Understanding Pole Pass Frequency
Pole pass frequency (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 electrical faults mula sa purong mechanical na mga 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 or misalignment. Kung basahin nang tama, ito ay nagsasabi sa analyst kung dapat buksan ang motor o tingnan ang ibang bahagi sa pagkakataon.
1. Pagkakalkula ng Pole Pass Frequency
Ang pangunahing formula
- PPF = P × (Nsync − N) / 60
- where P = bilang ng mga poles,
- Nsync = synchronous speed in RPM (Nsync = 120 × line frequency / P),
- N = aktwal na bilis ng rotor sa 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 Harmonic Frequency Calculator ay nagiging RPM sa Hz sa mga order na 1×–10×, at ang Motor Electrical Defect Frequency Calculator ay naglalabas ng electromagnetic frequencies nang magkasama.
Mga halimbawang ginawa na
Motor na may 4 polos sa 1750 RPM (60 Hz supply):
- 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.
Motor na may 6 polos sa 970 RPM (50 Hz supply):
- 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 spectral analysis.
2. Ang Pisikal na Mekanismo
Paano nabubuo ang electromagnetic force
Ang hanay ng mga kaganapan na lumilikha ng PPF ay napakaginhawa:
- Ang mga winding ng stator ay lumilikha ng isang magnetic field na umiikot sa synchronous speed.
- Ang field na iyon ay naayos sa magnetic poles sa isang N–S–N–S pattern.
- 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.
Uniform air gap — isang malusog na motor
- Ang rotor ay nakasentro sa stator bore.
- Ang air gap ay uniform sa buong circumference.
- Ang mga magnetic forces ay balansado at kumukuha ng isa't isa.
- PPF-related modulation is consequently very low in amplitude.
Eccentric air gap — isang defective na motor
- Ang rotor ay nakalagay na off-centre mula sa bearing wear, a bent shaft, o isang manufacturing error.
- Ang air gap ay mas maliit sa isang panig at mas malaki sa kabaligtaran.
- Ang mga magnetic forces ay nagiging hindi balansado — mas malakas kung saan ang gap ay mas maliit.
- A net radial force appears — hindi balanseadong magnetic pull — 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. Mga Sideband at Diagnostic Patterns
Static na eccentricity
Dito ang rotor centre ay offset ngunit stationary na may kaugnayan sa 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.
Dynamic na eccentricity
Dito ang rotor centre ay umiikot, o umiikot, sa paligid ng stator centre:
- 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.
Pinagsama-samang eccentricity
- Isang kombinasyon ng static at dynamic effects.
- Ito ang pinaka-karaniwang estado na makikita sa tunay na mga motor.
- Lumilikha ito ng komplikadong sideband patterns.
- Kailangan ng maingat na pagsusuri upang maintindihan ito nang tama.
4. Interpretasyon ng Diagnostic
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
- Isang normal na kondisyon.
- Isang pare-parehong air gap at magandang rotor–stator na concentricity.
- Walang kinakailangang aksyon na pangkorreksyon.
Moderate — sidebands clearly visible but far below the carrier peak
- Slight air-gap non-uniformity or early rotor-circuit degradation.
- Subaybayan ang trend at suriin ang kondisyon ng bearing.
- I-verify ang posisyon ng rotor kung ito ay accessible.
- Hindi agad kritikal, ngunit nangangailangan ng atensyon.
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.
- Isang peligro ng rotor-to-stator na pakikipag-ugnayan.
- Tumataas na electromagnetic forces na nagpapabilis ng sira.
- Planuhin ang pagkukumpuni o pagpapalit.
Sa praktika ang analyst ay bihira na naghuhusga ng PPF nang mag-isa. Isang portable na dalawang-channel analyser tulad ng 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 maaari mong itama sa pamamagitan ng on-site na pagbabalanse ang rotor sa lugar.
5. Relasyon sa Ibang Motor Frequencies
Ang PPF ay isang tunog sa isang pusong motor spectrum, at kinikilala kung saan ito nakaupo kaugnay ng mga kapit-bahay nito ay kalahati ng labanan. Ang isang tipikal na hierarchy para sa isang 4-pole, 1750 RPM motor sa isang 60 Hz supply ay:
- Slip frequency: (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× line frequency: 120 Hz (or 100 Hz) — the key electromagnetic component for eccentricity.
- Rotor bar pass frequency: 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. Mga Paraan ng Pagwawasto
Para sa mechanical eccentricity
- Palitan ang mga nagsimulang gumalaw nang maluwag na bearing upang maibalik ang wastong pagsentro ng rotor.
- Tukuyin ang baluktot na shaft o palitan ang rotor.
- I-mount muli ang rotor kung ang fault ay isang installation error.
- I-verify ang end-bell alignment at bolt tightness.
Para sa manufacturing eccentricity
- Ang mga matinding kaso ay maaaring mangailangan ng reboring ng rotor o stator.
- Palitan ang motor kung ito ay economically justified.
- Tanggapin ang kondisyon kung ang vibration ay manatili sa loob ng acceptable limits.
- I-document ito bilang isang baseline para sa hinaharap na paghahambing.
Para sa air-gap issues
- Suriin ang kondisyon ng bearing at palitan kung sira na.
- I-verify ang axial position ng rotor’s.
- Siyasatin para sa frame distortion o end-bell problems.
- Sukatin ang aktwal na air gap kung ito ay accessible.
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.
