A brownout is a drop in the magnitude of

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

in an electrical

power system An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The e ...

. Unintentional brownouts can be caused by excessive electricity demand, severe weather events, or a malfunction or error affecting electrical grid control or monitoring systems. Intentional brownouts are used for load reduction in an emergency, or to prevent a total grid

power outage A power outage, also called a blackout, a power failure, a power blackout, a power loss, a power cut, or a power out is the complete loss of the electrical power network supply to an end user. There are many causes of power failures in an el ...

due to high demand. The term ''brownout'' comes from the dimming of incandescent lighting when voltage reduces. In some countries, the term ''brownout'' refers not to a drop in voltage but to an intentional or unintentional

(or blackout).

Effects

Different types of electrical apparatus will react in different ways to a voltage reduction. Some devices will be severely affected, while others may not be affected at all.

Resistive loads

The heat output of any resistive device, such as an electric space heater,

toaster A toaster is a small electric appliance that uses radiant heat to brown sliced bread into toast, the color caused by the Maillard reaction. It typically consists of one or more slots into which bread is inserted, and heating elements, o ...

oven upA double oven A ceramic oven An oven is a tool that is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been use ...

, and incandescent bulbs is equal to the power consumption, which is directly proportional to the square of the applied voltage if the resistance stays constant. Therefore, a significant reduction of heat output will occur with a relatively small reduction in voltage. An

incandescent lamp An incandescent light bulb, also known as an incandescent lamp or incandescent light globe, is an electric light that produces illumination by Joule heating a filament until it glows. The filament is enclosed in a glass bulb that is eith ...

will dim due to lower heat creation in the filament, as well as lower conversion of heat to light. Generally speaking, no damage will occur but functionality will be impaired.

Motors

Commutated electric motors, such as

universal motor The universal motor is a type of electric motor that can operate on either AC or DC power and uses an electromagnet as its stator to create its magnetic field. It is a commutated series-wound motor where the stator's field coils are connected ...

s, will run at reduced speed or reduced torque. Depending on the motor design, no harm may occur. However, under load, the motor may draw more current due to the reduced back-EMF developed at the lower armature speed. Unless the motor has ample cooling capacity, it may eventually overheat and burn out. An

induction motor An induction motor or asynchronous motor is an AC motor, AC electric motor in which the electric current in the rotor (electric), rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding ...

will draw more current to compensate for the decreased voltage, which may lead to overheating and burnout. If a substantial part of a grid's load is electric motors, reducing voltage may not actually reduce load and can result in damage to customers' equipment.

Power supplies

An unregulated DC supply will produce a lower output voltage. The output voltage ripple will decrease in line with the usually reduced load current. In a

cathode-ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms on an oscilloscope, a ...

television, the reduced output voltage will make the screen image smaller, dimmer and fuzzier. A linear DC regulated supply will maintain the output voltage unless the brownout is severe and the input voltage drops below the drop out voltage for the regulator, at which point the output voltage will fall and high levels of ripple from the rectifier/

reservoir capacitor A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as ''rectification'', since it "straightens" t ...

will appear on the output. A

switched-mode power supply A switched-mode power supply (SMPS), also called switching-mode power supply, switch-mode power supply, switched power supply, or simply switcher, is an electronic power supply that incorporates a switching regulator to electric power conversio ...

will be affected if the brownout voltage is lower than the minimum input voltage of the power supply. As the input voltage falls, the current draw will increase to maintain the same output voltage and current, until such a point that the power supply malfunctions or its under-voltage protection kicks in and disables the output.

Digital systems

Brownouts can cause unexpected behavior in systems with digital control circuits. Reduced voltages can bring control signals below the threshold at which logic circuits can reliably detect which state is being represented. As the voltage returns to normal levels the logic can latch at an incorrect state; to the extent that even " can't happen" states become possible. The seriousness of this effect and whether steps need to be taken by the designer to prevent it depends on the nature of the equipment being controlled; for instance, a brownout may cause a motor to begin running backwards.

References