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.
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.
