WARNING: Don’t Buy A Static Voltage Regulator Before Reading This

1 Apr 2022

Est Reading time: 16 minutes


Static Voltage Regulators may seem affordable, but can they deliver the results you need? This blog exposes 10 industry secrets manufacturers hide from you.


Fully electronic, fast response, with little maintenance required and no moving parts. The Static Automatic Voltage Regulator (Static AVR) is considerably cheaper than other types of Automatic Voltage Regulators, plus it also gets the job done. On paper, this option seems to check all the boxes… 


But have you considered if “Fast, Cheap and Good” typically come as a whole package? Let’s find out…

Scouring through the internet, you often notice a lack of explanation or clarification on the working principles in many manufacturers’ catalogs.


Instead, you would see an Automatic Voltage Regulator being described using attention-grabbing words like “Static Type, No Maintenance, Instant Voltage Regulation”.


Throw in a considerably cheaper price than those of its counterparts, and Static Voltage Regulators appear like the best option.


But we’re here to tell you that for the price you are paying, they may not be the product you’re expecting.

Here’s why…

Behind the Science

Before we dive deeper, here’s a brief explanation of how a Static Automatic Voltage Regulator (Static AVR) works.

The name ‘Static Electronic Voltage Regulator’ or ‘Electronic Tap Switching Voltage Regulator’ is a general name that refers to products used in voltage regulation in the mains supply—where voltage fluctuates and requires correction.


There are 2 main types of Static AVRs: 

  1. Full power semiconductor (FPS) 
  2. Series Transformer (ST) 


Static AVRs get the term ‘Static’ from the fact that they do not have any moving parts like switching relays and contactors, and instead use thyristors or Silicon Controlled Rectifiers (SCRs).

Click to enlarge.

Most Static AVRs are offered in 3 or 6 tap configurations.


They mainly consist of the following 7 components:


  1. Sensor Control Board/PCB
  2. Compensating Transformer
  3. Bypass Contactor
  4. SCRs or Thyristors
  5. Semiconductor Fuses
  6. SCR Firing Control Board
  7. Microprocessor
  1. Sensor PCB: It detects any change in output voltage. Through the use of SCRs, it corrects any change in output voltage by selecting the right tap on the autotransformer, which is connected to the Compensating Transformer.
  2. Compensating Transformer: It helps correct the incoming voltage in order for the output voltage to be maintained constant.
  3. Bypass Contactor: This allows for the SCRs to be safely switched OFF to allow for the tapping on the autotransformer to be changed. This is done by energising or switching on the bypass contact, which cuts off supply to the thyristor circuit in case high current causes burn damage to the autotransformer windings. This also allows one to change the tap safely on the autotransformer. Only after the tap has been changed and the correct one is selected will the Bypass Contactor switch back to its normal OFF position.
  4. Thyristors: Also known as Silicon Controlled Rectifiers (SCRs), they have a high speed and act as a switching device and control electric power and current by acting as a switch. They are connected directly to the autotransformer windings at selected tapping points and are calculated according to the output accuracy designed by the manufacturer.
  5. Semiconductor fuses: Being connected in series, the semiconductors are no ordinary fuses. They are fast-acting and are designed to open the circuit immediately upon detecting high current flow, to protect the SCRs and autotransformer windings from shorting.
  6. SCR Firing Control Board: They help switch the SCRs on or off and manage the firing and triggering of the SCRs, by ensuring it is done at the exact position of the zero-crossing
  7. Microprocessor: In order to ensure firing or triggering the SCR precisely at zero voltage crossing, manufacturers use microprocessors to decide which of them to switch ON or OFF.

Did You know?


The spaces and number of SCRs between each of the windings differ based on the accuracy. The higher the accuracy, the further apart the SCRs will be connected on the windings and the more SCRs would be required.

10 Frightening Flaws in a Static AVR

Now that we understand the operations and science behind a Static AVR, here are some potential flaws to be aware of.



