Razumijevanje frekvencije prolaska pola
Frekvencija prolaska pola (PPF) is the low-frequency vibration component generated in an AC induction motor as the rotating magnetic field slips past the rotor. It is calculated as the number of poles multiplied by the slip frequency: PPF = number of poles × (synchronous speed − actual speed) / 60. Because slip is small, PPF is a very low frequency — typically only a few hertz — and it usually shows up not as a stand-alone peak but as sidebands spaced at PPF around the running-speed and line-frequency components. That slip-related modulation is greatly amplified when the motor has air-gap eccentricity, broken rotor bars, or a rotor-to-stator alignment problem. Because of this, PPF is one of the most useful tools for separating električnih kvarova od čisto mehaničkih problema.
PPF matters diagnostically because elevated sidebands spaced at this frequency point squarely at an electromagnetic problem — an eccentric rotor, broken rotor bars, a non-uniform air gap, or dynamic rotor–stator interaction — rather than at unbalance ili misalignment. Ako se pravilno pročita, analitičaru govori da li treba otvoriti motor ili da traži problem drugdje u sistemu.
1. Izračunavanje frekvencije prolaska polova
Osnovna formula
- PPF = P × (Nsync − N) / 60
- where P = broj polova,
- Nsync = synchronous speed in RPM (Nsync = 120 × line frequency / P),
- N = stvarna brzina rotora u RPM,
- and the result is in Hz. The term (Nsync − N) / 60 is the slip frequency expressed in Hz.
Note that PPF is built on the difference between synchronous and actual speed — the slip. At no load an induction motor runs almost at synchronous speed, so PPF approaches zero; as load grows, slip and therefore PPF grow with it, typically ending up in the range of roughly 0.5–3.5 Hz at rated load. Do not confuse PPF with the purely mechanical order P × running speed (for a 4-pole motor at 1750 RPM that is about 116.7 Hz) — that is a harmonic of shaft rotation, not pole pass frequency. When you need to convert running speed into a family of orders quickly, our Kalkulator harmonijskih frekvencija turns RPM into Hz across the 1×–10× orders, and the Kalkulator Frekvencije Električnog Defekta Motora prikazuje elektromagnetske frekvencije jednu pored druge.
Radni primjeri
4-polni motor sa 1750 RPM (50 Hz napajanje):
- Synchronous speed Nsync = 120 × 60 / 4 = 1800 RPM.
- Slip frequency = (1800 − 1750) / 60 = 0.83 Hz.
- PPF = 4 × (1800 − 1750) / 60 = 3.33 Hz
- In the vibration spectrum it appears as sidebands spaced ±3.33 Hz around the 1× running-speed peak (29.2 Hz) and around 2× line frequency (120 Hz) — a pattern diagnostic for eccentricity and broken rotor bars.
6-polni motor sa 970 RPM (50 Hz napajanje):
- Synchronous speed Nsync = 120 × 50 / 6 = 1000 RPM.
- Slip frequency = (1000 − 970) / 60 = 0.5 Hz.
- PPF = 6 × (1000 − 970) / 60 = 3.0 Hz
- Sidebands spaced only ±3.0 Hz around the 1× peak (16.2 Hz) or around 2× line frequency (100 Hz) are easy to miss.
- Resolving them demands careful, high-resolution spektralne analize.
2. Fizički mehanizam
Kako se generiše elektromagnetna sila
Niz događaja koji kreiraju PPF je jednostavan:
- Namotaji statora stvaraju magnetno polje koje rotira sinhronom brzinom.
- To polje je organizirano u magnetne polove u N–S–N–S obrascu.
- The rotor runs slightly slower because of slip, so the field poles continuously drift past the rotor rather than staying locked to it.
- Each time a field pole slips past a given spot on the rotor (a high-resistance bar, an eccentric side of the air gap), the magnetic force acting on that spot is modulated.
- With P poles, that spot is passed P times per slip cycle.
- The frequency of the resulting modulation is therefore P × slip frequency = PPF — a slow “beating” superimposed on the running-speed and line-frequency vibration.
Ujednačen vazdušni raspor — zdrav motor
- Rotor je centriran u otvoru statora.
- Vazdušni raspor je ujednačen oko cijele obim.
- Magnetne sile su uravnotežene i poništavaju jednu drugu.
- PPF-related modulation is consequently very low in amplitude.
Ekscentričan vazdušni raspor — neispravan motor
- Rotor je pozicioniran van centra od bearing wear, a bent shaft, ili proizvodna greška.
- Vazdušni raspor je manji na jednoj strani i veći na suprotnoj.
- Magnetne sile postaju nebalansirane — jače gdje je raspor manji.
- A net radial force appears — nebalansirane magnetne privlačnosti — and it is modulated at PPF as the field poles slip past the narrow side of the gap.
- Sidebands spaced at PPF develop around the main spectral peaks and grow with severity.
3. Bočni spektri i dijagnostički obrasci
Statička ekscentričnost
Ovdje je centar rotora pomjeren ali nepokretan u odnosu na stator:
- Pattern: an elevated 2× line frequency component (100/120 Hz); pole-pass sidebands are weak or absent because the offset does not rotate with the shaft.
- Cause: bearing wear, frame or end-bell distortion, or a machining error in the stator bore.
