Understanding Electrical Transformers: Delta and Wye Configurations and Three-Phase Power

Overview

This video provides a practical introduction to electric transformers, explaining how they convert AC voltages and how different winding connections affect voltage, current, and phase relationships.

Key Concepts

• Delta primary and Y secondary configurations
• Single-phase versus three-phase power
• Voltage transformations and power conservation
• Tap changers and voltage regulation

Takeaways for viewers

Understand how utilities move power efficiently, why voltage levels matter, and how open and high-leg deltas differ in capacity and risk.

Introduction to Transformers

Transformers are devices that transfer AC power between windings on a magnetic core. When current flows in the primary winding, it creates a changing magnetic field that induces voltage in the secondary winding. The frequency remains the same, but the voltage and current adjust according to the turns ratio. A transformer can be configured in different ways to suit voltage levels and loading, and the video walks through practical designs used in homes and industries.

Single-Phase versus Three-Phase Power

In distribution grids, most homes receive single-phase power, typically via two hot wires and a neutral, producing 240 V split into two 120 V legs. Commercial buildings often require three-phase power, generated as three sine waves 120 degrees apart. The generation source can produce multiple coils to form three phases, which are then delivered to customers through delta or Y connections.

Delta and Y Configurations

A transformer’s primary windings may be connected in delta or in a Y (wye) configuration, and the secondary windings can be delta or Y as well. The nameplate indicates the configuration. In a Y configuration, one end of each coil is tied at a common point to provide a neutral and ground; in a delta, windings form a closed triangle with no neutral. The configuration affects how voltages are measured, how many wires leave the transformer, and whether the neutral is available for single-phase service.

Real-World Examples

Three typical setups are highlighted: a small pad-mounted transformer with a 12,470 V delta primary and a 208/120 V Y secondary rated at 150 kVA; a larger pad-mounted unit delivering 12,470 V to 7200/480/277 V with a 500 kVA rating; and a smaller dry-type step-down that could be 480 V delta primary to 208/120 V secondary rated around 30 kVA. These examples illustrate how utilities scale voltage for different customers and loads.

Voltage, Current, and Power

The conductor current and winding current are related by the turns ratio. If the secondary has more turns than the primary, the output voltage is higher and the current lower, with the total apparent power (VA) roughly conserved. Conversely, a step-down reduces voltage and increases current, again keeping VA roughly constant. The relationship between line voltage and phase voltage in a three-phase system depends on the winding configuration and involves the square root of three for line-to-line voltages in a balanced system.

Tap Changers and Voltage Regulation

Some transformers include taps to adjust turns and compensate for supply voltage variation. Primary windings may be accessible on the outside of pad-mounted units for easier tapping, while larger oil-filled transformers use internal tap changers with the windings submerged for cooling. These taps ensure the desired secondary voltage remains at 120 or 208/277 V even when the upstream supply fluctuates.

Open Delta and High Leg Delta

Open delta uses two transformers to provide three-phase power, but with a reduced capacity compared to three transformers in a closed delta. The high leg delta includes a center tap on one coil, creating a neutral with 120 V from two phases but 208 V from the high leg to neutral, which is often marked with a warning color in panels to avoid damage to 120 V appliances.

Open Delta Capacity Example

Three transformers rated 20 kVA each would yield 60 kVA total, but with only two coils in an open delta, the practical capacity may drop to about 34.6 kVA, or roughly 57.7% of the nominal capacity. This makes open delta cheaper to install but sometimes limited in use for continuous three-phase loads.

Key Takeaways

transformers come in many designs, but at their core they are two coils on a core. The number of windings, how they are connected, and the presence of taps determine the voltages that can be produced, the neutral available, and the overall reliability of delivering power to homes and businesses.