Understanding Broken Rotor Bars

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

Parts

Vibration sensor

€90.00 + VAT (if applicable)

Parts

Optical Sensor (Laser Tachometer)

€124.00 + VAT (if applicable)

Devices

Balanset-4

€6,803.00 + VAT (if applicable)

Parts

Magnetic Stand Insize-60-kgf

€46.00 + VAT (if applicable)

Parts

Reflective tape

€10.00 + VAT (if applicable)

Devices

Dynamic balancer “Balanset-1A” OEM

€1,735.00 + VAT (if applicable)

Broken rotor bars are complete fractures of the conductor bars in the squirrel-cage rotor of an induction motor. The condition is essentially the same as a rotor bar defect, but the term emphasises a full break rather than a crack or a high-resistance joint. When one or more bars sever, current can no longer flow through them, and the resulting electromagnetic asymmetry produces distinctive vibration and current signatures — sidebands spaced at the slip frequency around the running speed.

Broken bars are especially insidious because they fail as a cascade. One broken bar forces extra current and stress into the bars beside it, which then begin to fail in turn. Caught early — at the single-broken-bar stage — the motor can run for months under monitoring; missed, the fault can accelerate to multiple broken bars and a catastrophic rotor failure that demands replacement.

1. How Rotor Bars Break

Thermal Fatigue (Most Common)

Repeated heating and cooling cycles are the leading cause, and the mechanism is worth following step by step:

• Startup current: during a start, the rotor carries 5–7× normal current in the locked-rotor condition.
• Thermal expansion: the aluminium bars expand strongly, with a coefficient around 23 µm/m/°C.
• Constraint: the iron core expands far less (about 12 µm/m/°C), restraining the bars.
• Stress: this differential expansion sets up high thermal stress in the bars.
• Fatigue: repeated start cycles drive low-cycle fatigue.
• Crack initiation: cracks usually begin at the bar-to-end-ring junction, the highest-stress point.

Mechanical Stress

• Centrifugal forces at high speed.
• Electromagnetic forces during running and starting.
• Vibration transmitted from external sources.
• Shock loading during starts or sudden load changes.

Manufacturing Defects

• Porosity: voids in cast-aluminium rotors.
• Poor bonding: inadequate bar-to-core bonding.
• Material inclusions: contaminants trapped in the casting.
• Weak end-ring joints: poor bar-to-end-ring connections.

Operating Conditions

• Frequent starting: every start is a thermal and mechanical stress event.
• High-inertia loads: long acceleration times prolong bar stress.
• Reversing service: plugging creates extreme currents.
• Single-phasing: running with one phase lost overloads the remaining rotor bars.

2. The Characteristic Sideband Signature

Why Sidebands Appear

The distinctive diagnostic pattern arises through a clear chain of cause and effect:

1. A broken bar cannot carry current, creating electrical asymmetry in the rotor.
2. That asymmetry rotates at the slip frequency — the difference between synchronous and rotor speed.
3. It produces a torque pulsation at twice the slip frequency.
4. The torque pulsation modulates the 1× vibration that comes from ordinary mechanical unbalance.
5. The result is sidebands spaced at running speed ± slip-frequency intervals.

Vibration Pattern

• Central peak: 1× running speed (f_r).
• Lower sideband: f_r − f_s (where f_s is the slip frequency).
• Upper sideband: f_r + f_s.
• Multiple sidebands: f_r ± 2f_s, f_r ± 3f_s as severity grows.
• Symmetry: the sidebands sit symmetrically around the 1× peak.

Worked Example

A 4-pole, 60 Hz motor at full load:

• Synchronous speed: 1800 RPM.
• Actual speed: 1750 RPM (29.17 Hz).
• Slip: 50 RPM (0.833 Hz).
• Vibration peaks at: 28.3 Hz, 29.17 Hz and 30.0 Hz.
• A broken bar is confirmed by the symmetric sidebands at ±0.833 Hz.

Because the slip frequency is the whole basis of this pattern, it pays to compute it precisely for the motor in question; the Motor Slip & Actual RPM Calculator does this directly from nameplate data.

