A variable transformer usually consists of a wiper or brush that can be rotated across the windings to create a variable turns ratio. Variable transformers are usually wired as autotransformers with no electrical isolation between the source and the output.

Variable transformers are often built with less than 1 V/turn, permitting fine voltage adjustment. The winding turns are evenly spaced, so the output voltage is proportional to the position of the knob. Almost all variable transformers use manual control to adjust voltages. Some models have motorized controls to allow adjustment at a distance.

Construction

A typical variable transformer consists of a copper winding on a toroidal core. This core consists of laminated, grain-oriented silicon steel to create low reluctance. This makes a variable transformer very efficient. A brush, usually made of carbon, rotates across the winding to take off voltage at any value from 0 V to the maximum design output voltage (see Figure 1).

Figure 1. Variable transformers have a brush that rotates across the winding to take off any voltage from 0V to the maximum design voltage

Variable transformers can have extremely low power losses with efficiencies as high as about 98%. Variable transformers can be built for either bench or panel mount with an adjustable shaft to allow for varying panel thicknesses.

Most 240V models also have a tap to allow for 120V input. This allows for normal output with only half of the normal input voltage. The output current must be reduced with the output voltage when it exceeds 125% of the rated input voltage.

Variable transformers are constructed to minimize losses. Therefore, these transformers can be operated at full current rating at ambient temperatures up to 122° F (50° C). In installations above 122° F, the output current must be reduced. On single transient loads and loads that are cycled on and off, a much higher output current may be carried for brief intervals. The specifications provided by the manufacturer should be consulted for temperature ratings.

 

Combinations

Most variable transformers are rated at 120 V or 240 V. Units with higher voltage requirements may be created by combining or ganging the standard units in series. Units with increased current requirements may be created by ganging the standard units in parallel.

For example, two units can be ganged in a 3-phase open delta. Three 120V units can be ganged in a wye connection to result in a 240V line-to-line 3-phase assembly(see Figure 2). In a system that typically has a common neutral or ground between source and load, the neutral or ground must be connected to the variable transformer common terminals. If the system has no neutral, the loads must be balanced, or the transformers will be damaged from circulating currents. 

 

Figure 2. Variable transformers can be ganged together to develop the desired output voltage.

In other applications of variable transformers, three 240 V units can be ganged in a wye connection, creating a 380V or a 480V line-to-line 3-phase assembly. A 480V single-phase application of variable transformers can be met with two 240V units ganged in a series connection.

 

Applications

Variable transformers are used in many applications requiring precise voltage control. A very common application of variable transformers is in voltage regulation. Other applications of variable transformers include temperature control, control lighting, and laboratory equipment testing for overvoltage and under-voltage conditions.

 

Voltage Regulation

The input voltage to a process can vary by the normal ±10% allowed by the electric utility. There are many applications that require more precise voltage control. Variable transformers can be used with buck-boost transformers to create a voltage regulation system (see Figure 3). The variable transformer is centre-tapped to allow a buck-boost transformer to apply a buck or boost as necessary.

 

Figure 3. Variable transformers can be used as part of a voltage regulation system.

A stepper motor is used to adjust the location of the output tap from the variable transformer. A controller measures the output voltage and adjusts the stepper motor to provide the desired amount of buck or boost to provide the required output regulated voltage.

For example, a plating application requires a stable voltage to ensure consistent plating thickness. A voltage regulator using a variable transformer is part of the power source to the plater.