Understanding Rotor Bar Defects

Devices

Portable balancer & Vibration analyzer Balanset-1A

Parts

Optical Sensor (Laser Tachometer)

Complete vibration diagnostics and balancing kit from Vibromera, including four vibration sensors with cables, a signal interface module, laser tachometer with magnetic stand, digital scale, USB cable, and a carrying case. The system is designed for field rotor balancing and condition monitoring on industrial equipment.

Devices

Dynamic balancer “Balanset-1A” OEM

Definition: What are Rotor Bar Defects?

Rotor bar defects (also called broken rotor bars or cracked rotor bars) are fractures, cracks, or high-resistance connections in the conductor bars of squirrel cage induction motor rotors. Squirrel cage rotors consist of aluminum or copper bars embedded in iron core slots, with both ends of the bars connected by shorting rings (end rings). When bars break or end ring connections crack, electrical current cannot flow properly through the damaged bars, creating electromagnetic asymmetry, pulsating torque, and characteristic vibration and current signatures with sidebands at slip frequency spacing.

Rotor bar defects account for 10-15% of motor failures and are particularly problematic because they can progress from a single broken bar to multiple failures, creating severe vibration, torque pulsation, and eventual motor failure if not detected and corrected.

Types of Rotor Bar Defects

1. Broken Rotor Bars

Description: Complete fracture of conductor bar
Location: Typically near end rings where thermal and mechanical stress concentrated
Progression: Usually starts with crack, progresses to complete break
Multiple Bars: One broken bar increases stress on adjacent bars, leading to progressive failures

2. Cracked End Rings

Description: Fractures in shorting rings that connect rotor bars
Effect: Similar to broken bars—disrupts current flow
Location: Often at bar-to-ring junction
More Common In: Large motors, motors with frequent starts, high-inertia loads

3. High-Resistance Joints

Description: Poor electrical connection between bars and end rings
Cause: Manufacturing defects, thermal cycling, corrosion
Effect: Similar symptoms to broken bars but may be intermittent
Detection: More subtle signatures than complete breaks

4. Rotor Porosity

Voids in cast aluminum rotors
Reduces effective conductor cross-section
Can progress to cracks and breaks
Manufacturing defect but may not manifest until later in life

Causes of Rotor Bar Failures

Thermal Stresses

Thermal Cycling: Expansion/contraction from startup/shutdown
Differential Expansion: Aluminum bars expand more than iron core
Hot Spots: Localized overheating from high resistance
Frequent Starts: Each start creates thermal shock

Mechanical Stresses

Centrifugal Forces: Particularly in high-speed motors
Electromagnetic Forces: Pulsating forces during operation
Starting Torque: High currents during startup create mechanical stress
Vibration: External vibration fatiguing bars

Manufacturing Defects

Porosity in cast rotors
Poor bar-to-end ring bonding
Material inclusions or voids
Inadequate heat treatment

Operating Conditions

Frequent Starting: Thermal and electromagnetic stress
High Inertia Loads: Long acceleration times increase bar stress
Locked Rotor Events: Extreme currents and forces
Single-Phasing: Operating with one phase lost creates asymmetric currents

Vibration Signature

Characteristic Pattern

The hallmark of rotor bar defects is sidebands around running speed:

Central Peak: 1× running speed (fr)
Sidebands: fr ± fs, fr ± 2fs, fr ± 3fs
Where fs = slip frequency (typically 1-3 Hz)
Pattern: Symmetrical sidebands spaced at slip frequency intervals

Calculation of Slip Frequency

fs = (Nsync – Nactual) / 60
Example: 4-pole, 60 Hz motor
Nsync = 1800 RPM, Nactual = 1750 RPM
fs = (1800 – 1750) / 60 = 0.833 Hz
Sidebands appear at 29.17 ± 0.833 Hz (28.3 Hz and 30.0 Hz)

Load Dependence

No Load: Minimal sidebands (low slip, low current through broken bars)
Light Load: Small sidebands beginning to appear
Full Load: Strong sidebands, most obvious diagnosis
Diagnostic Strategy: Test under load for best sensitivity

Current Signature (MCSA)

Motor current analysis shows same pattern as vibration:

Sidebands around line frequency (not running speed)
Pattern: fline ± 2fs (twice slip frequency in current)
For 60 Hz motor with 1 Hz slip: sidebands at 58 Hz and 62 Hz
Amplitude increases with number of broken bars
Can detect earlier than vibration in some cases

Detection and Diagnosis

Vibration Analysis Procedure

Calculate Expected Pattern: Determine synchronous speed, measure actual speed, calculate slip frequency
High-Resolution FFT: Use fine resolution (< 0.2 Hz) to resolve sidebands
Look for Sidebands: Search for peaks at 1× ± slip frequency
Under Load: Test with motor under normal operating load
Confirm Pattern: Verify symmetric sidebands at correct spacing

Severity Assessment

Sideband < 40% of 1× peak: Possible single broken bar, monitor
40-60% of 1×: Confirmed broken bar(s), plan replacement
> 60% of 1×: Multiple broken bars, urgent replacement needed
Sidebands > 1× peak: Severe condition, immediate action required

Consequences and Progression

Initial Failure (Single Bar)

Slight torque pulsation
Small sidebands appearing
May run for months with single broken bar
Performance degradation minimal

Progressive Failures (Multiple Bars)

Adjacent bars overheat from increased current
Thermal stress causes additional failures
Torque pulsations increase
Vibration becomes severe
Can progress from single to multiple bars in weeks

Severe Condition

Multiple adjacent broken bars
Severe torque pulsation
High vibration and noise
Overheating of rotor
Risk of complete rotor failure
May damage stator from excessive current

Corrective Actions

Upon Detection

Increase monitoring frequency (monthly → weekly)
Perform MCSA to confirm diagnosis
Plan motor replacement or rotor replacement
Prepare spare motor if critical application
Consider root cause (why bars broke)

Repair Options

Rotor Replacement: Most reliable solution for large motors
Complete Motor Replacement: Often most economical for small motors
Rotor Recasting: Specialized shops can recast aluminum rotors
Temporary Operation: Single broken bar may allow continued operation with monitoring

Prevention

Minimize frequent starts (use soft starters or VFDs)
Avoid single-phasing conditions
Ensure adequate ventilation and cooling
Use motors rated for duty cycle (frequent start motors for high-cycle applications)
Monitor for early detection before multiple failures

Rotor bar defects are among the most diagnostically distinctive motor faults, with their characteristic slip frequency sidebands enabling reliable detection through both vibration and current analysis. Early identification allows planned motor replacement before progression to multiple bar failures that can cause catastrophic rotor damage and extended unplanned downtime.

← Back to Main Index

What are Rotor Bar Defects? Broken Bars in Motors

Published by admin on October 31, 2025 October 31, 2025