Transformers are widely used in electronics and power, so it is important to have an understanding of how transformers work, what they are composed of and test and analysis techniques. The most common type of transformer is called an AC input line, or Mains transformer. Their job is to provide a reliable, inexpensive conversion of 120/240 VAC down to a lower AC voltage that a bridge rectifier circuit in a power supply is able to handle.

Step-down effect
To help visualize the two effects that occur with a step-down transformer, refer to Figure 1. The voltage in the system is reduced by using a specific ratio defined by the transformer turns ratio. The current available is also increased by using the same ratio.

With 1 amp of available current, the primary side of the 120 VAC transformer would magnetically couple, or transform, into an output at roughly 12 VAC. In this example, there is 10:1 step-down ratio, so the output into the bridge input is now 12 VAC. The available current on the secondary is now at 10 amps available, so the current is stepped up. In a perfect world, there would be no effect of any losses from the core, wire, or self-heating, however in practice, transformers typically operates at around 98 percent efficiency.

Turns ratio and phasing
The polarity of the input and output, signified by the red dot on the primary or secondary side in Figure 1, can be changed by simply winding the transformer turns on the laminate core in a clockwise or counterclockwise manner depending on the wound direction of the primary side.

Changing the polarity creates a 180-degree change in the actual measured voltage. Measuring the turns ratio and phasing can be done simply with two channels on a power analyzer. With Channel 1 as the reference, one can setup a math equation (Channel 1/Channel 2) to measure the actual turns ratio and phase. A power analyzer can also be used to measure the Mains primary winding and up to three secondary windings providing the magnitude and phase of each winding. This can be visually displayed in vector/polar form on the instrument for deeper analysis.

3-phase transformers
Three-phase transformers can be configured in either a Delta or Wye (Y) configuration. Figure 2 shows a common transformer test application where the primary (input side) is configured in the Delta shape while the secondary (output side) is in the Wye (Y) shape.  Other than the shape of the transformer, the configuration don’t differ much in effectively serving their purpose.

Measuring this system requires appropriate setup of the power analyzers: 3Phase 3Wire mode for the primary side, and 3Phase 4Wire mode for the secondary side. Both modes are almost identical, except the fourth wire is connected to the neutral circuit.

The closed circuit of the Delta transformer side provides some benefits: voltages on the secondary have improved balance. Also it cancels third harmonics since these are not supported on the three-wire system (they would be in phase on all three wires). A Delta connection on the secondary side introduces the possibility of large circulating currents if the characteristics of the windings are not perfectly balanced.  A Wye connection avoids this possibility. The concept of balance in any transformer leg is just conveying that the load currents are equal, or balanced, in each leg. Also note, if the line to line voltage is 208 Vrms L - L, then the line to neutral voltage is (L - L)/Ö3. So 208 Vrms/Ö3 = 120 Vrms L - N. This equation is important as it lets you transfer from L - L measurement to L - N measurements.