3. Current Signature Analysis (MCSA)

Motor current analysis reveals a closely related pattern around the line frequency:

• Central peak: line frequency (50 or 60 Hz).
• Sidebands: f_line ± 2f_s — note that this is twice the slip frequency in current, not once.
• Example: a 60 Hz motor with 1 Hz slip shows sidebands at 58 Hz and 62 Hz.
• Advantage: non-invasive and well suited to continuous monitoring.
• Sensitivity: often detects broken bars earlier than vibration. The Motor Electrical Defect Frequency Calculator predicts these exact current sidebands.

4. Progression Stages

Single Broken Bar

• Small sidebands appear, around 20–40% of the 1× peak.
• Slight torque pulsation, often unnoticeable.
• Motor performance is nearly normal.
• The motor can run for months under monitoring.
• Replacement should nonetheless be planned.

Multiple Adjacent Broken Bars

• Strong sidebands, greater than 50% of the 1× peak.
• Noticeable torque pulsation.
• Increased slip and temperature.
• Progression accelerating as the adjacent bars overheat.
• Replacement becomes urgent — a matter of weeks.

Severe Condition

• Sidebands may exceed the 1× peak amplitude.
• Severe torque pulsation reaching the driven equipment.
• High vibration and temperature.
• Risk of end-ring failure or complete rotor breakdown.
• Immediate replacement is required.

5. Detection in the Field

Vibration Analysis

The defining challenge is resolution: the sidebands sit less than 1 Hz from the 1× peak, so the analyser must separate them cleanly.

• Use a high-resolution FFT — better than 0.2 Hz resolution — to resolve the sidebands; the FFT Resolution Calculator helps you choose the line count and span.
• Test the motor under load, since the sidebands grow with current flow.
• Calculate the expected slip frequency for the motor in advance.
• Search the spectrum for symmetric sidebands at ±f_s around the 1× peak.
• Trend the sideband amplitude over time.

This work is well within reach of a portable instrument. A two-channel analyser such as the Balanset-1A captures the vibration spectrum at the motor bearing while its optical laser tachometer reads true shaft speed, letting you fix the precise 1× frequency, compute the slip, and look for the slip-spaced sidebands that confirm broken bars — all with the motor running under its normal load. Because the same instrument also measures 1× amplitude and phase, it cleanly separates a genuine rotor-bar signature from a simple running-speed unbalance that would call for balancing rather than a rotor replacement.

MCSA Testing

• Clamp current probes onto the motor leads.
• Acquire the current waveform and compute its FFT.
• Look for sidebands at f_line ± 2f_s.
• Compare against a healthy-motor baseline.
• This can flag a problem before the vibration symptoms become clear.

6. Corrective Actions

Immediate Response

• Increase the monitoring frequency — monthly, then weekly, then daily.
• Track the growth rate of the sideband amplitude through trend analysis.
• Order a spare motor or plan the rotor replacement.
• Reduce the duty cycle if possible, minimising starts.
• Document the progression for failure analysis.

Repair Options

• Rotor replacement: the most reliable choice for large motors (over 100 HP).
• Rotor recasting: specialised shops can recast aluminium rotors.
• Motor replacement: often the most economical route for small motors (under 50 HP).
• Root-cause investigation: determine why the bars broke to prevent a recurrence.

Prevention

• Use soft starters or VFDs to cut starting current and thermal stress.
• Limit the starting frequency for high-inertia loads.
• Specify motors rated for the actual duty cycle — frequent-start designs for high-cycle service.
• Ensure adequate motor ventilation and cooling.
• Protect against single-phasing conditions.

Broken rotor bars account for only about 10–15% of motor failures, yet they leave an unmistakable slip-frequency sideband signature that supports reliable early detection by vibration or current analysis. Understanding the thermal-fatigue mechanism, recognising the characteristic sideband pattern, and embedding the checks in a condition-monitoring programme allow a motor to be replaced on a planned basis — before a single broken bar cascades into multiple bar failures and extended unplanned downtime.

---

← Back to Main Index