1. Electrical Fundamentals for the Vibration Analyst

Before diagnosing motor defects from vibration spectra, it is essential to understand the key electrical frequencies that drive motor vibration.

1.1. Line Frequency (LF)

The AC supply frequency: 50 Hz in most of Europe, Asia, Africa, and Russia; 60 Hz in North America and parts of South America and Asia. All electromagnetic forces in the motor are derived from this frequency.

1.2. Twice Line Frequency (2×LF)

The dominant electromagnetic force frequency in AC motors. In a 50 Hz system, 2×LF = 100 Hz; in a 60 Hz system, 2×LF = 120 Hz. The magnetic attraction force between stator and rotor peaks twice per electrical cycle, making 2×LF the fundamental "electrical vibration" frequency of every AC motor.

1.3. Synchronous Speed and Slip

The stator magnetic field rotates at synchronous speed:

where P is the number of poles. An induction motor rotor always rotates slightly slower. This difference is slip:

Typical full-load slip for standard induction motors: 1–5%. For a 2-pole motor at 50 Hz: N_s = 3000 RPM, actual speed ≈ 2940–2970 RPM.

1.4. Pole Pass Frequency (F_p)

The rate at which rotor poles "slip past" stator poles. The result is universal — independent of pole count:

For a motor running at 50 Hz with 2% slip: F_p = 2 × 0.02 × 50 = 2 Hz. This frequency appears as characteristic sidebands in spectra of broken rotor bars.

1.5. Rotor Bar Pass Frequency

Where R is the number of rotor bars. This frequency and its sidebands become significant when rotor bars are damaged.

1.6. Key Frequency Reference Table

StatorRotorWindingsAir gapMechanicalAxial Any air-gap distortion directly changes magnetic pull, and that immediately changes the vibration pattern. Symbol ± denotes sidebands (modulation).

2. Overview of Diagnostic Methods

No single technique can detect all electric motor defects. A robust diagnostic program combines multiple complementary methods:

VibrationMCSAESAMCAThermography No single method gives full coverage. A combined diagnostic approach is strongly recommended.

2.1. Vibration Spectral Analysis

The primary tool for most rotating equipment diagnostics. Accelerometers on bearing housings capture spectra revealing mechanical defects (unbalance, misalignment, bearing wear) and some electrical defects (uneven air gap, loose windings). However, vibration analysis alone cannot detect all motor electrical faults.

2.2. Motor Current Signature Analysis (MCSA)

A current clamp on one phase captures the current spectrum. Broken rotor bars produce sidebands at LF ± Fp. MCSA is performed online and is completely non-invasive.

2.3. Electrical Signature Analysis (ESA)

Analyzes both voltage and current spectra simultaneously at the MCC. Detects supply voltage asymmetry, harmonic distortion, and power quality issues.

2.4. Motor Circuit Analysis (MCA)

An offline test measuring phase-to-phase resistance, inductance, impedance, and insulation resistance. Essential during maintenance shutdowns.

2.5. Temperature Monitoring

Stator winding temperature and bearing temperature trending provide early warning of overload, cooling problems, and insulation degradation.

3. Stator Defects

Stator defects are responsible for approximately 23–37% of all motor failures. The stator is the stationary part containing the laminated iron core and windings. Defects produce vibration primarily at 2×LF (100 Hz / 120 Hz) and its multiples.

3.1. Stator Eccentricity — Uneven Air Gap

The air gap between rotor and stator is typically 0.25–2 mm. Even a 10% variation creates measurable electromagnetic force imbalance.

Causes

• Soft foot — the most common cause
• Worn or damaged bearing housings
• Frame deformation from improper transport or installation
• Thermal distortion under operating conditions
• Poor manufacturing tolerances

Spectral Signature

• Typically dominant 2×LF in the radial velocity spectrum
• Often accompanied by minor increase of 1X and 2X due to unbalanced magnetic pull (UMP)
• Static eccentricity: 2×LF dominates with little modulation
• Dynamic component: sidebands at 2×LF ± 1X may appear

Severity Assessment

Note: These are illustrative guidelines, not a formal standard. Always compare against the machine's own baseline.

Confirmation Test

Power-off test (snap test): While monitoring vibration, de-energize the motor. If the 2×LF peak drops sharply — within seconds, much faster than mechanical coastdown — the source is electromagnetic.

3.2. Loose Stator Windings

Stator windings are subjected to electromagnetic forces at 2×LF during every operating cycle. Over years, mechanical fixation (epoxy, varnish, wedges) can degrade. Loose windings vibrate at 2×LF with increasing amplitude, accelerating insulation wear through fretting.

