BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Supply frequency
- What it is
- The frequency of the electrical supply feeding the motor, or the output frequency of its variable speed drive if it has one.
- Units and format
- Enter a number in Hz: hertz (cycles per second). The form accepts 0.000001 Hz to 400 Hz. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 Hz as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Synchronous speed rises in direct proportion to frequency.
- Get it right
- If the motor is on a drive, use the drive’s output frequency, not the mains frequency.
What this means: Number of poles
- What it is
- The number of magnetic poles in the motor winding, which with the frequency fixes the synchronous speed.
- Units and format
- Choose one of 6 options from the list: 2; 4; 6; 8; 10; 12.
- Example
- The form opens with “4” selected as a worked example. Choose the option that matches your case.
- Get it right
- If it is not on the nameplate, pick the pole count whose synchronous speed is just above the rated speed.
What this means: Measured speed
- What it is
- The actual shaft speed of the running motor, measured with a tachometer or strobe under the present load.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1475 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower measured speed means more slip and so a higher estimated load.
- Get it right
- Measure at the shaft under the load you want to estimate; it must be below synchronous speed.
What this means: Nameplate (rated) speed
- What it is
- The full-load speed printed on the motor nameplate, used to find the rated slip.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1460 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the nameplate speed for the supply frequency actually used, not the synchronous speed.
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.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
What a drive does to an induction motor
In Variable-Speed Drives by the Numbers
What it covers: How an induction motor's speed follows supply frequency, how a drive's rectifier, DC link and PWM inverter make a variable frequency and voltage, and how to read motor load on and off a drive.
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