A Definitive Guide to Buying a Variac Transformer


Need help reducing the speed of an AC motor? Or seeking a quick fix to undervoltage issues? In such cases, a variac transformer is an ideal solution to reduce or, in certain cases, increase the AC mains voltage. All you need to do is to turn the dial knob on the variac transformer to get the desired output voltage.


But how do you choose a variac transformer? Are there drawbacks to the voltage-tuning convenience it offers? Does it require heavy maintenance?


We’ll answer these questions and more in this in-depth variac transformer guide. 



What is a Variac Transformer? 

A variac transformer is a transformer that provides continuous output voltage adjustment. The variac transformer features a single winding, or inductor coil, connected to both the input and output. It is a type of autotransformer that allows the winding turns ratio to be changed. These transformers are available for single and 3-phase installations. 


How does the variac transformer work


Variac transformers behave like step-down or step-up voltage regulators by applying basic electrical principles. Instead of isolated windings, it shares the same winding between the AC input and output. The input connects to both ends of the windings, while the output has an adjustable arm that glides along the windings. The adjustable end connects to a carbon brush, which the user turns via a dial knob.


The variable autotransformer’s output voltage changes as the secondary winding’s turn position shifts. This follows a simple electronics principle where the output voltage is determined by the secondary to primary inductance ratio. For example, if the input is 240 VAC and the winding has 100 turns, adjusting the output to 50 turns will reduce the output voltage to 120 VAC.


While variac transformers are commonly used to reduce the incoming voltage, most can increase the output voltage marginally. This is because the transformer’s input does not take up the entire length of the windings. Many auto transformers allow the output voltage to be regulated upwards by an additional 17% of its input voltage. For example, a variac transformer can boost 240 VAC to 280.8 VAC with a variac transformer. 



Types of variac transformers


There are two types of variac transformers – manual and motorised. The manual transformer consists of the fundamental parts that provide continuous voltage adjustments, including the transformer winding, brush and dial knob. Meanwhile, the motorised variac transformer has a feedback mechanism that senses the output voltage and automatically adjusts the brush position with a motor. 



Common applications of variac transformers


Due to their simplicity, variac transformers are widely used in many industrial applications: 

  • It alters the temperature of a heating or cooling element by controlling the load current.
  • Variac transformers provide step-down AC to DC conversion by connecting the output to a rectifier circuit.
  • It helps to compensate for voltage drops or swell due to variables in the power distribution network.
  • Installing a variac transformer allows buildings to introduce dimming effects for certain lightings.
  • It prevents overheating AC motors when running below the rated value by increasing the supplied voltage. 



How long do variac transformers last?


Due to its carbon brush, you might assume that variac transformers would have a short lifespan or require frequent maintenance. This is, however, a common misassumption. The transformer’s carbon brush usually lasts for years as long as you adhere to the recommended usage guidelines. For example, avoid turning the dials in sudden forceful movements as it will speed up its degradation.




Importance of Variac Transformers 


Variac transformers generally play two key roles in industrial applications:

  • Shifting the mains AC supply to a lower or higher level.
  • Providing flexibility to change the voltage and current to the connected load spontaneously.




Voltage step up/step down

Continuous voltage adjustment


The first use case is prevalent where there is a disparity between the measured input supply and the nominal rating of the connected equipment. For example, the large resistance of a lengthy power cable causes the supplied voltage to drop by 10%. The variac transformer compensates for the drop by boosting the voltage by the same percentage.


Likewise, not all electrical equipment runs at standard mains voltage value. For example, some CCTV cameras operate on 24 VAC and can’t be connected directly to the 240 VAC outlet. In this case, installing a variac transformer steps down the mains AC to a level that safely powers the cameras. You do so by adjusting the transformer’s dial to 10% of its turn ratio.


The variac transformer’s versatility in regulating voltage to the nominal value is crucial in industrial premises. The measured voltage at incoming distribution points often deviates from the rated value, preventing equipment from connecting directly to the AC mains or risking damage. 


The second use case relates to changing the behaviour of specific equipment by adjusting the supplied power spontaneously. For example, altering the power supplied to an AC fan changes its speed. The same applies to heating elements, motors and dimmable lights. You can alter their output according to real-time requirements with a variac transformer. 


Voltage level adjustment via a variac transformer ensures smoother switching between different voltages compared to digital or servo-based methods. For the latter, it is vital to use high resolutions servo or DAC modules to provide small and incremental voltage steps. 


Whether correcting voltage levels or enabling dimming capabilities, the variac transformer is a simple and cost-effective power regulating solution. It spares companies from setting up complicated power systems while ensuring electrical equipment receives the correct rated power. Besides, variac transformers have a very efficient power transfer rate.



Owning a Variac Transformer 


Variac transformers work with simple electric and mechanical parts and are relatively easy to set up and use. That said, there are important considerations when purchasing, installing and maintaining variac transformers. 



The input and output voltage are the basic parameters to consider when choosing a variac transformer. Besides that, you must pay equal attention to the transformer’s current limit. Connecting a load that exceeds the maximum current limit might cause safety risks. To be safe, choose a variac transformer with twice the current capacity of the electrical load.




Variac transformers are rugged units but are not built to withstand harsh elements. For example, most variac transformers will not operate reliably on altitudes above 1,000 feet or temperatures beyond 40°C. If you require such setups, consult Ashley Edison for a customised variac transformer. 


During installation, it’s essential to level the variac transformer on a flat, stable base. Also, provide adequate space between transformers for ventilation purposes. By itself, the transformer is not protected against overloads, surges and short circuits. Therefore, you must install proactive devices, such as circuit breakers and fuses, to prevent damage during transient events.




Using a variac transformer is straightforward, but we recommend only qualified technicians do so. Besides adjusting the voltage with the knob, it’s important to never go beyond the transformer’s current rating.

While the transformer is rated by its nominal output current, you should not breach the continuous current limits. Continuous current includes the heat losses at the winding turn and varies according to output voltage levels. The continuous current is usually 20% to 30% lower than the nominal current at most winding turn ratios.




Variac transformers are maintenance-friendly devices. Most parts require almost no maintenance and could last years. The lifespan of the brush, however, depends on usage. Even if you’re a meticulous user, you’ll likely need a brush replacement after several years. We recommend periodically wiping it with a dry cloth to keep it dust-free. 



Disadvantages of Variac transformers 



Despite its versatility, variac transformers suffer from certain drawbacks. Its single-coiled winding means no physical separation between the incoming AC and the connected load. Thus, an internal circuit fault can result in the fullvoltage being transferred to the output. Not only this damages the load, but it also endangers humans using the equipment. 

The variac transformer also lacks short circuit protection. If the load is short-circuited, the variac transformer draws a very high current that might be lethal. 


To minimise safety risks, install electrical protective components, such as circuit breakers and fuses, to protect equipment and personnel operating them. Doing so allows the variac transformer to disconnect from the load when the voltage or current exceeds its safety ratings. 


A variac transformer might poorly regulate voltage when electrical interferences and harmonics are present. It lacks a filtering circuit that removes electrical noises or automatically corrects the AC waveform. In such cases, you’ll need to install an automatic voltage stabiliser before stepping the voltage down or up with the variac transformer. 





Variac transformers are useful voltage regulating modules to adjust incoming AC supply to one that safely powers equipment. We’ve shown you how variac transformer works and key considerations when purchasing one. Generally, they are easy-to-use and maintenance-friendly units when you need quick and spontaneous voltage adjustments.


Explore Ashley Edison’s specialised variac transformers for accurate and reliable voltage control.




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