KEVOS® MAKEOS · Power Transmission
Motor Efficiency Test with a Prony Brake (Torque, Output, Efficiency)
From a brake load test, what torque and output does this motor give, what efficiency does that mean against the measured electrical input, and does it match the nameplate?
Load test of a three-phase motor with a Prony brake or arm-and-scale dynamometer: net scale force, torque and shaft output at the measured speed (as the Prony brake tool), electrical input from line voltage, current and power factor (as the motor power tool), efficiency, load as a share of rated output, and the efficiency check against the nameplate within your tolerance.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Brake arm length | m | Distance from the shaft centre to the scale contact on the brake arm. |
| Scale reading under load | N | Scale reading with the brake loading the motor. |
| Scale reading with the brake slack (tare) | N | Scale reading with the brake slack (arm weight). |
| Measured speed | rpm | Shaft speed during the reading. |
| Line voltage | V | Line-to-line voltage during the test. |
| Line current | A | Average line current during the test. |
| Power factor | Power factor during the test (from a power analyser). | |
| Rated output | kW | Rated output from the nameplate. |
| Nameplate efficiency | % | Efficiency on the nameplate at this load. |
| Allowed shortfall from nameplate | points | How many percentage points below the nameplate you accept (test accuracy plus standard tolerance). |
Method and formulas
- Net force W = reading − zero; torque T = W L; output P = 2π n T ÷ 60 (as the Prony brake tool).
- Electrical input = √3 V I pf (as the motor power tool).
- Efficiency = output ÷ input; load = output ÷ rated output.
- Example: 70 N·m at 1460 rpm is 10.702 kW out for 11.914 kW in, 89.829% efficient at 97.294% load.
Limitations
- Power factor must be measured; nameplate power factor gives only an estimate of input.
- Brake friction heating changes the load during a reading: take readings quickly at steady speed.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a rebuilt motor’s efficiency
- Verify a motor’s output under load
- Teach or demonstrate motor testing
Who it is for
- Electrical engineers
- Maintenance engineers
- Test technicians
What you get
- Torque and shaft output
- Electrical input and efficiency
- Load share and nameplate check
Interpreting the result
- Efficiency well below the nameplate after a rewind usually points to core damage or wrong winding data.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2998–2999, 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 Motor Efficiency Test with a Prony Brake (Torque, Output, Efficiency) calculator
