What are Broken Rotor Bars? Squirrel Cage Motor Failure • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What are Broken Rotor Bars? Squirrel Cage Motor Failure • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What are Broken Rotor Bars? Squirrel Cage Motor Failure

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Understanding Broken Rotor Bars

Definition: What are Broken Rotor Bars?

Broken rotor bars are complete fractures of the conductor bars in squirrel cage induction motor rotors. This is essentially the same condition as rotor bar defects but specifically emphasizes complete bar breakage rather than cracks or high-resistance connections. When one or more bars break, electrical current cannot flow through those bars, creating electromagnetic asymmetry that produces characteristic vibration and current signatures with sidebands at slip frequency spacing around the running speed.

Broken rotor bars are particularly insidious because they create a cascading failure mode: one broken bar increases current and stress in adjacent bars, causing them to fail progressively. If not detected early (single broken bar), the condition can rapidly deteriorate to multiple broken bars and catastrophic rotor failure requiring motor replacement.

How Rotor Bars Break

Thermal Fatigue (Most Common)

Repeated heating and cooling cycles:

  • Startup Current: During motor start, rotor current 5-7× normal (locked rotor condition)
  • Thermal Expansion: Aluminum bars expand significantly (coefficient 23 µm/m/°C)
  • Constraint: Iron core expands less (12 µm/m/°C), constraining bar expansion
  • Stress: Differential expansion creates thermal stress in bars
  • Fatigue: Repeated start cycles cause low-cycle fatigue
  • Crack Initiation: Typically at bar-to-end ring junction (high stress point)

Mechanical Stress

  • Centrifugal forces at high speeds
  • Electromagnetic forces during operation and starting
  • Vibration from external sources
  • Shock loading during starts or load changes

Manufacturing Defects

  • Porosity: Voids in cast aluminum rotors
  • Poor Bonding: Inadequate bar-to-core bonding
  • Material Inclusions: Contaminants in casting
  • Weak End Ring Joints: Poor bar-to-end ring connections

Operating Conditions

  • Frequent Starting: Each start is thermal and mechanical stress event
  • High-Inertia Loads: Long acceleration times increase bar stress
  • Reversing Service: Plugging creates extreme currents
  • Single-Phasing: Operating with one phase lost overloads rotor bars

The Characteristic Sideband Signature

Why Sidebands Appear

The distinctive diagnostic pattern:

  1. Broken bar cannot carry current, creating electrical asymmetry
  2. Asymmetry rotates at slip frequency (difference between synchronous and rotor speed)
  3. Creates torque pulsation at 2× slip frequency
  4. Torque pulsation modulates 1× vibration from mechanical unbalance
  5. Result: sidebands at running speed ± slip frequency intervals

Vibration Pattern

  • Central Peak: 1× running speed (fr)
  • Lower Sideband: fr – fs (where fs = slip frequency)
  • Upper Sideband: fr + fs
  • Multiple Sidebands: fr ± 2fs, fr ± 3fs as severity increases
  • Symmetry: Sidebands symmetric around 1× peak

Example

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, 30.0 Hz
  • Broken bar confirmed by symmetric sidebands at ±0.833 Hz

Current Signature (MCSA)

Motor current analysis shows similar pattern:

  • Central Peak: Line frequency (50 or 60 Hz)
  • Sidebands: fline ± 2fs (note: 2× slip frequency in current, not 1×)
  • Example: 60 Hz motor with 1 Hz slip → sidebands at 58 Hz and 62 Hz
  • Advantage: Non-invasive, can monitor continuously
  • Sensitivity: Often detects broken bars earlier than vibration

Progression Stages

Single Broken Bar

  • Small sidebands appearing (20-40% of 1× peak)
  • Slight torque pulsation (may not be noticeable)
  • Motor performance nearly normal
  • Can operate for months with monitoring
  • Replacement should be planned

Multiple Adjacent Broken Bars

  • Strong sidebands (> 50% of 1× peak)
  • Noticeable torque pulsation
  • Increased slip and temperature
  • Progression accelerating as adjacent bars overheat
  • Replacement urgent (weeks time frame)

Severe Condition

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

Detection Best Practices

Vibration Analysis

  • Use high-resolution FFT (< 0.2 Hz resolution) to resolve sidebands
  • Test motor under load (sidebands more prominent with current flow)
  • Calculate expected slip frequency for motor
  • Search spectrum for symmetric sidebands at ±fs around 1×
  • Trend sideband amplitude over time

MCSA Testing

  • Clamp current probes on motor leads
  • Acquire current waveform and calculate FFT
  • Look for sidebands at fline ± 2fs
  • Compare to healthy motor baseline
  • Can detect before vibration symptoms clear

Corrective Actions

Immediate Response

  • Increase monitoring frequency (monthly → weekly → daily)
  • Track sideband amplitude growth rate
  • Order spare motor or plan rotor replacement
  • Reduce duty cycle if possible (minimize starts)
  • Document progression for failure analysis

Repair Options

  • Rotor Replacement: Most reliable for large motors (> 100 HP)
  • Rotor Recasting: Specialized shops can recast aluminum rotors
  • Motor Replacement: Often most economical for small motors (< 50 HP)
  • Root Cause Investigation: Determine why bars broke to prevent recurrence

Prevention

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

Broken rotor bars, while accounting for only 10-15% of motor failures, create distinctive slip frequency sideband signatures that enable reliable early detection through vibration or current analysis. Understanding the thermal fatigue mechanism, recognizing the characteristic sideband pattern, and implementing condition monitoring enables planned motor replacement before single bar failures progress to catastrophic multiple bar failures and extended unplanned downtime.

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