BIZOMAT MAKEOS · Power Transmission
Three-Phase Motor Power, Current and Torque
What current does this three-phase motor draw, or what power does a measured current mean?
Input power, output power, line current and shaft torque of a three-phase induction motor from voltage, power factor and efficiency.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Calculate | choice | Current from power, or power from a measured current. |
| Line voltage | V | Line-to-line supply voltage (e.g. 400 or 415 V). |
| Shaft output power | kW | Rated or required shaft power (current mode). |
| Measured line current | A | Measured line current (power mode). |
| Power factor | Power factor at this load from the motor data sheet (falls at part load). | |
| Efficiency | % | Efficiency at this load from the data sheet. |
| Shaft speed | rpm | Shaft speed (rated speed for full load). |
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: Calculate
- What it is
- Which way the relation is solved: line current from a required shaft power, or shaft power from a measured line current.
- Units and format
- Choose one of 2 options from the list: Current from output power; Output power from measured current.
- Example
- The form opens with “Current from output power” selected as a worked example. Choose the option that matches your case.
- Get it right
- In current mode the measured current is ignored; in power mode the shaft power input is ignored.
What this means: Line voltage
- What it is
- The voltage between any two of the three supply lines (line-to-line) at the motor terminals.
- Units and format
- Enter a number in V: volts. The form accepts 0.000001 V to 100000 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
- At a given power, current falls in proportion as voltage rises.
- Get it right
- Use the line-to-line voltage, not the line-to-neutral (phase) voltage.
What this means: Shaft output power
- What it is
- The mechanical power delivered at the motor shaft, rated or required. Used only in current mode.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 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
- Line current rises in proportion.
- Get it right
- This is output power; the tool divides by efficiency to find input power.
What this means: Measured line current
- What it is
- The current measured in one supply line with the motor running at the load of interest. Used only in power mode.
- Units and format
- Enter a number in A: amperes. The form accepts 0 A to 1000000 A. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 A as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Input and output power rise in proportion.
- Get it right
- Measure with a clamp meter on one line conductor at a steady load.
What this means: Power factor
- What it is
- The power factor of the motor at this load: the ratio of real power to apparent power, from the motor data sheet.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 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.
- Effect on the result
- A lower power factor raises current for the same power.
- Get it right
- It falls at part load; use the value for the actual load, not the full-load nameplate value.
What this means: Efficiency
- What it is
- The motor efficiency at this load: shaft output power as a share of electrical input power, from the motor data sheet.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% 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
- Lower efficiency raises current for the same shaft power.
- Get it right
- Type the percentage itself, not a fraction.
What this means: Shaft speed
- What it is
- The speed of the motor shaft, used only to convert output power into shaft torque.
- 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.
- Effect on the result
- At constant power, torque falls in proportion as speed rises.
- Get it right
- Use the loaded (rated) speed, not the synchronous speed.
Method and formulas
- Input power P_in = √3 V I pf.
- Output power = P_in × efficiency; current I = P_out ÷ (√3 V pf η).
- Torque = 9549 × P_out (kW) ÷ n.
- Example: 11 kW at 415 V, pf 0.85 and 91% efficiency draws about 19.784 A.
Limitations
- Balanced three-phase supply; power factor and efficiency change with load.
- Not a substitute for the motor nameplate current or electrical code sizing rules.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate full-load current for cable and protection sizing
- Convert a measured current into load
- Find shaft torque from rated power and speed
Who it is for
- Electrical and maintenance engineers
- Design engineers
What you get
- Line current
- Electrical input power
- Shaft output power
- Shaft torque
Interpreting the result
- Size cables and protection from the nameplate current and your wiring rules (e.g. AS/NZS 3000 and 3008).
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
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In Distribution Transformers by the Numbers
What it covers: How a transformer's windings carry current, where its load losses come from, what its impedance means, how far its voltage falls under load, and how to add a new load correctly in kVA, not kW.
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In Variable-Speed Drives by the Numbers
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