2× Line Frequency

Definition

2× line frequency is a vibration component occurring at twice the electrical power supply frequency, commonly observed in electric motor-driven equipment.

For example:

• 50 Hz system → 2× line frequency = 100 Hz
• 60 Hz system → 2× line frequency = 120 Hz

Physical Mechanism

2× line frequency is primarily generated by electromagnetic forces inside an electric motor.

It is commonly associated with:

• Magnetic pull variations in the air gap
• Stator and rotor magnetic field interactions
• Rotor eccentricity (static or dynamic)
• Power supply asymmetry or imbalance in magnetic fields

These electromagnetic forces produce periodic mechanical vibration at twice the supply frequency.

Signal Signature

2× line frequency typically appears in vibration data as:

• A discrete peak at 2× electrical line frequency (100 Hz or 120 Hz)
• Often most visible in radial vibration measurements
• May increase under electrical or air gap irregularities
• Can be stable or modulated depending on motor condition

In waveform:

• Smooth periodic vibration component (not impact-based)

Diagnostic Relevance

Elevated 2× line frequency is commonly associated with:

• Rotor eccentricity (air gap variation)
• Stator core looseness or distortion
• Voltage imbalance or phase issues
• Electromagnetic force asymmetry
• Motor structural resonance excited by electrical forces

It is especially important in motor diagnostics because it indicates electrical or magnetic origin rather than mechanical imbalance.

Interpretation Notes

• 2× line frequency is not related to shaft rotational speed; it is tied to electrical supply frequency.
• It should not be confused with mechanical 2× running speed components (misalignment-related).
• Electrical faults often show strong stability at line-related frequencies regardless of mechanical load changes.
• Comparison between electrical signature (current analysis) and vibration is often required for confirmation.

Summary

2× line frequency is a vibration component generated by electromagnetic forces in electric motors, occurring at twice the electrical supply frequency. It is primarily used to diagnose electrical and air-gap related issues rather than mechanical faults.