Induction-Motor Slip & Spectral-Candidate Worksheet
Calculate motoring slip and selected frequency candidates from the contemporaneous supply fundamental,even pole count and measured rotor speed. A frequency match is not a fault diagnosis or severity assessment.
Document one steady motoring state
The model is restricted to a rotating induction motor below synchronous speed with slip less than50%. Use the supply fundamental and speed measured during the same stable acquisition. It is not a start-up,plugging,generating or synchronous-motor model.
Synchronous speed Nₛ
Shaft frequency fᵣ
Slip s
Mechanical speed difference
Rotor electrical slip frequency
Classical MCSA spacing Δf
Lower current candidate
Upper current candidate
2× supply reference
Rotor slot/bar passing
Equations and distinct meanings
| Количество | Equation | Граница |
|---|---|---|
| synchronous speed | Nₛ=120fₛ/P | r/min for fₛ in Hz and even pole count P. |
| slip ratio | s=(Nₛ−N)/Nₛ | Dimensionless;this worksheet accepts steady motoring0<s<0.5. |
| mechanical speed difference | fΔm=(Nₛ−N)/60 | Hz of mechanical rotational-speed difference. |
| rotor electrical slip frequency | fslip,e=s fₛ=(P/2)fΔm | Electrical rotor frequency;not the broken-bar sideband spacing. |
| classical first-order MCSA spacing | Δf=2s fₛ | Distance from fₛ to each candidate sideband. |
| classical first-order candidates | fL,U=fₛ(1∓2s) | Candidate current components;no amplitude or fault conclusion. |
| rotor slot/bar passing | fZ=Z N/60=Zfᵣ | Geometric passing frequency when Z is verified;not a defect frequency by itself. |
Sources and applicability
IEC Electropedia defines synchronous speed as the rotational speed resulting from system frequency and pole count,and IEV411-46-07 defines slip as the synchronous-to-actual rotor-speed difference expressed per unit or percent of synchronous speed. These vocabulary definitions support the kinematic portion;they do not prescribe a fault diagnosis.
Peer-reviewed experimental MCSA research reports broken-bar-related current candidates at (1±2s)fₛ and emphasizes that amplitudes depend on fault severity,load and excitation frequency. See Zhu et al.,“Practical Aspects of Broken Rotor Bars Detection in PWM Voltage-Source-Inverter-Fed Squirrel-Cage Induction Motors,”2013,DOI 10.1155/2013/128368. More recent experimental/review literature also documents leakage,load sensitivity and the need for corroborating features.
These equations are ordinary induction-machine kinematics and research-derived spectral candidates,not ISO10816,ISO1940 or an ISO fault-severity method. The former related links to withdrawn ISO10816/ISO1940 did not establish the calculator’s formulas or diagnostic rules and were removed.
Что было исправлено
The former page calculated rotor electrical slip as s·fₛ and then multiplied it by the full pole count to create “pole pass frequency.” Classical broken-bar current spacing is 2s·fₛ. The former result was correct only by coincidence for two poles;it was2× too high for four poles,3× for six and4× for eight.
It then labelled fₛ±n·PPF as rotor-bar sidebands inside a section that required bar count,although those sidebands did not use bar count. The replacement separates the first-order MCSA candidates from the optional geometric slot/bar passing frequency and removes higher-order unsourced tables.
Defaults,presets,URL/history persistence,prefix parsing,typical-slip bands,“always present”2×line claims,automatic fault lists,dB severity claims and the unsafe instant power-off diagnostic rule were removed. The result now starts blank,requires contemporaneous acquisition evidence and rejects odd/non-integer poles,supersynchronous speed,standstill/start-up and slip at or above50%.
Use and interpretation
- Frequency resolution must separate a sideband spaced only2sfₛ from the supply fundamental;document window length,window function,sampling and leakage.
- Use the actual supply/converter fundamental and measured rotor speed from the same load state. Nominal50/60Hz and nameplate speed can move the candidates outside a narrow spectral bin.
- A visible candidate is not specific to one fault,and an absent candidate does not prove a healthy rotor. Compare load-matched baselines and multiple observables.
- Do not perform ad-hoc energized or power-interruption tests from this page. Follow the equipment owner’s approved electrical-safety and diagnostic procedure.
