KEVOS® MAKEOS · Power Transmission
Induction Motor Load Assessment (Current, Slip, Load, Energy)
From the nameplate and a measured speed, how heavily loaded is this induction motor, what current and torque is it running at, and what does it cost to run?
Assessment of a three-phase induction motor: rated line current, input power and torque from the nameplate (as the motor power tool), synchronous speed, slip and the load estimated from measured against rated slip (as the motor slip tool), the estimated output, torque and current at that load, the yearly energy and cost, and checks for overload and for an oversized (lightly loaded) motor.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Line voltage | V | Line-to-line supply voltage. |
| Rated output | kW | Rated shaft output from the nameplate. |
| Rated power factor | Power factor at rated load from the nameplate. | |
| Rated efficiency | % | Efficiency at rated load from the nameplate. |
| Supply frequency | Hz | Supply frequency. |
| Poles | choice | Number of poles (sets synchronous speed). |
| Rated speed | rpm | Full-load speed from the nameplate. |
| Measured speed | rpm | Speed measured in service (tachometer or strobe). |
| Running hours per year | h | Hours the motor runs each year. |
| Electricity price | $/kWh | Price of electricity. |
| Load below which the motor is oversized | % | Load share below which you treat the motor as oversized (your value). |
Method and formulas
- Rated current I = P ÷ (√3 V pf η); rated torque = 9549 P ÷ n (as the motor power tool).
- Synchronous speed n_s = 120 f ÷ poles; slip = (n_s − n) ÷ n_s; load ≈ measured slip ÷ rated slip (as the motor slip tool).
- Output ≈ load × rated output; input ≈ output ÷ η (rated efficiency assumed); torque = 9549 × output ÷ n.
- Current at part load estimated as load × rated current (an upper bound at light load, where the magnetising current dominates).
- Example: 1475 rpm means about 62.5% load: 6.875 kW and 44.509 N·m, costing 7,554.9 a year.
Limitations
- The slip method is approximate (±20%), and voltage and temperature change the slip.
- Efficiency and power factor fall at light load; measure input power for accuracy.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check whether a motor is overloaded or oversized
- Estimate running cost of a motor
- Find torque at the operating point
Who it is for
- Maintenance engineers
- Energy managers
- Electrical engineers
What you get
- Rated current, input and torque
- Slip and estimated load
- Estimated output, current, torque and energy cost
Interpreting the result
- A motor running under about 40% load wastes energy in poor power factor and efficiency: consider a smaller motor.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2569–2580. Topic reference; no source tables or recommended settings are reproduced.
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Induction Motor Load Assessment (Current, Slip, Load, Energy) calculator