If you want to switch the SCRs in your system, brace yourself for what could be a looong, complicated process.


For starters, to maintain a constant output voltage: 

The SCRs can only be switched ‘ON or OFF’ one tap at a time at the zero-crossing, until it reaches the required voltage tap. You cannot simply jump tap and must follow this sequence religiously.

Not doing so could damage the SCRs and short the transformer windings. 


Additionally, it is also not easy to turn OFF these SCRs, except with the help of a microprocessor. Because the mechanical components of an old Static AVR design have been replaced with electronic power devices, this increases the points of failure and means that if the microprocessor fails, the whole regulator would stop functioning.


As for the changing of the SCRs, its tappings can also only be done one tap at a time, not just by jumping from one tap to a few taps more. Failing to do so could cause the output voltage to suddenly increase in voltage on the output side, which could negatively affect the output loads connected to this regulator. 


The switching procedure between the SCR and bypass contactor will continue after the tap had changed and the correct one is selected.

For Example:


If the regulator has to be switched up to 4 tappings, the switching back and forth will happen until it reaches the 4th tap, which could take up a lot of time depending on the number of SCRs your system has.



If you want to switch the SCRs in your system, brace yourself for what could be a looong, complicated process.


For starters, to maintain a constant output voltage: 

The SCRs can only be switched ‘ON or OFF’ one tap at a time at the zero-crossing, until it reaches the required voltage tap. You cannot simply jump tap and must follow this sequence religiously.

The design of the Static AVR puts several of its components at a huge risk of damage. This increases the vulnerability of several individual parts of the Static AVR and even the entire system to suffer from costly breakdowns.  

Let’s explore why…


The 2 main areas at highest risk are: 

  1. The SCRs
  2. The Mains Supply line.


The SCRs are affected primarily because the design of a Static AVR means that if the supply mains continue to fluctuate, the switching of SCRs will continue. This could potentially impact other SCRs. Additionally, they can also be accidentally switched ON or OFF by outside electrical noise caused by the supply line connected to other loads. This could potentially cause the parts of voltage regulators like the autotransformer to burn up in flames!

As for the Mains supply line, the main problem is due to a phenomenon known as Waveform distortion, which results in the mains supply line not having a non-linear load behaviour. Even microprocessors cannot function properly with distorted waveform (it can only function with clean waveform). This causes waveform distortion to occur and not even the mains supply can guarantee that the waveform is not distorted, as rectifiers and inverters are commonly used in many other types of equipment such as UPS, Inverters, Frequency Converters, Computers, Speed Controllers, and more.



Many manufacturers will preach to you about the extra long-lasting lifespan and the ultra-high reliability of the SCRs connected to each of the taps in the voltage regulators. 




“Saying is one thing, but proving is another…”

As talked about in Point 1, because of the way a Static AVR is designed, the SCRs can only be switched off at the zero-voltage-crossing. This process depends on the firing control circuit board or the microprocessor board. Mis-firing or mis-triggering of the thyristors can cause nuisance tripping or the Static AVR to go to bypass mode, thereby resulting in disastrous consequences.


Another problem arises when too many SCRs are used. As previously noted, systems with higher accuracies would require more SCRs. Static AVR systems are designed as single-phase units, which means that systems with a wider voltage range or input swing would require even more SCRs.


However, with so many SCRs, it would only be fair to question the reliability of your Static AVR. What happens after the warranty period is a worrying feeling for your team and your operations. At best, this could lead to more frequent breakdowns or extremely expensive repair costs that could break your maintenance budgets.



Another thing that many manufacturers will say to promote Static AVR is the fact that it is very durable. If you take their word for it, you might have thought that it is durable for all environments


However, that cannot be further from the truth.


This is because a Static AVR may not be rugged enough to be installed in certain environments. With this in mind, do you think that the Static AVR could sustain or provide a long-lasting supply to your operations?

