Johnson Electric Coil Company

Frequently Asked Questions

What Is a Three-Phase Transformer and Where Are Three-Phase Transformers Used?

An electric power grid distributes power from the electrical utility to residential buildings, schools, and commercial and industrial properties along the way. While single-family homes typically require only single-phase power, buildings with electrical needs involving high-power equipment like industrial machinery, heavy-duty power protection equipment and uninterruptible power supplies, semiconductor and ion beam systems, or large-scale information systems consistently use three-phase power.
As the name suggests, three-phase transformers are designed to transform the line or input voltage supplied by a three-phase source to the voltage required by a three-phase load on the output side. Where step down transformers deliver a lower secondary or output voltage than the primary or input voltage, step up transformers do the opposite. In other cases, where the available line voltage matches that of a machine or system, a transformer is often needed to isolate and therefore protect the equipment under load or other downstream components in the circuit from various disturbances and electrical noise on the line side that might otherwise compromise performance or shorten their useful life. A three-phase transformer can also be designed to deliver a given output voltage across a range of input voltages to accommodate variations in line voltage, step-up or step-down, from one location to another. A custom-designed three-phase transformer can be configured to supply a combination of relatively balanced single-phase loads or a low-power single-phase component in addition to its main three-phase load as well.

Can a Transformer Convert Single Phase to Three Phase?

Although single-phase power can be derived from a three-phase power source, a transformer cannot convert single-phase power to three-phase power. Converting single-phase power to three-phase power requires either a phase converter or a variable frequency drive.
Although a transformer does not function as a phase-changing device or transform single-phase input power into three-phase out, transformers can be designed for connection to other phase configurations besides single-phase and three-phase, including two-phase, six-phase, or even 24-phase for certain DC rectification transformers. Commonly, a single-phase component can be connected to only one of three phases of a three-phase power source. In addition to powering a large three-phase load, for instance, one leg of a three-phase transformer is often used to feed a low power single-phase load, such as a cooling fan.

Voltage into a three-phase application can be stepped up or down as required by either:

Connecting three single-phase transformers—having one coil and core each—
together, thus forming a “three-phase transformer bank,” or,
Using a single three-phase transformer—having three coils, one for each phase—
assembled onto a single laminated core.
To be compatible with a three-phase supply, three single-phase transformers can be connected together in a way that functions electrically like a three-phase transformer. That is, if the primary windings of three single-phase transformers are properly interconnected into a delta or wye configuration and the same is done with the secondary windings, they will operate much like a three-phase transformer, using three-phase power in to deliver three-phase power out.

The benefits of using a three-phase transformer compared to three single-phase transformers? The three-phase design utilizes windings, core, and available space more effectively, where using three single-phase transformers to accomplish the same electrical design objectives typically only increases labor, size, weight, and cost.

What Are The Advantages Of Using A Toroidal Transformer?

There are many different transformer options on the market, and there are various trade-offs from one to the next: Size, shape, weight, cost, efficiency, power range, and the application’s circuit characteristics are all factors to be considered. Especially where available space is limited and/or minimizing weight is critical, the choice of a toroidal transformer offers several benefits or advantages over other core configurations. A toroid design is particularly suitable for high frequency, low current, low voltage applications.


A common advantage for OEMs is the comparatively light weight design that a toroidal core affords. Not only are toroids typically constructed of high-performance electrical grade steel, but also the shape allows for compact winding that loops through the core’s center as it is wound around the periphery, minimizing losses and maximizing output power and efficiency.

The toroid’s lightweight, compact nature can be critical in electronic equipment and machinery, particularly those designed to be mobile.

Low Electromagnetic Field Radiation

The toroidal design is also ideal in applications where the transformer’s leakage inductance and electromagnetic interference or compatibility are concerns.

Typically, toroid transformers produce significantly less stray magnetic energy, which means less risk of the transformer’s magnetic field interfering with other components within the system. And with the magnetic energy flux largely contained due to the configuration of windings and core, there is limited leakage inductance as well.

Low Noise

Due to the toroid’s “tape-wound” core, compact design, and efficient use of space, the constant hum often produced by other types of transformers in operation is also reduced.


Finally, a toroidal transformer can draw power from almost any power source application, further streamlining design possibilities and potential.

For more information about toroidal transformers, call us at 715.627.4367.

Common Questions About Isolation Transformers

What is the difference between an isolation transformer and an autotransformer?

Whether an isolation transformer or an autotransformer, a transformer typically steps the supply or line voltage coming into the unit up or down to whatever voltage the load or downstream component or circuit requires. While sometimes confused, there is a critical difference between the isolation transformer and the autotransformer:
By definition, the input and output windings of an isolation transformer are separate windings—physically isolated and electrically insulated from each other by distance, an insulation barrier, or both.

An autotransformer, on the other hand, is uniquely designed to accommodate the range of both the available input and required output voltages in a single winding. And because the “common” section of that single winding is shared between the input and output of the unit, there is no physical isolation or electrical insulation between them.

Where and why are isolation transformers used?

An autotransformer can be a remarkably efficient and versatile solution, particularly when the ratio of voltage transformation between input and output is low. Advantages of weight and overall size typically yield a lower cost design than an isolation transformer designed to the same electrical parameters.
In contrast, the isolation transformer, by its very nature, affords a degree of electrical safety/protection to people and equipment alike that the autotransformer cannot provide. And, unlike an autotransformer, an isolation transformer can accommodate an electrostatic shield between input and output, which a) prevents electrical noise on the load side from filtering back onto the line side, b) provides an additional degree of isolation between input and output windings, and c) can provide for an electrical path to ground, or protective earthing, in the event of a fault. Isolation transformers, like autotransformers, can be used to create a step up or step down. The isolation transformer can also mitigate potentially damaging effects of poor power quality on sensitive equipment over time.

Our engineering team will help you determine which type—isolation or autotransformer—is best suited to your unique application. Call us at 715.627.4367.

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