Understanding Pole Pass Frequency
Pole pass frequency (PPF, and called slot pass frequency in some contexts) is the vibration frequency generated in an AC motor as the rotor passes the stationary magnetic poles of the stator. It is calculated as the number of stator poles multiplied by the rotor running speed, PPF = (number of poles × RPM) / 60. The pole-passing action sets up electromagnetic forces that produce vibration, and that vibration is greatly amplified when the motor has air-gap eccentricity or a rotor-to-stator alignment problem. Because of this, PPF is one of the most useful tools for separating electrical faults from purely mechanical ones.
PPF matters diagnostically because elevated amplitude at this frequency, together with its sidebands, points squarely at an electromagnetic problem — an eccentric rotor, a non-uniform air gap, or dynamic rotor–stator interaction — rather than at unbalance ili misalignment. Read correctly, it tells the analyst whether to open the motor or to look elsewhere on the train.
1. Calculating Pole Pass Frequency
The basic formula
- PPF = P × N / 60
- where P = number of poles,
- N = actual rotor speed in RPM,
- and the result is in Hz.
Note that N is the actual shaft speed, not the synchronous speed of the field. An induction motor always runs slightly slower than its field because of slip, so using the nameplate synchronous speed introduces a small but real error. When you need to convert running speed into a family of orders quickly, our Harmonic Frequency Calculator turns RPM into Hz across the 1×–10× orders, and the Motor Electrical Defect Frequency Calculator lays out the electromagnetic frequencies side by side.
Worked examples
4-pole motor at 1750 RPM (60 Hz supply):
- PPF = 4 × 1750 / 60 = 116.7 Hz
- This component will appear in the vibration spectrum.
- Sidebands at ±1× running speed (±29.2 Hz) are diagnostic for eccentricity.
6-pole motor at 970 RPM (50 Hz supply):
- PPF = 6 × 970 / 60 = 97 Hz
- This sits close to 2× line frequency (100 Hz) and can overlap it.
- Distinguishing the two may demand careful, high-resolution spectral analysis.
2. The Physical Mechanism
How the electromagnetic force is generated
The chain of events that creates PPF is straightforward:
- The stator windings create a magnetic field that rotates at synchronous speed.
- That field is organised into magnetic poles in an N–S–N–S pattern.
- The rotor, running slightly slower because of slip, passes by each of those poles.
- Every pole passage exerts a magnetic force on the rotor.
- With P poles, the rotor feels P force pulses per revolution.
- The frequency of those pulsations is therefore P × rotor speed = PPF.
Uniform air gap — a healthy motor
- The rotor is centred in the stator bore.
- The air gap is uniform around the full circumference.
- The magnetic forces are balanced and cancel one another.
- PPF vibration is consequently very low in amplitude.
Eccentric air gap — a defective motor
- The rotor sits off-centre from bearing wear, a bent shaft, or a manufacturing error.
- The air gap is smaller on one side and larger on the opposite.
- The magnetic forces become unbalanced — stronger where the gap is smaller.
- A net radial force appears at PPF, an effect closely related to unbalanced magnetic pull.
- PPF amplitude rises and sidebands develop.
3. Sidebands and Diagnostic Patterns
Static eccentricity
Here the rotor centre is offset but stationary relative to the stator:
- Pattern: PPF with sidebands at ±1× running speed.
- Example: PPF ± fr, where fr is rotor speed.
- Cause: bearing wear, a bent shaft, or rotor eccentricity.
- Amplitude: the sideband amplitude indicates the severity of the eccentricity.
Dynamic eccentricity
Here the rotor centre orbits, or whirls, around the stator centre:
- Pattern: PPF with a complex sideband structure.
- Causes: rotor-to-stator rub or bearing looseness.
- More severe: it signals an active dynamic interaction rather than a fixed offset.
Mixed eccentricity
- A combination of static and dynamic effects.
- This is the most common state found in real motors.
- It produces complex sideband patterns.
- Careful analysis is needed to interpret it correctly.
4. Diagnostic Interpretation
Amplitude at PPF is best read as a continuum, alongside the strength of its sidebands:
Low PPF amplitude (under 0.5 mm/s)
- A normal condition.
- A uniform air gap and good rotor–stator concentricity.
- No corrective action needed.
Moderate PPF (0.5–2.0 mm/s)
- Slight air-gap non-uniformity.
- Monitor the trend and check bearing condition.
- Verify rotor position if it is accessible.
- Not immediately critical, but it warrants attention.
High PPF (above 2.0 mm/s)
- Significant eccentricity or an air-gap problem.
- Strong sidebands present.
- A risk of rotor-to-stator contact.
- Rising electromagnetic forces that accelerate damage.
- Plan repair or replacement.
In practice the analyst rarely judges PPF in isolation. A portable two-channel analyser such as the Balanset-1A, used at the bearing housings, captures the spectrum and resolves the sidebands around PPF — 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 you can correct by field balancing the rotor in place.
5. Relationship to Other Motor Frequencies
PPF is one tone in a crowded motor spectrum, and recognising where it sits relative to its neighbours is half the battle. A typical hierarchy for a 4-pole, 1750 RPM motor on a 60 Hz supply is:
- Running speed (1×): about 29 Hz.
- Slip frequency: typically 1–3 Hz.
- Line frequency: 50 or 60 Hz.
- PPF: P × running speed — about 117 Hz here.
- 2× line frequency: 100 or 120 Hz.
- Rotor bar pass frequency: number of rotor bars × running speed.
The close spacing of PPF, 2× line frequency, and the higher-order harmonics of running speed is exactly why electromagnetic faults are so easily confused — and why 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. Correction Methods
For mechanical eccentricity
- Replace worn bearings to restore proper rotor centring.
- Correct a bent shaft or replace the rotor.
- Remount the rotor if the fault is an installation error.
- Verify end-bell alignment and bolt tightness.
For manufacturing eccentricity
- Severe cases may require reboring the rotor or stator.
- Replace the motor where that is economically justified.
- Accept the condition if vibration stays within acceptable limits.
- Document it as a baseline for future comparison.
For air-gap issues
- Check bearing condition and replace if worn.
- Verify the rotor’s axial position.
- Inspect for frame distortion or end-bell problems.
- Measure the actual air gap where it is accessible.
Pole pass frequency is, in summary, a motor-specific vibration component that opens a window onto rotor–stator electromagnetic interaction and air-gap uniformity. Mastering its calculation, recognising its sideband signatures, and reading its 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.
