AC Synchronous Motor Vibration Frequency Calculator

Calculator Inputs

Enter nameplate values first. Add optional geometry for deeper spectral matching.

Formula Used

Synchronous speed: Ns = 120 × f / P.

Shaft frequency: Fs = RPM / 60.

Mechanical order frequency: Fo = order × Fs.

Electrical harmonic: Fe = harmonic × f.

Slot pass frequency: Fslot = slot count × Fs.

Blade pass frequency: Fblade = blade count × Fs.

Bearing estimates: FTF, BPFO, BPFI, and BSF use ball count, ball diameter, pitch diameter, contact angle, and shaft frequency.

How to Use This Calculator

1. Enter the supply frequency in hertz.
2. Enter the motor pole count from the nameplate.
3. Enter measured RPM when tachometer data is available.
4. Enter a vibration order for shaft based checks.
5. Add slot, blade, and bearing values when available.
6. Press the calculate button and review the result table.
7. Compare calculated frequencies with measured vibration spectrum peaks.

Example Data Table

Motor Vibration Frequency Guide

AC synchronous motors run at a speed fixed by supply frequency. The pole count sets that speed. When the rotor locks with the rotating magnetic field, slip becomes nearly zero. That stable speed makes frequency prediction practical. Vibration analysis then links measured motion to known forcing sources. Common sources include shaft rotation, magnetic pull, slot pass, blade pass, and bearings.

Why Frequency Matters

Each mechanical fault creates a pattern. Unbalance often appears near one times shaft speed. Misalignment may show strong two times speed. Loose parts can create many harmonics. Electrical or magnetic problems often appear at line frequency or twice line frequency. A synchronous motor may also show tones near pole pass frequency. Comparing calculated tones with measured spectra helps reduce guesswork.

Core Speed Relationship

The main relationship is simple. Synchronous speed equals one hundred twenty times line frequency divided by pole count. A four pole motor on fifty hertz runs at fifteen hundred revolutions per minute. A four pole motor on sixty hertz runs at eighteen hundred revolutions per minute. The shaft frequency equals revolutions per minute divided by sixty. Every order is then a multiple of shaft frequency.

Advanced Diagnostic Inputs

This calculator includes optional advanced inputs. Rotor and stator slot counts estimate slot pass tones. Fan or pump blade count estimates blade pass frequency. Bearing geometry estimates cage, outer race, inner race, and ball spin frequencies. These values help when vibration data contains several close peaks. They also help when machine drawings provide geometry before field testing.

Interpreting the Results

Use the result table as a comparison map. It does not prove a fault alone. Real diagnosis also needs amplitude, phase, trend history, loading, mounting, and sensor position. A peak close to a calculated tone deserves attention. A peak that grows over time deserves faster action. If several related tones rise together, inspect the matching system first.

Good Measurement Practice

Record motor nameplate data before testing. Confirm the supply frequency. Confirm pole count from rated speed. Measure actual speed with a tachometer when possible. Place sensors firmly on bearing housings. Take readings in horizontal, vertical, and axial directions. Compare loaded and unloaded operation. Keep notes about temperature, coupling condition, base tightness, and driven equipment.

Practical Maintenance Value

Calculated vibration frequencies support early screening. They turn raw spectral peaks into useful clues. They also help teams share findings in clear language. A planner can decide which parts to inspect. A technician can check alignment, balance, bearings, or electrical supply. Clean inputs make vibration checks faster and more dependable.

Limits And Safety

Use estimates as screening data. Stop equipment only through approved plant procedures. Do not open energized cabinets for speed checks. High voltage and rotating parts can injure workers. When a peak is severe, follow site alarm rules. Ask a qualified vibration specialist first. Confirm critical faults before planning major shutdown decisions safely.

FAQs