- 1). Find the nameplate rating of the current and voltage. The nameplate rating is the maximum rating applied to the motor, and is usually given by the manufacturer. As an example, assume the nameplate ratings are 15A and 10V for current and voltage, respectively.
- 2). Calculate the root mean square (RMS) current and voltage for the three phases. Do this by squaring all three individual values (i.e., multiplying the number by itself), taking the average of the squared values, and finally taking the square root of this number.
For example, assume the currents for the three phases to be 7A, 8A and 11A. Take the average of the squared values by adding them together (49 + 64 + 121) and dividing by 3, which equals 78. Take the square root of that value, which gives 8.8A. Assume the calculated value for the RMS voltage of 6.2V. - 3). Divide the calculated RMS current by the nameplate rated current. Using the values given above, this would give 8.8A divided by 15A, which equals 0.59. Do the same for the RMS voltage and nameplate rated voltage. Using the values above, this gives 6.2V divided by 10V, which equals 0.62.
- 4). Multiply these two numbers together, then multiply by 100. This will give you the output power as a percentage of the rated power (i.e., the motor load). In this example, 0.59 multiplied by 0.62, then multiplied by 100, equals 36.5. Therefore, the motor has a load of 36.5 percent.
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