Wednesday, March 23, 2011

Automatic Voltage Regulator (AVR) is a device that is basically intended to improve the voltage regulation of an electrical system – adjust, control or maintain a constant voltage level through the use of either an electromechanical mechanism or electronic components (active or passive). Automatic Voltage Regulators are important since sensitive loads have stringent voltage limits to operate properly and such loads are generally efficient when supplied with voltage near their rated voltage. Accordingly, AVRs protect electrical and electronic devices from the possible damage due to overvoltage and undervoltage conditions. Downtime and equipment damage caused by poor voltage regulation could in turn result to financial costs that would have been avoided by using AVRs.

Automatic Voltage Regulator
Automatic Voltage Regulator
Automatic Voltage Regulators come in different sizes and designs depending on its application. For example, in electronics the voltage regulators are small (mounted on printed circuit boards) and usually have a DC output. Meanwhile, distribution utilities use large voltage regulators (up to the size of a small house) to maintain a constant AC voltage. This post will discuss more on the basic operation of AC Automatic Voltage Regulators.

Common types of AVRs for steady-state applications are:

Step-Voltage Regulator
Utility Step-Voltage Regulator

Basic AVR Operation Process

An automatic voltage regulator basically functions almost the same regardless of type and size. It takes in a range of voltage levels and automatically outputs a voltage with a much narrower range of voltage levels. A voltage regulator may have a symmetrical input voltage range (e.g. ±10% of nominal) or an asymmetrical input voltage range. The choice of symmetrical versus asymmetrical input voltage range is dictated by purpose and design of the voltage regulator. To illustrate, a typical AVR for power quality application may have an input voltage range of +10% to -25% of the nominal input voltage and convert this to a regulated voltage range of ±3% of the nominal output voltage. The output voltage regulation range is almost universally symmetrical (e.g. ±3% of nominal output voltage).

Moreover, some AVRs may also perform a voltage step up or step down by converting incoming voltage to a new voltage level of output (i.e. a step up from a 120 V input to a 240 V output) and have the input and output voltage ranges applied to the input and output voltages, respectively. For example, an input voltage of 120 V (variation of +10% to -25%) to an output voltage of 240 V (±3% regulation).

(Next posts about AVR will describe each of the common types of voltage regulators including applications and sizing)

For sample DC voltage regulator projects (click here)


Clark, J. (1990). AC Power Conditioners Design and Application
Dugan, R., McGranaghan, M., Santoso, S. and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.). New York: McGraw-Hill.
Utility Systems Technologies, Inc. (2009). AC Automatic Voltage Regulators

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About Me

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.