BIZOMAT 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). |
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: Brake arm length
- What it is
- The distance from the shaft centre to the point where the brake arm presses on the scale: the lever arm for the torque.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 10 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque and output rise in direct proportion.
What this means: Scale reading under load
- What it is
- The force shown on the scale under the brake arm while the brake is loading the motor at steady speed.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 1000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 190 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Scale reading with the brake slack (tare)
- What it is
- The scale reading with the brake slack, mainly the weight of the arm; it is subtracted from the loaded reading.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 1000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Measured speed
- What it is
- The shaft speed measured with a tachometer during the loaded reading.
- 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
- Shaft output rises in direct proportion.
What this means: Line voltage
- What it is
- The line-to-line voltage of the three-phase supply, measured during the test.
- 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.
- Get it right
- Line-to-line, not line-to-neutral; the tool uses √3 × V × I × pf.
What this means: Line current
- What it is
- The average of the three line currents measured during the test.
- Units and format
- Enter a number in A: amperes. The form accepts 0.000001 A to 100000 A. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 19.5 A as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Power factor
- What it is
- The power factor measured during the test with a power analyser.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 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
- The nameplate power factor applies only at rated load, so it gives just an estimate.
What this means: Rated output
- What it is
- The rated shaft output on the motor nameplate, used to show the load at which the test ran.
- 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.
What this means: Nameplate efficiency
- What it is
- The efficiency the nameplate or data sheet gives at the load tested.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 30% 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.
What this means: Allowed shortfall from nameplate
- What it is
- How many percentage points the measured efficiency may fall below the nameplate value before the motor is flagged.
- Units and format
- Enter a number in points: points. The form accepts 0 points to 30 points. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 points as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
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.
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Open the Motor Efficiency Test with a Prony Brake (Torque, Output, Efficiency) calculator