and frequency s.f^ and causes positive sequence currents to flow in the rotor cir cuit. Each individual rotor mesh is, inessence, a short-pitched single-turn single-phase winding. If the rotor circuit is unsymmetr ical, the flow of posi tive sequence current through it generates negative sequence voltages of the same frequency at its terminals, so that a negative sequence magnetic field is also produced in the air- gap of the machine. When the unsymmetry in the rotor circuit is small, the negative sequence field will be relatively small. This field rotates backwards relative to the rotor with a velo city s.cju^ and backwards relative to the stator with a velo city (2s-l).ui£. The negative sequence voltages induced in the stator windings by this field have a frequency (2s-l).f£ and produce negative sequence currents in the symmetrical stator winding. The path for the stator currents of frequency (2s-l>, is through the impedance of the stator winding and the power lines supplying the induction motor. Since the impedance of the power lines which complete the circuit is usually small com pared to the stator impedance, it can be assumed in most cases that these currents are short-circuited through the stator impedance. It will be seen that the stator currents now consist of the normal mains frequency component, together with a compo nent of <2s-l) times main frequency.
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