What is 2× line frequency?

2× line frequency (100 Hz for 50 Hz systems, 120 Hz for 60 Hz systems) is the vibration frequency caused by the alternating magnetic field in an electric motor. It is always present at some level. Elevated 2× line frequency can indicate electrical problems such as stator eccentricity, shorted laminations, unbalanced phase voltages, or loose iron.

What indicates broken rotor bars?

Broken rotor bars are indicated by sidebands around 1× line frequency spaced at pole pass frequency (2 × slip × line frequency). In current spectrum analysis (MCSA), look for sidebands at f_line ± n × pole_pass. In vibration, look for rotor bar pass frequency with sidebands at 2× line frequency.

How to distinguish electrical from mechanical unbalance?

Key test: turn off power and observe coast-down. If the 1× vibration drops instantly at power cutoff, the source is electrical (magnetic unbalance). If it decreases gradually with speed, the source is mechanical. Electrical unbalance at 1× appears at exactly line frequency or 2× line, while mechanical unbalance appears at shaft running speed.

What is slip frequency?

Slip frequency is the difference between the synchronous speed and actual rotor speed, expressed as a frequency: f_slip = s × f_line, where s = (Ns - N)/Ns. Slip is necessary for torque production in induction motors. Typical full-load slip is 1-5% depending on motor size and design.

How to distinguish electrical from mechanical faults?

Use the power-off test: electrical vibration disappears instantly when power is removed, while mechanical vibration decreases gradually with coast-down. Electrical faults typically produce vibration at exact multiples of line frequency (100/120 Hz), while mechanical faults produce vibration at shaft speed and its multiples. Current signature analysis (MCSA) can further isolate rotor-specific faults.

Free Engineering Tool #039

Motor Electrical Defect Frequency Calculator

Calculate synchronous speed, slip, 2× line frequency, pole pass frequency, rotor bar pass frequency, and diagnostic sidebands for electric motor analysis.

Slip 2× Line Pole Pass Rotor Bars

Results

Synchronous Speed

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Slip

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Slip Frequency

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1× Line Frequency

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2× Line Frequency

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Pole Pass Frequency

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Shaft Frequency (1×)

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Synchronous Speed

The synchronous speed of an AC induction motor depends on line frequency and number of poles:

Slip

Slip is the difference between synchronous and actual rotor speed, expressed as a fraction or percentage:

Typical full-load slip for standard induction motors is 1–5%.

Slip Frequency

2× Line Frequency

This frequency (100 Hz at 50 Hz supply, 120 Hz at 60 Hz) is always present in motor vibration due to the alternating magnetic field. Elevated 2× line indicates electrical issues: unbalanced phases, air gap eccentricity, or stator winding faults.

Pole Pass Frequency

Pole pass frequency is a key indicator for broken rotor bar analysis. Sidebands at pole pass frequency around 1× line frequency in current spectra (MCSA) are a classic rotor bar fault signature.

Rotor Bar Pass Frequency

Practical Example

Example — 4-pole motor, 50 Hz, 1475 RPM

Given: f_line = 50 Hz, Poles = 4, N = 1475 RPM

N_s = 120 × 50 / 4 = 1500 RPM

s = (1500 − 1475) / 1500 = 1.67%

f_slip = 0.0167 × 50 = 0.833 Hz

2× line = 2 × 50 = 100 Hz

Pole pass = s × f_line × Poles = 0.0167 × 50 × 4 = 3.33 Hz

⚠️ Note: Motor speed varies with load. Ensure you use the actual measured speed under operating conditions, not the nameplate speed. A tachometer or strobe measurement gives the most accurate results for slip-dependent calculations.

Motor Electrical Defect Frequency Calculator

Results

Rotor Bar Analysis

Rotor Bar Sidebands (f_line ± n × f_{pole pass})

Synchronous Speed

Slip

Slip Frequency

2× Line Frequency

Pole Pass Frequency

Rotor Bar Pass Frequency

Practical Example

Motor Electrical Defect Frequency Calculator

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Rotor Bar Analysis

Rotor Bar Sidebands (fline ± n × fpole pass)

Synchronous Speed

Slip

Slip Frequency

2× Line Frequency

Pole Pass Frequency

Rotor Bar Pass Frequency

Practical Example

Rotor Bar Sidebands (f_line ± n × f_{pole pass})