- Amplitude: the 2× line frequency amplitude indicates the severity of the eccentricity.
Dinamička ekscentričnost
Ovdje centar rotora orbitira, ili se vrti, oko centra statora:
- Pattern: 1× running speed and 2× line frequency peaks with sidebands spaced at PPF (e.g. ±3.33 Hz in the 4-pole example above).
- Causes: a bent shaft, rotor eccentricity, rotor-to-stator rub or bearing looseness.
- More severe: it signals an active dynamic interaction rather than a fixed offset. The same pole-pass sideband family around 1× and its harmonics is the classic signature of broken rotor bars.
Mješovita ekscentričnost
- Kombinacija statičkih i dinamičkih efekata.
- Ovo je najčešće stanje pronađeno u realnim motorima.
- Proizvodi složene obrasce bočnih spektara.
- Potrebna je pažljiva analiza za ispravan pristup.
4. Dijagnostička interpretacija
Because PPF is only a few hertz, it is judged through the strength of its sidebands around the 1× running-speed and 2× line-frequency peaks, read as a continuum:
Low — pole-pass sidebands barely visible above the noise floor
- Normalno stanje.
- Ujednačeni razmak zraka i dobar stupanj koncentričnosti rotora i statora.
- Nema potrebe za korektivnom akcijom.
Moderate — sidebands clearly visible but far below the carrier peak
- Slight air-gap non-uniformity or early rotor-circuit degradation.
- Pratite trend i provjerite stanje ležaja.
- Ako je dostupno, provjerite poziciju rotora.
- Nije odmah kritično, ali zaslužuje pažnju.
High — sidebands approaching the carrier peak in amplitude
- Significant eccentricity, broken rotor bars, or an air-gap problem.
- A pronounced low-frequency “beat” may be felt or heard at PPF.
- Rizik od kontakta rotora sa statorom.
- Rastući elektromagnetni procesi koji ubrzavaju oštećenje.
- Planirajte popravku ili zamjenu.
U praksi analitičar rijetko procjenjuje PPF u izolaciji. Prijenosni dvokanalnog analizator kao što je Balanset-1A, used at the bearing housings, captures the spectrum and resolves the pole-pass sidebands around the 1× and line-frequency peaks — and, just as importantly, confirms whether the dominant component is electromagnetic or the simple 1× peak of a mechanical fault. That distinction decides everything that follows: an electromagnetic signature sends you inside the motor, while a clean 1× peak that disappears the instant power is cut points to unbalance možete ispraviti pomoću field balancing rotor na mjestu.
5. Veza sa drugim frekvencijama motora
PPF je jedan ton u prepunom spektru motora, i prepoznavanje gdje se nalazi u odnosu na svoje susjedne tonove je pola bitke. Tipična hijerarhija za motor sa 4 pola, 1750 RPM na 60 Hz napajanju je:
- Frekvencija klizanja: (1800 − 1750) / 60 ≈ 0.83 Hz.
- PPF: P × slip frequency — about 3.3 Hz here, the lowest tone in the family.
- Running speed (1×): about 29.2 Hz.
- Line frequency: 60 Hz (or 50 Hz in Europe).
- Rotor mechanical order P × running speed: about 117 Hz — often mistaken for PPF, but it is a shaft-speed harmonic.
- 2× linijska frekvencija: 120 Hz (or 100 Hz) — the key electromagnetic component for eccentricity.
- Frekvencija prolaska šipki rotora: number of rotor bars × running speed, up in the kilohertz range.
The close spacing of 2× line frequency (120 Hz) and the nearby harmonics of running speed (the 4th order sits at about 117 Hz here) is exactly why electromagnetic faults are so easily confused with mechanical ones — and why the fine pole-pass sideband structure, not amplitude alone, is the deciding clue. Where the picture remains ambiguous, switching off the supply is the definitive test: an electromagnetic component vanishes instantly with the field, whereas a mechanical one decays only as the rotor coasts down.
6. Metode Korekcije
Za mehanički ekscentricitet
- Zamijenite istrošene ležajeve kako biste obnovili ispravnu centriranost rotora.
- Ispravite savijenu osovinu ili zamijenite rotor.
- Ponovno montirajte rotor ako je greška u instalaciji.
- Provjerite poravnanje krajnjeg poklopca i čvrstinu vijaka.
Za proizvodni ekscentricitet
- Teški slučajevi mogu zahtijevati ponovno bušenje rotora ili statora.
- Zamijenite motor ako je to ekonomski opravdano.
- Prihvatite stanje ako vibracija ostaje unutar prihvatljivih granica.
- Dokumentirajte to kao referentnu vrijednost za buduće uspoređivanje.
Za probleme s razmakom zraka
- Provjerite stanje ležaja i zamijenite ako je istrošen.
- Provjerite aksijalnu poziciju rotora’s.
- Ispitajte stator na znakove deformacije okvira ili problema s krajnjim poklopcima.
- Izmjerite stvarni razmak zraka gdje je dostupan.
Pole pass frequency is, in summary, a slip-related, motor-specific low-frequency component that opens a window onto rotor–stator electromagnetic interaction and air-gap uniformity. Mastering its calculation, recognising its sideband signatures, and reading their amplitude trends lets an engineer diagnose electromagnetic faults with confidence — and direct maintenance effort to the right place instead of chasing a mechanical cause that was never there.
