How is measuring the current a motor draws used when selecting a new motor?

The measured current is the most practical figure for showing how heavily the existing motor is really loaded. If the current under load is close to the rated current, the power was chosen correctly; if it is clearly below, the motor may be oversized; if it is above, the motor is overloaded. Whether the new motor should be the same, smaller or larger is decided by this comparison.

Conditions for a correct measurement

1. Take the measurement when the machine is working at its heaviest and during normal production; the current of a machine running idle is misleading.
2. Measure the three phases separately; a clear difference between phases points to supply imbalance or a winding problem.
3. Note the supply voltage at the same time; at low voltage the current may appear high.
4. If the load fluctuates, record several different moments or use a logging instrument.

Since the measurement is taken in a live panel or terminal box, it must be carried out by qualified electrical personnel.

Care when interpreting the result

The relationship between current and load is not linear. Even at no load, the motor draws a significant current for magnetisation; so a motor showing half its rated current may be running not at half load but at a lower one. For an exact load figure, measuring active power with a power analyser is more reliable. If the old motor has been rewound, the current on its nameplate may not reflect reality.

Turning it into a selection

• If the current is close to the rated value and the motor does not overheat, continuing with the same power makes sense.
• If the current constantly exceeds the rated value, the next size up is considered.
• If the current is very low, downsizing may be considered; however, starting torque and sudden loads must be checked separately.

Compare the expected currents by power and pole number in the motor current table, then review suitable models in the three-phase motor shop.