On August 25, 2011 at 18:48, KRAZYK said...
Power=Current*Voltage
During a low voltage condition, current increases in order to maintain the correct power output. An increase in current results in a increase in heat to components.
The above comments are true... If the load happens to consume a constant power. The issue, is that a great many loads do not act like a constant power load.
AC motors vary greatly. The ones most common in homes fall into two categories.
A "squirrel cage" motor will consume more current when the load on the motor is high. These are common for AC compressors, and blower fans on HVAC equipment. If these motors are run at low voltages, high currents can result, and damage the motor, or trip breakers. This follows the "constant power load", comments made above.
In some modern appliances, AC motors that implement special motor control circuitry are present. I have seen these most on home washing machines. The circuit works to change the power available at various speeds. In a washing machine this can result in handling a larger load, with a smaller motor. The circuitry in most, is smart enough that with an undervoltage condition, the motor will simply not attempt to run. So such motors have no problem or damage from undervoltage events.
A light bulb is more of a constant resistance load. As a constant resistance load, as the voltage drops, the current will also drop. This is apparent when you look at ohms law: V = I * R. If the resistance stays constant, as voltage drops, so must current. An oven or the heating element in an electric dryer looks like a constant resistance as well.
Very few power supplies will look like a constant current load. So few, that I can't think of an example, other than small areas in circuits specifically designed to produce or consume constant currents.
Most power supplies will fall into one of two types, linear or switching.
In a linear supply, a transformer is used to couple energy from the primary to the secondary. A lower voltage on a linear supply will result in the primary attempting to draw a greater current to allow for the same power output on the secondary side. Most linear supplies typically have protection on either the primary or secondary side of the transformer, and when the current gets above a level that would cause damage to the components, will typically interrupt the flow of current. One way this is done, is with a fuse. Obviously, replacing fuses in lots of things every time a brown out occurs would get tiring. It typically is in fairly inexpensive items where this is done, where the current consumption is typically small, and the fuses are there to provide safety during extended brown out conditions (brown outs that last in the multiple seconds.) Typically such devices can handle such long brown out events, because the output powers are small enough, that the electric company powers the circuit down, long before the brown out event could damage the device. Electronic circuits which notice this condition, and use an electronic switch are quite common. Linear supplies have become fairly uncommon today, as they tend to be large and heavy for a given power output. They were commonly used in the past as they are simple to design and build. The vast majority of linear supplies used in consumer goods will not be damaged by brown outs.
Switching power supplies use a control circuit that carefully controls the current and voltage. Every switching supply I've ever seen has circuitry to recognize an undervoltage condition (which could occur during a brown out), and shut down the supply during an event. Without such a circuit the possibity would exist to cause damage to the supply. I have not seen a switching supply that would be damaged by brown outs.
So, the only items I would worry about during a brown out, are AC motors for appliances (refrigerators, AC, blower motors, etc.), and the cooling for a bulb in a projector.
