Enrichment · STEM Explorations · Engineering

Thirty watts in.
Eighteen turn
the wheel.

Type what the meters read at the motor and at the shaft. The calculator compares electrical power in with mechanical power out and shows where the rest goes.

Test my motorWhat you get

The motor efficiency calculator

The worked example is a 12-volt robot drive motor on a test bench. Nothing you type leaves this browser.

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What you get

Enrichment · STEM ExplorationsFree

What you finish with
Input and output power, horsepower, efficiency, the heat loss and the yearly energy and cost of the losses, with an illustrative look at other loads. It prints as a lab record.
Who it is for
Engineering and physics students running a lab, robotics teams choosing motors, and makers sizing a motor for a project.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The STEM Explorations space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Pick the supply type.
  2. Measure volts and amps under load.
  3. Measure torque and speed.
  4. Read the efficiency.
  5. Repeat at other loads.

Test my motor

How efficiency is measured

Power in, power out, and the difference.

The worked example: a robot drive motor on the benchInput30.0 W12 V × 2.5 AOutput18.1 W0.15 N·m at 1,150 rpmEfficiency60.2%output ÷ inputHeat11.9 Wthe differenceThe worked example: a robot drive motor on the benchInput30.0 W12 V × 2.5 AOutput18.1 W0.15 N·m at 1,150 rpmEfficiency60.2%output ÷ inputHeat11.9 Wthe difference
Electrical power in is volts times amps; mechanical power out is torque times angular speed; efficiency is their ratio.

Power in and out

Electrical power is volts times amps for DC. For AC, the power factor accounts for current that flows back and forth without doing work, and a three-phase supply adds a factor of the square root of three. Mechanical power is torque times angular speed: newton-meters times revolutions a minute times 2π ÷ 60 gives watts.

Why load matters

A motor has losses that stay roughly fixed (friction, magnetizing the iron) and losses that grow with the square of the current (heating in the windings). It is usually most efficient somewhere near its rated load and poor at light load. Federal rules set minimum efficiencies for many industrial motors for this reason.

Sources

  1. U.S. Department of Energy. Determining electric motor load and efficiency, Motor Systems tip sheet (energy.gov).
  2. Electronic Code of Federal Regulations. 10 CFR Part 431, Subpart B: energy conservation standards for electric motors (ecfr.gov).
  3. National Institute of Standards and Technology. SI Units: the watt and the newton-meter, Special Publication 811 (nist.gov).
  4. OpenStax. University Physics Volume 1, rotational work and power (openstax.org).

Readings decide the result; meters on a hobby bench have their own error. The load table is a teaching model, not a measurement. Your entries stay in this browser. Support: team@caveatacademic.com.