Mechanical Voltage Regulator Advantages & Disadvantages


The advantages of mechanical AVRs, either tap changing or magnetic induction type, vary by design but generally include:

  • Very accurate output voltage regulation
  • Wide choice of kVA rating, voltage and configuration
  • Easily applicable to outdoor application
  • High tolerance to system faults and overload
  • High tolerance to power factor and frequency deviations
  • Good line isolation
  • Relatively inexpensive

In addition to these advantages, the servo induction voltage regulator also provides a very smooth output voltage compared to the discrete switching that occurs in tap changers.


  • Mechanical drive components, brushes and contractors require regular maintenance and/or replacement
  • Frequent overloads can damage brushes
  • Speed of voltage correction correct may not be fast enough for electronic loads

Because mechanical voltage regulators must physically move components, their speed in correcting output voltage fluctuations is very slow. Mechanical AVRs measure correction time in volts per second so the larger the voltage correction required, the longer the correction will take. Large voltage corrections can take one-half second to tens of seconds – much longer than many modern electronic loads can tolerate.

Tap switching voltage regulators tend to have higher maintenance requirements than magnetic induction units since the magnetic induction voltage regulator has few or no brushes or contactors to wear out. With either type, the maintenance requirements are directly related to frequency of voltage correction – the more it moves the more maintenance will be required.

Because maintenance on utility, oil-filled on-load tap changers can be quite expensive, they often have time delays and other capability built into their controls to minimize the frequency of tap changing. These units typically have recommended overhaul intervals ranging from 500,000 to 1,000,000 operations.

In power quality applications, it would be counterproductive to have delays or other limitations on the operation of tap switching or servo induction voltage regulators. Fortunately, these units are usually air-cooled, smaller in size and relatively accessible compared to the utility OLTC.