Places with harsh electrical environments cause the SCRs in Static AVR to be damaged due to high inrush current. On the other hand, high-temperature environments are also unsuitable because Static AVRs need to be kept cool at all times. 


Since they cannot withstand the heat generated in such environments, they cannot operate in outdoor or non-air-conditioned environments. In the long run, the lack of a cool, air-conditioned environment could cause your system to be unreliable, and be at greater risk of unplanned downtime.



“Never Give Safety a Day Off” is a saying we practice as engineers. And one situation where you should REALLY follow this—is while operating a Static AVR. If you perform certain processes incorrectly, your machine could be at huge risk of catastrophe.

Here’s why…

 Power Relays now replaced by SCRs, but with the same ARCING effect.

As previously mentioned in Point 1, the SCR can only be switched OFF at the zero-crossing of the voltage waveform, otherwise substantially high voltage and/or high current can occur. In normal everyday switches, arcing can occur when you turn switches OFF.

In this case, it can lead to

  • Serious Damage to SCRs
  • Shorting or Burning of Transformer Windings
  • Blowing of the fuse.



If you thought a Static AVR had a super quick response time, we’re sorry to break the bad news to you…

It has been proven that this topology is not much faster than the time-tested Servo Automatic Voltage Regulators, especially in cases you are correcting a change in the voltage system.

While SCRs are fast-acting and can quickly switch ‘ON and OFF’, correcting the change in voltage is an extremely slow process. This is especially when there is low, fluctuating incoming voltage, causing this switching back and forth between the bypass mode and SCRs to continue. This process is silent and invisible to the naked eye, and you could be unaware that such a process is happening. This could cause your operations to be grounded for a long time. 


What’s worse, if you do it wrongly, this switching can cause a high current to flow through the transformer windings (across tapping), creating excessive burn and damage to the transformer.



During a motor load start-up, the inrush current will be high and the voltage will dip. It is especially during this time that the regulator will try to correct the change. But with this sudden voltage drop, there will be many more taps to jump. 


Since the regulator switches between bypass mode and the SCR switch (one tap at a time), it could take a longer time to correct up to its set output value. This exposes the loads to low voltages, which many of them may not be able to withstand, causing some equipment to fail.

Additionally, when the fuses blow and Static AVR are in bypass mode, loads are subject to an erratic incoming voltage supply. This can lead to catastrophic damages and expensive emergency repair costs.



At the start of this article, we talked about how most static tap regulators are offered in 3 or 6 taps configurations. However, they are not good enough if your electrical system demands for precise voltage regulation. This is because a static regulator with 3 taps configured only offers around 10% tolerance.

Static AVR

If you think that a static regulator with 6 taps could do the trick, think again. For the price you are paying, you are getting a 5% increase (or more) in its voltage tolerance from the nominal value—hence the incredibly poor voltage accuracy.


Put simply, due to their dependence on a limited number of taps, when compared to other types of automatic voltage regulators, Static AVRs are simply not precise enough for most electrical systems.



You might have thought of buying a Static AVR because of its cost may be the main draw of the product. In fact, it might even have been the factor that led you to consider buying one in the first place. 


However, while they may be cheap on the surface, once you add in the other costs associated with it like repairs or additional parts, the final costs may leave you shocked.

One of the ways is in the semiconductor fuses used in the Static AVR, because depending on the voltage range or swing of your electrical system, there may be more or fewer SCRs needed.


For example, if your electrical system needs a 3% output accuracy, according to calculations, there will be at least 14 nos. of SCRs with semiconductor fuses connected in-line on one phase.


Thus, you would need 3 times the number of SCRs in a 3 phase unit, in this case—a whopping  42 nos. of SCRs


Plus, there will also be 14 nos. of semiconductor fuses since these are connected in-line with the thyristors, and for a 3 phase unit, a total of 42 nos. fuses as well. 


However, these semiconductor fuses, which are not similar to ordinary fuses, are not always easily available and cost much more than ordinary fuses.



On the surface, two Static AVRs may look the same.


