The calculation, without hidden assumptions
Turn a tachometer reading into transparent induction-motor speed diagnostics. Enter supply frequency, even pole count and measured shaft RPM; MAXScanner calculates synchronous speed, signed slip, rotor-frequency magnitude and an optional comparison with nameplate full-load slip without pretending slip alone measures load.
How to use this calculator
Enter the actual fundamental supply frequency. For a VFD-fed motor, use the drive output frequency at the operating point rather than assuming 50 or 60 Hz.
Enter the motor pole count as an even whole number and the measured shaft speed from a suitable tachometer or trusted drive feedback.
Optionally enter the nameplate full-load RPM to compare measured slip with rated full-load slip.
Read signed slip separately from rotor-frequency magnitude: speed above synchronous produces negative slip rather than being silently clamped to zero.
Use current, terminal voltage, mechanical condition and manufacturer data before diagnosing overload or rotor faults; slip by itself is an indicator, not a diagnosis.
Where people use it
- •Checking induction-motor slip during preventive maintenance.
- •Calculating synchronous RPM for 50 Hz, 60 Hz or VFD operation.
- •Converting measured shaft RPM into rotor electrical frequency for motor diagnostics.
- •Comparing operating slip with nameplate full-load slip without asserting a direct load percentage.
Example: 4-pole, 60 Hz motor at 1,750 rpm
Ns = 120 × 60 ÷ 4 = 1,800 rpm. Slip speed = 50 rpm. Slip = 50 ÷ 1,800 × 100 = 2.778%. Rotor electrical frequency = 0.02778 × 60 ≈ 1.667 Hz.
What the result does not assume
- •This owner models induction-machine slip. Synchronous, permanent-magnet and electronically commutated machines require different interpretation.
- •Pole count must be an even whole number. Use actual configured motor poles rather than guessing from a nominal speed label.
- •Nameplate full-load RPM is a rated operating point, not a universal prediction of present load. Slip-to-load proportionality is only approximate over limited operating regions and depends on voltage, temperature and motor design.
- •For VFD operation, frequency must match the drive output at the same instant as the speed measurement.
- •Negative slip can occur in generating/regenerative operation; this calculator preserves the sign instead of mislabeling it as ordinary motoring slip.
Frequently asked questions
How do I calculate motor slip percentage?+
Calculate synchronous speed from 120 × frequency ÷ poles, subtract measured rotor RPM, divide that difference by synchronous RPM and multiply by 100.
What is synchronous speed for a 4-pole 50 Hz motor?+
1,500 rpm because 120 × 50 ÷ 4 = 1,500.
Why does an induction motor run below synchronous speed?+
Positive slip creates relative motion between the rotating stator field and rotor, allowing rotor current and torque to be produced.
What is rotor or slip frequency?+
Its magnitude is supply frequency multiplied by the magnitude of per-unit slip. A 60 Hz motor at 2.778% slip has about 1.667 Hz rotor electrical frequency.
Can motor slip tell me exact load percentage?+
No. Slip often rises with load in the normal operating region, but voltage, temperature and motor design also matter. MAXScanner reports rated-slip comparison only as diagnostic context.
What does negative slip mean?+
The shaft is rotating faster than the synchronous field in the selected direction, which is generating/regenerative rather than ordinary induction-motor motoring operation.
Semantic next steps
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