Spectral Signature

• Predominantly radial vibration
• 2×LF may be less stable — slight amplitude fluctuations
• Severe cases: harmonics at 4×LF, 6×LF

Consequences

This is destructive for winding insulation — leads to accelerated degradation, unpredictable ground faults, and complete stator failure requiring rewind.

3.3. Loose Power Cable — Phase Asymmetry

A poor contact creates resistance asymmetry. Even 1% voltage asymmetry causes approximately 6–10% current asymmetry. The unbalanced currents create a backward-rotating magnetic field component.

Spectral Signature

• 2×LF amplitude increases due to unbalanced magnetic pull
• In some cases, sidebands near ±⅓×LF (~16.7 Hz in 50 Hz systems) around the 2×LF peak
• In current spectrum (MCSA): elevated negative-sequence current

Practical Checks

• Check all cable terminations, bus bar connections, contactor contacts
• Measure phase-to-phase resistance — within 1% of each other
• Measure supply voltage on all three phases — asymmetry should not exceed 1%
• IR thermography of cable termination box

3.4. Shorted Stator Laminations

Damage to inter-lamination insulation allows eddy currents to circulate, creating localized hot spots. Not always detectable in vibration spectra — IR thermography is the primary detection method. Offline: electromagnetic core test (EL-CID test).

3.5. Inter-Turn Short Circuit

A turn-to-turn short creates a localized circulating current loop, reducing effective turns in the affected coil. Produces increased 2×LF, elevated 3rd harmonic of LF in current, and phase current asymmetry. Best detected via MCA surge test offline.

2×LF1X / 2XSidebands The power-off test confirms electromagnetic origin: if 2×LF drops sharply upon de-energization (much faster than coastdown), the source is electromagnetic.

4. Rotor Defects

Rotor defects account for approximately 5–10% of motor failures but are often the most challenging to detect early.

4.1. Broken Rotor Bars and Cracked End Rings

When a bar breaks, current redistribution creates local magnetic asymmetry — effectively a "magnetic heavy spot" that rotates at slip frequency relative to the stator field.

Vibration Signature

• 1X peak with sidebands at ± F_p. For 50 Hz / 2% slip: sidebands at 1X ± 2 Hz
• Severe cases: additional sidebands at ± 2F_p, ± 3F_p
• 2×LF may also show F_p sidebands

MCSA Signature

MCSA Severity Scale

Important: MCSA requires steady load near rated conditions. At partial load, sideband amplitude drops.

Time Waveform

Broken rotor bars produce a characteristic "beating" pattern — amplitude modulates at the pole pass frequency. Often visible before spectral sidebands become prominent.

1X±Fp sidebandsMCSA sidebands Broken rotor bars are best confirmed via MCSA. The vibration spectrum suggests the defect; MCSA provides quantitative severity assessment.

4.2. Rotor Eccentricity (Static and Dynamic)

Static Eccentricity

Shaft centerline offset from stator bore. Produces elevated 2×LF. In current: rotor slot harmonics at f_RBPF ± LF.

Dynamic Eccentricity

Rotor center orbits around stator bore center. Produces 1X with 2×LF sidebands and elevated rotor bar pass frequency. In current: sidebands at LF ± f_rot.

In practice, both types are usually present simultaneously — the pattern is a superposition.

4.3. Thermal Rotor Bow

Large motors can develop a temperature gradient causing temporary bow. Produces 1X that varies with time after startup — typically increasing for 15–60 minutes, then stabilizing. The phase angle drifts as the bow develops. Distinguish from mechanical unbalance (which is stable) by monitoring 1X amplitude and phase for 30–60 minutes post-startup.

4.4. Electromagnetic Field Displacement (Axial Shift)

If the rotor is axially displaced relative to the stator, the electromagnetic field distribution becomes asymmetric axially. The rotor experiences an oscillating axial electromagnetic force at 2×LF.

Causes

• Incorrect rotor axial positioning during assembly or after bearing replacement
• Bearing wear allowing excessive axial play
• Shaft thrust from the driven machine
• Thermal expansion during operation

Axial EM forceShift / overhangStator CLDetection Axial 2×LF that vanishes instantly on power-off is the key differentiator from mechanical causes.

5. Bearing-Related Electrical Defects

5.1. Bearing Currents and EDM

Voltage between shaft and housing causes current flow through bearings. Sources: magnetic asymmetry, VFD common-mode voltage, static charge. Repeated discharges create microscopic pits (Electrical Discharge Machining) leading to fluting — evenly spaced grooves on races.

