Types of Losses in Electrical Machines

Definition

Losses in electrical machines are the portions of input electrical or mechanical energy that are not converted into useful output energy and are instead dissipated as heat, sound, friction, magnetic hysteresis, eddy currents, or other unwanted forms of energy.

Main Content

1. Copper Losses

Copper losses are the losses that occur due to the resistance of conducting windings in a machine.
When electric current flows through armature windings, stator windings, rotor windings, or field windings, some energy is lost as heat according to the formula $I^2R$ , where $I$ is current and $R$ is resistance.

Copper losses are also called winding losses or I²R losses. They are present in most electrical machines because every practical conductor has some resistance. In DC machines, copper loss occurs in the armature and field windings. In induction motors, it occurs in both stator and rotor windings. In transformers, copper loss occurs in the primary and secondary windings.

A major feature of copper losses is that they vary with load. As the load increases, current increases, and therefore copper loss increases rapidly. For example, if the load current doubles, copper loss becomes four times because it depends on the square of current. This makes copper loss one of the most important variable losses in electrical machines.

To reduce copper loss, manufacturers use conductors with low resistance such as copper or aluminum, increase conductor cross-sectional area, and ensure proper cooling to prevent excessive temperature rise.

2. Core Losses

Core losses are the losses that occur in the magnetic core of electrical machines due to alternating magnetic flux.
These losses mainly include hysteresis loss and eddy current loss, both of which occur in iron or steel parts of the machine.

Core losses are also called iron losses or magnetic losses. They are common in machines that operate with alternating flux, such as transformers, induction motors, and alternators. Unlike copper losses, core losses depend mainly on supply voltage, frequency, and the nature of the magnetic material rather than directly on load.

Hysteresis Loss

Hysteresis loss occurs because the magnetic material is repeatedly magnetized and demagnetized during each AC cycle.
Energy is used to reverse the magnetic domains in the core, and this energy is dissipated as heat.

The amount of hysteresis loss depends on the type of material, frequency, and maximum flux density. Soft magnetic materials such as silicon steel are used in machine cores to reduce this loss because they have narrow hysteresis loops.

Eddy Current Loss

Eddy current loss is caused by circulating currents induced in the metallic core by changing magnetic flux.
These currents flow in closed loops within the core and produce heating due to resistance.

Eddy currents can become large if the core is solid. To reduce this loss, the core is built using thin laminated sheets insulated from each other. Lamination increases resistance to eddy current flow and greatly reduces the loss.

Core losses are nearly constant at rated voltage and frequency, which is why they are often called constant losses. They are very important in transformer efficiency and in machines running continuously for long durations.

3. Mechanical Losses

Mechanical losses are the losses associated with the moving parts of a machine.
They mainly include friction loss and windage loss, which occur due to the motion of rotating parts against air and mechanical supports.

Mechanical losses are significant in rotating electrical machines such as motors and generators. Since these machines have shafts, bearings, fans, and rotating rotors, some input energy is always consumed in overcoming mechanical resistance.

Friction Loss

Friction loss occurs in bearings, brushes, commutators, and other contact surfaces.
It is caused by rubbing between moving parts and produces heat.

In DC machines, brush and commutator friction can be considerable. In all machines, bearing friction contributes to the mechanical loss.

Windage Loss

Windage loss occurs due to air resistance against the rotating parts of the machine.
It increases as rotor speed increases and is more noticeable in high-speed machines.

Mechanical losses generally depend on speed rather than load, which is why they are often treated as constant losses for a given operating condition. Proper lubrication, smooth bearing design, balanced rotors, and efficient cooling fans help reduce these losses.

4. Stray Load Losses

Stray load losses are the additional losses that occur due to leakage flux, harmonic effects, and non-ideal operating conditions.
They are not easily classified into copper, core, or mechanical losses and usually appear under load.

These losses arise from several small effects such as eddy currents induced in machine parts by leakage flux, current distribution distortion in conductors, and harmonic flux in the air gap. Because they are caused by complex phenomena, stray load losses are difficult to measure directly and are often estimated as a percentage of output or total losses.

Stray load losses are important in large machines and under heavy load conditions. Although individually small, they can significantly affect overall efficiency. Engineers minimize them by improving magnetic design, using better winding arrangements, reducing leakage flux, and maintaining proper air-gap construction.

Working / Process

Electrical power is supplied to the machine, and current flows through the windings, causing copper losses due to resistance.
Magnetic flux is established in the iron core, producing core losses through hysteresis and eddy currents.
In rotating machines, mechanical motion produces friction and windage losses, while additional stray load losses appear under practical load conditions, reducing the useful output power.

Advantages / Applications

Helps in calculating machine efficiency accurately and predicting performance under different load conditions.
Assists engineers in selecting proper materials, cooling methods, and machine designs to reduce energy waste.
Useful in maintenance and fault diagnosis because abnormal increase in losses can indicate wear, insulation failure, overload, or core problems.

Summary

Electrical machines suffer from copper losses, core losses, mechanical losses, and stray load losses during operation.
Copper losses depend on current, core losses depend on magnetic behavior, and mechanical losses depend on rotation.
Reducing these losses improves efficiency, reliability, and service life of electrical machines.