Understanding Motor Bar Pass Frequency

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Definition: What is Motor Bar Pass Frequency?

Motor bar pass frequency (also called rotor bar pass frequency, rotor slot frequency, or simply bar pass) is the frequency at which the rotor bars in a squirrel cage induction motor pass by the stator slots or stator windings. It is calculated as the number of rotor bars multiplied by the rotor rotational frequency (Bar Pass Frequency = Number of Rotor Bars × RPM / 60). This frequency is typically in the range of 200-2000 Hz depending on motor size and speed.

While normally a low-amplitude component in motor vibration spectra, elevated bar pass frequency can indicate rotor-to-stator eccentricity, air gap problems, or electromagnetic irregularities. It is distinct from but related to rotor bar defects, which produce sidebands at slip frequency rather than at the bar pass frequency itself.

Calculation

Formula

• RBPF = Nb × N / 60
• Where RBPF = Rotor Bar Pass Frequency (Hz)
• Nb = Number of rotor bars
• N = Rotor speed (RPM)

Typical Values

Small Motor Example

• Number of rotor bars: 28
• Speed: 1750 RPM
• RBPF = 28 × 1750 / 60 = 817 Hz

Large Motor Example

• Number of rotor bars: 56
• Speed: 3550 RPM
• RBPF = 56 × 3550 / 60 = 3313 Hz

Finding Number of Bars

• Consult motor nameplate or manufacturer data
• Visual count (if rotor accessible)
• Identify from vibration spectrum peak
• Typical range: 16-80 bars depending on motor size and poles

Physical Mechanism

Rotor-Stator Interaction

Bar pass frequency arises from magnetic interaction:

1. Rotor bars carrying current create local magnetic field perturbations
2. As rotor rotates, each bar passes by stator slots in sequence
3. Magnetic reluctance varies as bars align with or pass between stator teeth
4. Creates small pulsating electromagnetic force
5. Force pulsation frequency = rotor bar passage rate

Uniform vs. Non-Uniform Air Gap

• Uniform Air Gap: Effects largely cancel, low RBPF amplitude
• Eccentric Rotor: Interaction asymmetric, RBPF amplitude elevated
• Diagnostic Value: RBPF amplitude indicates air gap uniformity

Diagnostic Significance

Normal Condition

• RBPF peak present but very low amplitude (< 0.5 mm/s)
• May be barely visible above noise floor
• No sidebands around RBPF
• Indicates uniform air gap and good rotor-stator concentricity

Elevated RBPF Indicates

Air Gap Eccentricity

• Rotor off-center in stator bore
• RBPF amplitude increases
• May have sidebands at ±1× running speed
• Similar to pole pass frequency elevation

Rotor-Stator Misalignment

• Rotor axis not parallel to stator axis
• Air gap varies along axial length
• RBPF and harmonics elevated

Broken or Damaged Rotor Bars

• Different diagnostic pattern than RBPF itself
• Creates sidebands around 1× at slip frequency spacing
• See rotor bar defects for details

Differentiation from Other Frequencies

RBPF vs. Bearing Frequencies

• RBPF: Typically 200-3000 Hz, depends on motor design
• Bearing Frequencies: Typically 50-500 Hz for motor bearings
• Distinction: Calculate both and compare to observed peaks
• Possible Overlap: Large motors may have RBPF overlapping bearing frequency range

RBPF vs. Stator Slot Frequency

• Stator Slot Pass: Number of stator slots × running speed (rarely significant)
• RBPF: Number of rotor bars × running speed (more commonly observed)
• Both Present: In some motors, both may be visible

Practical Application

When to Monitor RBPF

• Suspected air gap problems
• After bearing replacement (verify proper rotor centering)
• Elevated 2× line frequency (may be related to eccentricity)
• Baseline establishment for new or rewound motors
• Quality verification after motor repair

Measurement Considerations

• Requires adequate frequency range (Fmax > 2× RBPF)
• May need accelerometers rather than velocity sensors for high frequencies
• Measurement on motor frame or bearing housing
• Compare to baseline or similar motors

Relationship to Broken Bar Detection

While RBPF itself indicates air gap issues, rotor bar defects create a different signature:

• Rotor Bar Defects: Sidebands around 1× running speed at ±slip frequency
• RBPF Issues: Elevated amplitude at RBPF itself (number of bars × speed)
• Both Can Coexist: Eccentricity AND broken bars possible simultaneously
• Comprehensive Analysis: Check both patterns for complete motor diagnosis

Motor bar pass frequency, while less commonly monitored than bearing frequencies or rotor bar defect signatures, provides valuable diagnostic information about air gap uniformity and rotor-stator concentricity. Understanding RBPF calculation and recognition in vibration spectra completes the diagnostic picture for induction motor condition assessment.

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