But dig deeper into how a typical Static AVR is made, its country of origin, and its flaws begin to reveal themselves.


You may have heard the terrible horror stories of devastating accidents caused by electrical equipment made of inferior quality…

Many Static AVRs assembled in the East are readily available at extremely tempting prices. Questionable workmanship is derived from the manufacturer’s need to satisfy budget-conscious consumers’ demands (the fast and cheap way), in a highly competitive industry. Beware of equipment manufactured with inferior grade and subpar components built into them.


Industry sources have even reported how manufacturers stay suspiciously competitive—by using recycled copper in the AVR’s wirings.

This cutting of corners could potentially put your electrical system and its components at huge risk of failure or damage. This is why you should do a thorough check on your Static AVR’s manufacturer country of origin. There is almost always a catch despite the low and tempting price of a Static AVR. Learn how to spot counterfeit AVRs here.

It’s simple: If you pay peanuts, you get Monkeys


A Static AVR may seem cheaper in comparison to other higher-quality designs, but because of its increased rate of failure, there could be higher repair costs, greater safety concerns, and expensive damages to your overall system.

In Summary

    SCRs can only be switched ON or OFF one tap at a time at the zero-crossing, anything outside would cause damage to your equipment. 
    Because of the Static AVR’s disadvantages in design, thyristors can be accidentally switched ON by external causes, resulting in devastating outcomes such as shorting the AVR’s transformer windings.
    Mis-firing or mis-triggering of thyristors could pose a safety threat, while being the weakest link in your electrical system—simply because there are too many potential points of failures.
    Thyristors cannot withstand harsh environments. Explore other suitable options in the market and select an ideal design that’s best suited for your environment.. 
    To avoid damaging your load equipment, be careful of arcing when switching OFF SCRs at the zero-crossing. This is because the SCRs cannot jump tap, and can only be switched ‘ON or OFF’ one tap at a time.
    Because of the Static AVR’s slow voltage correction speed, switching between bypass and on-load regulation exposes load to sudden change in voltage. In sudden voltage drops, your load equipment is exposed to harmful low voltages.
    Nothing is perfect, while fast response time is typically advertised for Static AVRs, in reality—it might take way longer than expected. Because the Static AVR regulates voltage one tap at a time, it is inherent that the Static AVR lacks adequate speed of response.
    The Static AVR’s “step” regulation means that the output voltage regulation is not smooth. So depending on how critical your application is, choose an AVR that fits your requirements. If you need help picking out the perfect AVR for your needs, here’s the Ultimate guide to choose the best AVR
    The Static AVR relies on multiple SCRs and semiconductor fuses to get the job done. Typically in any electronic equipment, having more components directly contributes to a higher risk of failure.
    Look beyond the tempting prices and scrutinize various factors such as the equipment’s country of origin, the scope of warranty and most importantly, the manufacturer’s credibility and technical capabilities. This could hint if corners were cut through the use of inferior grade or subpar components in the manufacturing process.


In Conclusion

This article serves to highlight the crucial factors to watch out for in a Static AVR. 


For applications where budgets are constrained, the Static AVR may seem like the most viable option, and there is nothing wrong with that. It is however important to recognise the advantages and disadvantages of a Static AVR, so you can make a better informed decision on your buying journey.


While the performance and reliability of a Static AVR do not satisfy requirements of critical applications, the cost would make up for it. 


And if the Static AVR is not a good fit for your needs, what are some other alternatives? 


Well, fret not.


There are other types of Automatic Voltage Regulators that are available in the Market like Servo and Magnetic AVRs.


Read our blog post to explore 4 Types of Automatic Voltage Regulators—that could possibly help you achieve maximum operational reliability for your electrical system.


If you’ve enjoyed this article, check out more insider tips and tricks on how you can take your operations to the next level, and effectively protect your precious uptime. 


P.S. Get notified with more educational content on our LinkedIn Page, as we engage with the community and share the latest electrical guides.

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