Brownout (electricity)
Encyclopedia
A brownout is an intentional drop in volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

age in an electrical power supply system used for load reduction in an emergency. The reduction lasts for minutes or hours, as opposed to short-term voltage sag or dip. The term brownout comes from the dimming experienced by lighting when the voltage sags. A voltage reduction may be an effect of disruption of an electrical grid, or may occasionally be imposed in an effort to reduce load and prevent a blackout
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...

.

Effects of a brownout

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

The heat output of any resistance device, such as an electric space heater will vary with the true power consumption, which is proportional to the square of the applied voltage. Therefore, a significant loss of heat output will occur with a relatively small reduction in voltage. Similarly, an incandescent lamp will dim due to the lower heat emission from the filament. Generally speaking, no damage will occur but functionality will be impaired.

Commutated electric motors, such as universal motors, whose mechanical power output also varies with the square of the applied voltage, will run at reduced speed and reduced torque. Depending on the motor design, no harm may occur. However, under load, the motor will draw more current
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

 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 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, attempting to reduce an overload by voltage reduction may not decrease load and can result in damage to customer's equipment.

An unregulated direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 supply will produce a lower output voltage for electronic circuits. The output ripple  voltage will decrease in line with the usually reduced load current. In a CRT television, the reduced output voltage can be seen as the screen image shrinking in size and becoming dim and fuzzy.

A linear direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 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 will appear on the output.

A switching power supply which has a regulated output, will be affected. As the input voltage falls, the current draw will increase to maintain the same output voltage and current, until such a point that the switching power supply malfunctions.

Brownouts can cause unexpected behaviour 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; 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.
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