KEVOS® MAKEOS · Power Transmission
Induction Motor Synchronous Speed, Slip and Load
What is this motor’s synchronous speed and slip, and roughly how loaded is it?
Synchronous speed from supply frequency and poles, slip at a measured speed, and an estimate of load from slip compared with rated slip.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Supply frequency | Hz | Supply frequency (50 Hz in Australia). |
| Number of poles | choice | Number of poles (from the nameplate or synchronous speed). |
| Measured speed | rpm | Measured shaft speed with a tachometer. |
| Nameplate (rated) speed | rpm | Full-load speed from the nameplate. |
Method and formulas
- Synchronous speed ns = 120 f ÷ poles.
- Slip = (ns − n) ÷ ns.
- Load ≈ measured slip ÷ rated slip × 100% (slip is roughly proportional to load below full load).
- Example: a 4-pole motor at 50 Hz running 1475 rpm has 1.6667% slip, about 62.5% load.
Limitations
- Slip–load proportionality is approximate and assumes rated voltage; nameplate speeds have tolerance.
- Not valid for synchronous or permanent-magnet motors.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate motor load from a tachometer reading
- Check pole count and nameplate speed
- Find speed at another frequency (VFD)
Who it is for
- Electrical and maintenance engineers
- Energy managers
What you get
- Synchronous speed
- Slip at measured and rated speed
- Estimated load
Interpreting the result
- Motors running well below 50% load are often oversized and less efficient.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2569–2580. Topic reference; no source tables or recommended settings are reproduced.
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