What does an oversized motor running at low load cost the business?

The loss does not appear in a single item but in several places at once: the purchase price is needlessly high, the motor runs with a poorer power factor at low load, and the starting current and the related cable and panel sizes grow. When the load falls well below the rated power, the share of the motor's own losses in the total also rises.

Power factor and reactive energy

To build its magnetic field, an induction motor draws a magnetising current that does not depend on the load. As the load decreases, the proportion of this current in the total current grows and cos φ falls. The result is an increase in the plant's reactive load and more work for the power factor correction system. If correction is insufficient, a reactive energy charge may appear on the bill.

Items that grow in the installation

• The starting current of a larger motor can cause a deeper voltage dip on the supply.
• Because cable cross-section, switchgear and protective devices are selected from the nameplate value, the panel cost increases.
• The starting arrangement may have to be selected in a larger size.

Possible effects on the mechanical side

A larger motor is heavier and longer; it may not fit the space in the machine and may require a change of base. Because it produces a higher torque at start, couplings, belts and gears may be stressed more harshly.

How much margin is reasonable

On machines with a fluctuating load or a heavy start, leaving a reasonable margin is correct. The problem arises when a separate margin is added at every stage to an estimate that is not firm. Measuring the current of the existing motor under load is the most reliable way to show the real need; you can follow the relationship between current and power in the motor current table.

If the measured load allows you to step down one power rating, compare the intermediate ratings among three-phase induction motors and check whether the frame and shaft dimensions suit the machine.