Automatic Voltage Regulators (AVR): How They Work, Types and How to Size One

In large parts of Africa, South Asia, Southeast Asia and the Middle East, grid supply voltage is not the stable 220–240V shown on the nameplate of imported appliances. Brownouts (sustained under-voltage) and surges are common, and they silently shorten the life of compressors, motors, LED drivers and electronics. An automatic voltage regulator (AVR) is the front-line protection device that corrects these fluctuations before they reach the load.

How an AVR works

An AVR continuously samples the incoming AC voltage and compares it to a set point (typically 220V or 230V). When the measured voltage deviates from the set point by more than the dead band (usually ±5–10%), the AVR corrects the output by switching taps on an autotransformer (relay type) or by adjusting a servo-driven variable transformer (servo type). The correction happens fast enough that connected loads see a nearly stable output voltage, even as the supply fluctuates over a wide range.

Relay-type vs servo-type: which is right for your load?

Feature	Relay-type AVR	Servo-type stabilizer
Regulation method	Stepped (relay switching)	Continuous (servo motor)
Correction speed	Fast (20–40 ms)	Slow (1–2 sec)
Output accuracy	±5–8%	±1–2%
Maintenance	None (solid state)	Motor brushes, every 2–3 years
Best for	Home appliances, office, retail	CNC machines, medical, precision labs
Price point	Lower	Higher

AVR vs voltage protector vs UPS: where each belongs

These three devices are often confused in procurement discussions:

• AVR / voltage stabilizer: regulates sustained voltage fluctuations continuously. No battery. Supply must be present.
• Voltage protector (over/under relay): monitors voltage and disconnects the load when limits are exceeded; reconnects when supply recovers. Does not regulate — it cuts and restores. No battery. Faster acting than an AVR on dangerous overvoltage events.
• UPS (uninterruptible power supply): provides battery backup during power cuts in addition to voltage regulation. Much more expensive. Battery adds weight, cost and replacement cycles.

For a market with unstable voltage but no extended power cuts, an AVR is sufficient and far more cost-effective than a UPS. If power cuts are also a problem, size a UPS to include an AVR function.

Sizing an AVR: the VA calculation

1. Identify the total rated wattage of all loads to be protected.
2. For motor loads (air conditioners, refrigerators, pumps): multiply the motor wattage by 3 to account for starting current inrush.
3. Sum all adjusted loads and multiply by 1.25 safety factor.
4. Round up to the next standard VA size.

Our TM and AVR relay series are available from 500VA to 30kVA in single-phase wall-mount and floor-stand configurations, and from 9kVA to 90kVA in three-phase floor configurations. The front-panel triple LED display shows input voltage, output voltage and output current simultaneously.

FAQ

What is an automatic voltage regulator (AVR)?

A device that monitors AC supply voltage and automatically corrects it to a stable output — protecting connected equipment from brownouts, surges and slow voltage drift.

What is the difference between an AVR and a voltage stabilizer?

In most markets the terms are interchangeable — both regulate AC output voltage automatically. Relay-type AVRs use switched taps; servo stabilizers use a continuously variable motor-driven autotransformer.

How do I size an AVR for an air conditioner?

Multiply the AC nameplate kVA by 3 (for starting current), then by 1.25 (safety factor). Round up to the next standard size. A 1.5 kVA AC needs at least a 5 kVA AVR.

What input voltage range do AVRs work with?

The TM series accepts 80–250V AC input, suitable for grids with severe brownouts. Check the nameplate input range against the worst-case voltage at your site.