Understanding and Minimising Power Loss in Electric Devices and Power Systems

September 16, 2019 by Sam Holland
Share

Many types of power losses can occur in electric circuits for a multitude of reasons: troubling electrical engineers (EEs), they cause a host of problems that affect the efficiency, reliability, and safety of the systems that EEs design and build. Understanding how and where these losses occur—including heating, eddy current, and switching losses—can help engineers to keep them at a bare minimum.

Power losses can occur in several forms for a multitude of reasons. Some important losses are heating (ohmic) losses, eddy current losses, switching losses, conduction losses, hysteresis losses, and corona discharge. Understanding how and where these losses occur can help engineers to keep them at a bare minimum.

Heating Power Loss

Heating losses, (aka ohmic losses), result from the heating effects of resistive elements in DC and AC circuits. Circuit elements, such as cables, capacitors, and coils offer varying degrees of resistivity or impedance when electric current flows through them, and some of the energy is dissipated as heat.

This phenomenon is justified by Joule-Lenz’s law, which states that the heating power in a conductor is directly proportional to the product of its resistance and the square of its voltage. Here’s the law expressed in an equation:

P ∝  I2 x R

(‘P’ represents ‘power’, ‘I’ is ‘current’, and ‘R’means ‘resistance’.

Going by this formula, the higher the resistance, the greater the heating effect—and therefore, the power loss.

Ohmic heating causes copper losses in the cores of electric transformers. It is also the main reason that power is transmitted at relatively low currents and high voltages in overhead lines. Engineers working at power system installations utilise thicker cables with lower resistances to minimise the heating effects in the conductors. Bear in mind that the resistance of a conductor is directly proportional to its cross-sectional area; therefore, the thinner it is, the greater the temperature rise.

An electric transformer. Image courtesy of Pixabay.

Eddy Current Loss

Eddy currents are swirling loops of electric current induced in conductors by varying magnetic fields. They are so called because they mimic the ‘eddy waves’ of a stream. In keeping with Joule-Lenz’s law, these induced currents must flow in a specific direction, namely one that opposes the magnetic field that produces such a flow. Eddy currents cause both the power loss and heating of conductors, such as soft iron cores in transformers and rotor windings of electric motors. Engineers can minimise eddy currents in transformers by increasing the resistance of the laminated cores.

Switching Loss

Switching losses are commonly observed in power semiconductors, such as diodes and transistors. They occur when the component is transitioning from its blocking (OFF) state to its conducting (ON) state, and vice versa. Switching losses are dependent on the switching frequency, which is the rate at which the component is turned on or off. (For more information on the effects of switching losses on diodes, power bipolar junction transistors (BJTs), and insulated-gate bipolar transistors (IGBTs), visit this article from Electronics Point’s sister site, All About Circuits.)

Conduction Loss

Conduction loss refers to electrical energy that is dissipated in a power semiconductor when in its conducting state. Conduction losses can be observed in BJTs, IGBTs, and MOSFETs (metal-oxide-semiconductor field-effect transistors). where they can impact

Engineers can minimise conduction losses by selecting semiconductors with the lowest drain-source on-resistance (RDS_ON) and a suitable gate driver.

A close-up shot of a transister. Image courtesy of Pixabay.

Hysteresis Loss

Hysteresis loss occurs due to rapid magnetisation and demagnetisation in an iron core (such as the primary winding of a transformer) when a flux-carrying AC is applied to it. It is also observed in the coils of AC motors. The magnetisation and demagnetisation of the core occurs due to the constantly-reversing direction of the current which produces some degree of heat, causing power loss.

Hysteresis loss is dependent on the nature of the iron core. Most transformer cores are made of ferromagnetic metals, which exhibit the tendency when a varying AC is applied. To minimise hysteresis, engineers can utilise cores made of metals with very low loss coefficients. A good example is cold rolled grain oriented steel—a material that has its electrical and magnetic properties significantly enhanced.

Corona Discharge

The corona effect is a fairly common problem that mostly affects power transmission lines. At high voltages (33kV and greater), the air (dielectric medium) around the conductors could become ionised, producing an audible noise and a blue/purple glow.

In severe cases, coronas can develop into a complete dielectric breakdown, wherein the surrounding air loses its insulative properties and becomes a conductor. When this happens, electric arcs form between the conductors. Corona discharge causes power losses in transmission, reduced efficiency of the overall power system, and interference in nearby data transmission circuits.

Engineers can prevent corona discharge by minimising the strength of the magnetic field set up between conductors at higher voltages. One way to achieve this is by using larger conductors. Increasing the radius of the conductors lowers the corona discharge tendency.

Other techniques include increasing the spacing between conductors, which increases the voltage needed to form a corona; and using conductors with smooth surfaces (consider that conductors having sharp edges or defects in the material exhibits greater corona discharge).

A part of a high-voltage electrical transformer. Image courtesy of Pixabay.

In view of all the points raised, there are, in all kinds of electric circuits, varying amounts of power losses are inevitable. But at least, by understanding how and where these losses occur, engineers will always be able to minimise them—altogether ensuring efficient and reliable devices and systems.