What is Pole Pass Frequency? Motor Electromagnetic Forces • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is Pole Pass Frequency? Motor Electromagnetic Forces • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is Pole Pass Frequency? Motor Electromagnetic Forces

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Understanding Pole Pass Frequency

Definition: What is Pole Pass Frequency?

Pole pass frequency (PPF, also called slot pass frequency in some contexts) is the vibration frequency generated in AC motors when the rotor passes by the stator magnetic poles. It is calculated as the number of stator poles multiplied by the rotor running speed (PPF = Number of Poles × RPM / 60). Pole pass frequency creates electromagnetic forces that produce vibration and can be significantly amplified when the motor has air gap eccentricity or rotor-to-stator alignment problems.

PPF is diagnostically important because elevated amplitude at pole pass frequency and its sidebands indicates electromagnetic problems such as eccentric rotor position, non-uniform air gap, or dynamic rotor-stator interaction, helping distinguish electromagnetic issues from purely mechanical faults.

Calculation of Pole Pass Frequency

Basic Formula

  • PPF = P × N / 60
  • Where P = number of poles
  • N = actual rotor speed (RPM)
  • Result in Hz

Examples

4-Pole Motor at 1750 RPM (60 Hz Supply)

  • PPF = 4 × 1750 / 60 = 116.7 Hz
  • This frequency will appear in vibration spectrum
  • Sidebands at ±1× running speed (±29.2 Hz) diagnostic for eccentricity

6-Pole Motor at 970 RPM (50 Hz Supply)

  • PPF = 6 × 970 / 60 = 97 Hz
  • Close to 2× line frequency (100 Hz), can overlap
  • May require careful spectrum analysis to distinguish

Physical Mechanism

Electromagnetic Force Generation

Understanding why PPF occurs:

  1. Stator windings create rotating magnetic field at synchronous speed
  2. Field organized into magnetic poles (N-S-N-S pattern)
  3. Rotor (running slightly slower due to slip) passes by these poles
  4. Each pole passage creates magnetic force on rotor
  5. With P poles, rotor experiences P force pulses per revolution
  6. Frequency of force pulsations = P × rotor speed = PPF

Uniform Air Gap (Healthy Motor)

  • Rotor centered in stator bore
  • Air gap uniform around circumference
  • Magnetic forces balanced, cancel each other
  • PPF vibration very low amplitude

Eccentric Air Gap (Defective Motor)

  • Rotor off-center from bearing wear, shaft bend, or manufacturing error
  • Air gap smaller on one side, larger on opposite
  • Magnetic forces unbalanced (stronger where gap smaller)
  • Net radial force at PPF
  • PPF amplitude elevated and creates sidebands

Sidebands and Diagnostic Patterns

Static Eccentricity

Rotor center offset but stationary relative to stator:

  • Pattern: PPF with sidebands at ±1× running speed
  • Example: PPF ± fr (where fr = rotor speed)
  • Cause: Bearing wear, bent shaft, rotor eccentricity
  • Amplitude: Sideband amplitude indicates eccentricity severity

Dynamic Eccentricity

Rotor center orbits (whirls) around stator center:

  • Pattern: PPF with complex sideband structure
  • Causes: Rotor-to-stator rub, bearing looseness
  • More Severe: Indicates dynamic interaction

Mixed Eccentricity

  • Combination of static and dynamic
  • Most common in real motors
  • Complex sideband patterns
  • Requires careful analysis to interpret

Diagnostic Interpretation

Low PPF Amplitude (< 0.5 mm/s)

  • Normal condition
  • Uniform air gap
  • Good rotor-stator concentricity
  • No corrective action needed

Moderate PPF (0.5-2.0 mm/s)

  • Slight air gap non-uniformity
  • Monitor trend
  • Check bearing condition
  • Verify rotor position if accessible
  • Not immediately critical but warrants attention

High PPF (> 2.0 mm/s)

  • Significant eccentricity or air gap problem
  • Strong sidebands present
  • Risk of rotor-stator contact
  • Increased electromagnetic forces accelerating damage
  • Plan repair or replacement

Relationship to Other Motor Frequencies

Frequency Hierarchy in Motor Spectra

  • Running Speed (1×): ~29 Hz for 1750 RPM motor
  • Slip Frequency: 1-3 Hz typically
  • Line Frequency: 50 or 60 Hz
  • PPF: P × running speed (e.g., 117 Hz for 4-pole at 1750 RPM)
  • 2× Line Frequency: 100 or 120 Hz
  • Rotor Bar Pass: Number of rotor bars × running speed

Correction Methods

For Mechanical Eccentricity

  • Replace worn bearings restoring proper rotor centering
  • Correct bent shaft or replace rotor
  • Remount rotor if installation error
  • Verify end bell alignment and bolt tightness

For Manufacturing Eccentricity

  • Severe cases may require rotor or stator reboring
  • Motor replacement if economically justified
  • Accept if vibration within acceptable limits
  • Document as baseline for future comparison

For Air Gap Issues

  • Check bearing condition and replace if worn
  • Verify rotor axial position
  • Inspect for frame distortion or end bell problems
  • Measure actual air gap if accessible

Pole pass frequency is a motor-specific vibration component that provides valuable diagnostic information about rotor-stator electromagnetic interaction and air gap uniformity. Understanding PPF calculation, recognizing its sideband patterns, and interpreting amplitude trends enables effective diagnosis of motor electromagnetic problems and guides appropriate maintenance actions.

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