BIZOMAT 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.
Open the calculatorRuns in your browser. No account needed.
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). |
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: Line voltage
- What it is
- The line-to-line voltage of the three-phase supply to the motor, from the nameplate or a measurement at the terminals.
- Units and format
- Enter a number in V: volts. The form accepts 0.000001 V to 15000 V. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 415 V as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rated current is inversely proportional to it.
- Get it right
- Use line-to-line, not line-to-neutral voltage.
What this means: Rated output
- What it is
- The rated mechanical output at the shaft, from the motor nameplate.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 11 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Estimated output and energy cost scale with it.
- Get it right
- Rated output, not electrical input; convert hp to kW if needed (1 hp = 0.7457 kW).
What this means: Rated power factor
- What it is
- The power factor at rated load, from the motor nameplate or data sheet. It is used for the rated current.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.85 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter a decimal fraction, not a percentage.
What this means: Rated efficiency
- What it is
- The efficiency at rated load, from the motor nameplate or data sheet. The tool applies it at part load too, to estimate input power.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 40% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 91% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher efficiency lowers estimated input power and energy cost.
- Get it right
- Type the percentage itself, for example 91, not 0.91.
What this means: Supply frequency
- What it is
- The frequency of the supply feeding the motor. With the number of poles it sets the synchronous speed.
- Units and format
- Enter a number in Hz: hertz (cycles per second). The form accepts 10 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.
- Get it right
- If a variable speed drive feeds the motor, the slip method here does not apply directly.
What this means: Poles
- What it is
- The number of magnetic poles of the motor winding, which with the supply 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
- Work it out from the rated speed: on 50 Hz a motor near 1450 rpm is 4-pole.
What this means: Rated speed
- What it is
- The full-load speed from the nameplate. Its slip from synchronous speed is taken as the slip at 100 % 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 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 full-load speed, not the synchronous speed.
What this means: Measured speed
- What it is
- The shaft speed measured with the motor running under its normal load, with a tachometer or strobe.
- 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
- Estimated load is proportional to measured slip, so a lower speed means a higher load.
- Get it right
- A few rpm of reading error changes the estimate a lot; measure carefully.
What this means: Running hours per year
- What it is
- The number of hours the motor runs each year, from operating records or the shift pattern.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 8784 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4000 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Annual energy and cost rise in proportion.
What this means: Electricity price
- What it is
- What you pay for one kilowatt-hour of electricity, from your power bill or contract.
- Units and format
- Enter a number in $/kWh: money per kilowatt-hour. The form accepts $0/kWh to $10/kWh. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $0.25/kWh as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Load below which the motor is oversized
- What it is
- The load percentage below which you treat the motor as oversized for its duty, set by your own policy.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
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.
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
Fans, pumps and energy savings
In Variable-Speed Drives by the Numbers
What it covers: How the fan and affinity laws predict the power saved by slowing a fan or pump, why static head cuts the saving, how to measure the before and after, and how to appraise a drive as an investment.
BIZOMAT 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