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.
Results
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
Given: fline = 50 Hz, Poles = 4, N = 1475 RPM
Ns = 120 × 50 / 4 = 1500 RPM
s = (1500 − 1475) / 1500 = 1.67%
fslip = 0.0167 × 50 = 0.833 Hz
2× line = 2 × 50 = 100 Hz
Pole pass = 2 × s × fline = 2 × 0.0167 × 50 = 1.67 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.
2× line frequency (2FL) is 100 Hz for 50 Hz systems and 120 Hz for 60 Hz systems. It is the vibration frequency generated by the alternating magnetic attraction in the motor air gap (the magnetic field reverses twice per cycle, creating a force at 2× the supply frequency).
It is always present in motor vibration at some level. Elevated 2FL can indicate:
- Stator eccentricity (uneven air gap)
- Shorted stator laminations
- Unbalanced supply voltage or phase problems
- Loose stator iron
Broken rotor bars produce characteristic patterns in both vibration and current spectra:
- Current spectrum (MCSA): Sidebands at fline ± n × fpole pass around the supply frequency peak
- Vibration: Rotor bar pass frequency with sidebands at 2× line frequency
- Amplitude modulation: The 1× vibration amplitude varies at pole pass frequency rate
- Speed variation: Slight cyclic speed fluctuation at pole pass frequency
The severity is judged by the amplitude difference (dB) between the line frequency peak and the pole-pass sidebands in the current spectrum.
Both electrical and mechanical unbalance produce vibration at or near 1× shaft speed. The key tests:
- Power-off test: Turn off power and observe. If vibration drops instantly at disconnect, it's electrical. If it coasts down with speed, it's mechanical.
- Frequency precision: Electrical unbalance peaks appear at exact line frequency or 2× line. Mechanical unbalance appears at shaft running speed.
- Sidebands: Electrical problems show sidebands at slip frequency or pole pass frequency around line frequency harmonics.
Slip frequency is the electrical frequency corresponding to the difference between synchronous and actual rotor speed:
fslip = s × fline
where s = (Ns − N) / Ns
Slip is essential for torque production in induction motors — without slip, no current is induced in the rotor. Typical full-load slip values:
- Large motors (>100 kW): 1–2%
- Medium motors (10–100 kW): 2–4%
- Small motors (<10 kW): 3–5%
Systematic approach to identifying fault origin:
- Step 1 — Power-off test: Instantly disappearing vibration = electrical source
- Step 2 — Frequency analysis: Peaks at exact multiples of line frequency = electrical; peaks at shaft speed multiples = mechanical
- Step 3 — Sideband analysis: Slip-related sidebands = rotor electrical fault; bearing frequencies = mechanical
- Step 4 — Current analysis: Use MCSA to identify rotor bar, eccentricity, or stator faults independently of vibration
- Step 5 — Phase analysis: Electrical 2× line is typically in-phase across all measurements; mechanical unbalance shows phase shift between positions
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional vibration analysis instruments for motor diagnostics, field balancing, and condition monitoring. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
