When the primary winding of a transformer is energized with an alternating current (AC), alternating magnetic lines of force, called "flux," circulates through the core, establishing a magnetic field. Photo: Quora

Transformers efficiently transfer **electrical energy** from one circuit to another by means of **magnetic induction**. Each phase of a **transformer** is composed of two separate **coil windings** wound on a **common core**.

The transformer **primary winding** receives electrical energy *from the power source*. When the primary winding is energized with an **alternating current (AC)**, alternating magnetic lines of force, called "flux," circulates through the core, establishing a magnetic field.

With a second winding wrapped around the same core, a voltage is induced by the magnetic field. This winding is called the **secondary winding**. The amount of voltage induced in each turn of the secondary winding **will be the same** as the voltage across each turn of the primary winding; this is referred to as the **transformer turns ratio**.

If the secondary winding has **fewer turns** than the primary, a *lower voltage* will be induced in the secondary. This type of transformer is called a step-down transformer.

A secondary coil with **twice as many turns** as the primary will be **cut twice as many times** by the magnetic flux, and twice the **applied primary voltage** will be *induced* in the secondary. This transformer is known as a **step-up transformer**.

Note that the **primary** is always connected to the *source of power*, and the **secondary** is always connected to *the load*. Either the high- or low-voltage winding can be the primary or the secondary.

## How TTR is Calculated

The *total induced voltage* in each winding is **proportional to the number of turns** in that winding and the **current is inversely proportional** to both *voltage and number of turns*.

E1 / E2 = N1 / N2 = I2 / I1

E1 is the primary voltage and I1 the primary current, E2 the secondary voltage and I2 the secondary current, N1 the primary turns and N2 the secondary turns. If **voltage** is *stepped up*, the **current** must be *stepped down* and vice versa. The **number of turns** remains constant unless there is a **tap changer**.

##### Example 1

If the primary voltage of a transformer is **110 volts** (V), the primary winding has **100 turns**, and the secondary winding has **400 turns**, what will the *secondary voltage* be?

110 / E2 = 100 / 400

100 E2 = 44,000

**E2 = 440 Volts**

##### Example 2

If the primary current is **20 amps**, what will the *secondary current* be?

440 x I2 = 110 x 20 = 2,200

**I2 = 5 amps**

Since there is a **ratio of 1 to 4** between the turns in the primary and secondary circuits, there must be a ratio of 1 to 4 between the *primary and secondary voltage* and a ratio of 4 to 1 between the *primary and secondary current*.

As **voltage is stepped up**, the **current is stepped down**, keeping volts multiplied by *amps constant*. This is referred to as "volt amps."

Calculate the ratio of *each three-phase winding* based on the **line to neutral voltage** of the **wye winding**.

Divide the **line-to-line** winding voltage by **1.732** to obtain the correct **line-to-neutral** voltage.

**Example:** 13200-480Y/277 would be 13200/277 = 47.653

Check the **tap changer position** to make sure it is set where the **nameplate voltage** is based. Otherwise, the turns ratio test information cannot be compared with the nameplate.

## How TTR is Measured

The **turns ratio test** is capable of detecting shorted turns in the winding, which indicate **insulation failure** by determining if the correct turns ratio exists. **Shorted turns** may result from short circuits or dielectric failures.

Measurements are taken by applying a known **low voltage** across *one winding* and measuring the **induced-voltage** on the corresponding winding. The low voltage is normally **applied across a high-voltage winding** so that the induced-voltage is *lower*, reducing hazards while performing the test.

Look at the nameplate phasor diagram to find out what winding on the primary corresponds to a winding on the secondary. Photo: Quora

The voltage ratio obtained by the test is **compared to the nameplate** voltage ratio. Look at the nameplate **phasor diagram** to find out what winding on the primary corresponds to a winding on the secondary.

The **ratio obtained from the field test** should fall within 0.5%, or whichever the manufacturer specifies.

New **transformers of good quality** normally compare to the nameplate *within 0.1%*. For three-phase **delta/wye** or **wye/delta** connected transformers, a *three-phase equivalency test* should be performed. The test is performed and **calculated across corresponding single windings**.

### References

- Transformers: Basic, Maintenance and Diagnostics – U.S. Department of the Interior
- NFPA 70B: Recommended Practice for Electrical Equipment Maintenance, 2016 Edition
- Instruction Manual for TTR
- How Do Transformers Work?

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