Page review information

Edition covered

Motor and drive engineering practice — vendor documentation governs application — n/a — practice module

Content status

Reviewed

Coverage

Training module: Motor Efficiency, Power Factor, and Losses — educational treatment; sizing and application decisions follow vendor data and the governing standards.

Last reviewed

August 2026

Primary reference

The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

Training — Motors, Drives, and Motion

Motor Efficiency, Power Factor, and Losses

Intermediate Time: 20 min Type: Concept Focus: Motor / Drive Engineering

After this module: Interpret IE efficiency classes, identify the dominant loss mechanisms, and understand when power factor correction is worth adding.

Prerequisites: Induction Motor Basics

Purpose

This module explains three motor concepts that are often mentioned together but should not be confused:

• efficiency
• power factor
• loss mechanisms

Efficiency

Efficiency measures how much electrical input becomes useful mechanical output.

Relationship:

eta = P_out / P_in

Low efficiency means more input power is being lost as heat or other internal losses.

Power factor

Power factor is the ratio of real power to apparent power.

PF = P / S

In practical terms:

• lower power factor means higher current is needed for the same real power
• higher current increases conductor and system burden

Induction motors commonly operate below unity power factor.

Main motor loss types

Common loss categories:

• stator copper loss
• rotor copper loss
• core loss
• mechanical loss
• stray losses

Stator copper loss

Caused by current flowing in stator windings.

Rotor copper loss

Important in induction motors because rotor current exists due to slip.

Core loss

Magnetic loss in the steel, often associated with hysteresis and eddy-current effects.

Mechanical loss

Includes bearing friction, windage, and other rotational mechanical losses.

Stray losses

Smaller losses not always grouped neatly into the main buckets.

Why engineers should care

These concepts affect:

• heating
• current demand
• enclosure temperature
• operating cost
• motor selection quality

Practical mistakes

Confusing efficiency and power factor

They are not the same:

• efficiency compares output power to input power
• power factor compares real power to apparent power

Ignoring losses when current seems acceptable

A motor can overheat because of cooling or mechanical issues even when measured current is not obviously excessive.

Practical takeaway

If a motor is running hot, do not stop at current alone. Review:

• actual load
• cooling path
• ambient temperature
• friction or bearing problems
• electrical loss and operating condition

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