SpecCalc Hub
Synchronous Speed Calculator: 120f/p, 50 Hz & 60 Hz
Find motor rotating-field speed from frequency and total poles. Compare 50 Hz and 60 Hz examples without confusing poles with pole pairs.
Enter the values
Client-side estimate. SpecCalc Hub does not store entered values.
Formula: n_s = 120 x f / p
Reference formula
This simplified formula block is educational and depends on the page status and limitations.
n_s = 120 x f / p
Assumptions
- Inputs are user-provided.
- The result is an informational estimate.
Limitations
- Loaded rotor speed is not calculated here.
What this page helps with
Find motor rotating-field speed from frequency and total poles. Compare 50 Hz and 60 Hz examples without confusing poles with pole pairs.
Calculate rotating-field speed as 120 × frequency / pole count. Enter poles, not pole pairs.
Formula or method
ns = 120 × f / p in rpm when f is in Hz and p is total poles. If a source gives pole pairs, multiply that count by two before entering it.
The working method on this page uses inputs such as Frequency and Poles and reports outputs such as Synchronous speed. The formula, assumptions and limitations stay visible so the result can be reviewed instead of simply trusted.
This makes the page useful for comparison and documentation, but it does not automatically include every manufacturer coefficient, installation condition, environmental factor or local-code requirement that may matter in a real project.
How to use
- Enter the page inputs, starting with Frequency, and confirm that the units match the real scenario you are checking.
- Read the main output Synchronous speed together with the other reported values instead of focusing on only one number.
- Before a real decision, review the assumptions, limitations, source links and related tools on the same page.
Practical examples
A common question, worked through
At 50 Hz: 2 poles → 3000 rpm, 4 → 1500, 6 → 1000, 8 → 750. At 60 Hz: 2 → 3600, 4 → 1800, 6 → 1200, 8 → 900. These are field speeds.
Worked example with the form defaults
Example inputs: Frequency: 50 Hz; Poles: 4. n_s = 120 x f / p. Calculated outputs: Synchronous speed: 1,500 rpm.
Common mistakes
- Using the page without checking units, document revision or the real operating context.
- Treating a preliminary result or reference table as a final engineering decision.
- Ignoring the source links, limitations and related tools needed for the next validation step.
Limitations
- A variable-frequency drive can change frequency, but permitted shaft speed, cooling and mechanical limits still come from the motor and machine documentation.
- An induction motor normally rotates below synchronous speed while motoring. Four poles at 50 Hz gives 1500 rpm for the field, not a promise of 1500 rpm on the loaded shaft.
- Loaded rotor speed is not calculated here.
- The page does not replace official manufacturer documentation, local rules or site measurements.
- Before practical use, confirm that the method matches the real equipment revision and operating conditions.
FAQ
What do I enter for two pole pairs?
Enter four poles. Entering two would double the calculated synchronous speed.
Does 1500 rpm guarantee a 1500 rpm shaft speed?
No. At 50 Hz and four poles, 1500 rpm is the field speed. A loaded induction motor has slip; a gearbox changes the driven-machine speed again.
Which frequency should I enter with a variable-frequency drive?
Use the drive output frequency applied to the motor for the scenario, not automatically the grid frequency. This arithmetic does not establish the motor’s allowable speed or cooling capability.
Related tools
Explanation updated: 2026-09-16
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This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.