Understanding Motor Bar Pass Frequency

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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 of a squirrel-cage induction motor sweep past the stator slots and windings. It equals the number of rotor bars multiplied by the rotor’s rotational frequency, so it lands far above running speed, typically between about 200 and 2000 Hz depending on motor size and speed. In a healthy motor it is a quiet, low-amplitude line in the vibration spectrum, but when it rises it points to eccentricity between rotor and stator, air-gap problems, or other electromagnetic irregularities. It is closely related to, yet distinct from, rotor bar defects — broken bars announce themselves with sidebands spaced at the slip frequency around 1×, not by raising the bar-pass line itself.

1. Calculating Bar Pass Frequency

The formula

RBPF = N_b × N / 60, where RBPF is the rotor bar pass frequency in Hz, N_b is the number of rotor bars, and N is the rotor speed in RPM.

The structure of the formula is the same one behind any tooth- or vane-counting frequency, such as gear mesh frequency or vane passing frequency: count the repeating features, multiply by how often they come round. Because the bar count is usually a large number, the result sits well into the high-frequency region of the spectrum.

Worked examples

Small motor: 28 rotor bars at 1750 RPM gives RBPF = 28 × 1750 / 60 = 817 Hz.

Large motor: 56 rotor bars at 3550 RPM gives RBPF = 56 × 3550 / 60 = 3313 Hz.

Note that the larger machine’s bar-pass line climbs past 3 kHz — a useful reminder that the measurement chain must reach that high to see it. If you want to translate running speed and order numbers into frequencies quickly, the harmonic frequency calculator handles the arithmetic.

Finding the number of bars

• Consult the motor nameplate or the manufacturer’s data sheet.
• Count them visually if the rotor is accessible during a repair.
• Back-calculate from a clearly identified peak in the vibration spectrum.
• Expect roughly 16 to 80 bars, depending on motor size and pole count.

2. The Physical Mechanism

Rotor-stator interaction

Bar pass frequency is born from magnetic interaction, not from mechanical contact:

1. The rotor bars carry induced current and create local perturbations in the magnetic field.
2. As the rotor turns, each bar sweeps past the stator slots one after another.
3. Magnetic reluctance rises and falls as the bars align with, then pass between, the stator teeth.
4. This produces a small pulsating electromagnetic force on the structure.
5. The pulsation rate equals the rate at which bars pass — the bar pass frequency.

Uniform versus non-uniform air gap

• Uniform air gap: the force contributions from bars on opposite sides of the rotor largely cancel, leaving a low RBPF amplitude.
• Eccentric rotor: the interaction becomes asymmetric, the cancellation breaks down, and the RBPF amplitude rises.
• Diagnostic value: the height of the RBPF line is therefore a direct indicator of how uniform the air gap is.

3. Diagnostic Significance

Normal condition

• An RBPF peak is present but very small — often below 0.5 mm/s.
• It may be barely visible above the noise floor.
• There are no sidebands flanking it.
• This signature confirms a uniform air gap and good rotor-stator concentricity.

What an elevated RBPF tells you

Air-gap eccentricity. The rotor sits off-centre in the stator bore, the RBPF amplitude climbs, and sidebands at ±1× running speed may appear. The pattern parallels the way pole pass frequency rises under the same fault.

Rotor-stator misalignment. When the rotor axis is not parallel to the stator axis, the air gap varies along the motor’s length, lifting both RBPF and its harmonics.

Broken or damaged rotor bars. This is a different signature entirely: broken bars create sidebands around 1× at slip-frequency spacing rather than raising the bar-pass line. See broken rotor bars and rotor bar defects for the full diagnostic detail.

4. Telling Bar Pass Apart from Other Frequencies

RBPF versus bearing frequencies

• RBPF: typically 200–3000 Hz, set by motor design.
• Bearing frequencies: typically 50–500 Hz for motor bearings.
• How to distinguish: calculate both and compare them with the observed peaks.
• Watch for overlap: on large motors the RBPF can fall into the same band as bearing fault frequencies, so confirm the source before acting.

RBPF versus 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 you will see each, and separating them depends on knowing the bar and slot counts.

5. Practical Application

When to monitor RBPF

• When an air-gap problem is suspected.
• After a bearing replacement, to verify the rotor is correctly centred.
• When 2× line frequency is elevated, which can accompany eccentricity.
• When establishing a baseline for a new or rewound motor.
• As a quality check after motor repair.

Measurement considerations

• The analyser’s frequency range must comfortably exceed twice the RBPF (F_max > 2 × RBPF) to capture it without aliasing.
• An accelerometer is usually needed rather than a velocity sensor, because the frequencies are high.
• Measure on the motor frame or bearing housing.
• Always compare against a baseline or against similar healthy motors.

6. Where Bar Pass Fits in Motor Diagnostics

It helps to keep two motor-rotor signatures clearly separated. The bar pass line speaks to air-gap geometry; broken bars speak to electrical integrity of the cage. In practice both can be present at once, so a thorough electrical-fault assessment checks for each:

• Rotor bar defects: sidebands around 1× running speed at slip-frequency spacing.
• RBPF issues: elevated amplitude at the bar-pass line itself (bars × speed).
• They can coexist: eccentricity and broken bars are not mutually exclusive.
• Comprehensive diagnosis: inspect both patterns to complete the picture.

This kind of high-frequency electromagnetic diagnosis is one half of induction-motor health assessment; the other half is the low-frequency mechanical world of unbalance and misalignment. A portable two-channel instrument such as the Balanset-1A covers that mechanical side, capturing the 1× amplitude and phase needed to balance the rotor in its own bearings and verify the result — a natural complement to the spectral motor checks described here.

Motor bar pass frequency, though less routinely watched than bearing frequencies or broken-bar signatures, carries genuine diagnostic value about air-gap uniformity and rotor-stator concentricity. Knowing how to calculate it and recognise it in a spectrum rounds out the diagnostic picture for condition assessment of squirrel-cage induction motors.

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