Spectral Signature

• Bearing defect frequencies (BPFO, BPFI, BSF) with very uniform, "clean" peaks
• Elevated high-frequency noise floor in acceleration spectrum
• Advanced: characteristic "washboard" sound

Prevention

• Insulated bearings (coated rings)
• Shaft grounding brushes (especially for VFD applications)
• Common-mode filters on VFD output
• Regular shaft voltage measurement — below 0.5 V peak

6. Variable Frequency Drive (VFD) Effects

6.1. Frequency Shifting

All motor electrical frequencies shift proportionally with VFD output frequency. If VFD runs at 45 Hz, 2×LF becomes 90 Hz. Alarm bands must be speed-adaptive.

6.2. PWM Harmonics

Switching frequency (2–16 kHz) and sidebands appear in spectra. Can cause audible noise and bearing currents.

6.3. Torsional Excitation

Low-order harmonics (5th, 7th, 11th, 13th) create torque pulsations that can excite torsional natural frequencies.

6.4. Resonance Excitation

As VFD sweeps through a speed range, excitation frequencies may pass through structural natural frequencies. Critical speed maps should be established for VFD-driven equipment.

7. Differential Diagnostics Summary

ElectricalMechanical2×LF analysisRotor defects The power-off snap test is the first fork in the diagnostic tree. Once electrical origin is confirmed, dominant frequency and direction narrow the diagnosis.

8. Instrumentation and Measurement Techniques

8.1. Vibration Measurement Requirements

8.2. Balanset-1A for Motor Diagnostics

The portable dual-channel vibrometer Balanset-1A (VibroMera) provides core capabilities for motor vibration diagnostics:

After diagnosing and correcting the motor defect, the Balanset-1A can be used for in-situ rotor balancing — completing the full diagnostic-to-correction workflow without removing the motor.

8.3. Measurement Best Practices

• Three directions — vertical, horizontal, and axial — on each bearing. Axial is critical for EM field displacement
• Prepare surfaces — remove paint, rust for reliable accelerometer coupling
• Steady-state conditions — nominal speed, load, temperature
• Record operating conditions — speed, load, voltage, current with each measurement
• Consistent timing — same conditions for trend comparisons
• Power-off test when electrical vibration is suspected — takes seconds, provides reliable source identification

9. Normative References

• GOST R ISO 20816-1-2021 — Vibration. Measurement and evaluation of machine vibration. Part 1. General guidelines.
• GOST R ISO 18436-2-2005 — Condition monitoring. Vibration condition monitoring. Part 2. Training and certification.
• ISO 20816-1:2016 — Mechanical vibration. Measurement and evaluation. Part 1: General guidelines.
• ISO 10816-3:2009 — Evaluation of machine vibration. Part 3: Industrial machines >15 kW.
• IEC 60034-14:2018 — Rotating electrical machines. Part 14: Mechanical vibration.
• IEEE 43-2013 — Recommended practice for testing insulation resistance.
• IEEE 1415-2006 — Guide for induction machinery maintenance testing.
• NEMA MG 1-2021 — Motors and generators. Vibration limits and testing.
• ISO 1940-1:2003 — Balance quality requirements for rotors.

10. Conclusion

1. 2×LF is the primary electromagnetic indicator. A prominent peak at exactly twice supply frequency strongly suggests an electromagnetic source. The power-off test provides confirmation.
2. Direction matters. Radial 2×LF → air gap / windings / supply. Axial 2×LF + 1X → electromagnetic field displacement — one of the most destructive defects.
3. Sidebands tell the story. ± ⅓×LF → supply cable problems. ± F_p → broken rotor bars. The sideband pattern is often more diagnostic than the main peak.
4. Spectral resolution is critical. For 2-pole motors at 50 Hz, 2X and 2×LF are only ~2 Hz apart. Resolution ≤ 0.5 Hz is mandatory.
5. Combine methods. Vibration + MCSA + MCA + Thermography. No single method covers all defects.
6. Talk to the electricians. Motor repair personnel possess irreplaceable knowledge about specific motors, their history, and supply conditions.

Recommended Workflow

Diagnostic stepsMCSAVerification Follow this sequence systematically. The power-off test (step 2) takes seconds and reliably distinguishes electrical vs. mechanical source.

Modern portable dual-channel vibrometers such as the Balanset-1A enable field engineers to perform spectral vibration analysis with the resolution and phase accuracy required for motor defect identification — from detecting uneven air gaps through cross-phase analysis to subsequent in-situ rotor balancing.