# Transformers

## Transformers Revision

**Transformers**

**Transformers** are essential to transform the **current and voltage** of the power supply to ones that are suitable for the device.

**Transformers**

A **transformer** consists of a soft iron core and two coils of wire. The coils are known as the **primary coil** and the **secondary coil**. The current enters the primary coil and causes a **magnetic field to flow** around the soft iron core. As the **changing magnetic field **reaches the secondary coil, an **EMF is induced** in the secondary coil.

Although the transformer changes the **voltage**, the** frequency of the alternating current** is not affected by the **transformer**.

The transformer equation states:

\dfrac{N_s}{N_p} = \dfrac{V_s}{V_p}

- N_s is the
**number of turns on the secondary coil**. - N_p is the
**number of turns on the primary coil**. - V_s is the
**voltage at the secondary coil**in volts \left(\text{V}\right). - V_p is the
**voltage at the primary coil**in volts \left(\text{V}\right).

**Example:** A transformer changes a 230 \: \text{V} power supply to 100 \: \text{V} output. If the primary coil has 120 turns, how many turns are on the secondary coil?

**[2 marks]**

\dfrac{N_s}{N_p} = \dfrac{V_s}{V_p}

\dfrac{N_s}{120} = \dfrac{100}{230}

N_s = \dfrac{100}{230} \times 120

N_s = 52 \: \text{turns}

A **transformer** may either be a **step-up** or **step-down transformer**. A step-up transformer **increases the voltage** and a step-down transformer **decreases the voltage supply**.

The most common use of a step-up transformer is to increase the voltage (decrease the current) of a power supply for **efficient transport** in transmission cables. It always has more turns on the secondary coil than the first.

A step-down transformer reduces the voltage to a level that is **safe** enough to use in a device or certain area. They always have more turns on the primary coil than the secondary.

**Efficiency**

As with almost all devices, **transformers** are not 100 \% efficient. Therefore the **power output** is always slightly less than the **power input**. **Efficiency **is a measure of how much of the **energy input** is transformed to **useful energy output**.

The transformers efficiency can be calculated by the equation:

## \text{Efficiency} = \dfrac{I_s V_s}{I_p V_p}

- V_s is the
**voltage at the secondary coil**in volts \left(\text{V}\right). - V_p is the
**voltage at the primary coil**in volts \left(\text{V}\right). - I_s is the
**current of the secondary coil**in amps \left(\text{A}\right). - V_p is the
**current of the primary coil**in amps \left(\text{A}\right).

**Inefficiencies in Transformers**

**Transformers** are not always 100 \% efficient. The majority of the energy loss in a **transformer** comes from the production of **Eddy currents**. When the **magnetic field** is induced in the iron core, a small EMF is also induced which acts **opposite in direction** to the change that caused it (**Lenz’s law**). The current flowing in the soft iron core causes energy to dissipate as heat.

The **Eddy currents** can be reduced by **laminating the iron core**. This is where the iron core is sliced into smaller pieces with insulating materials wedged between the slices of iron. The higher the** resistivity** of metal also reduces eddy currents.

## Transformers Example Questions

**Question 1:** Describe the role of the soft iron core in a transformer.

**[1 mark]**

**To conduct magnetic fields between the primary coil and the secondary coil.**

**Question 2:** Use the transformer equation to calculate the output voltage of a transformer when the input voltage is 230 \: \text{V}, number of turns on the primary coil is 120 and the number of turns on the secondary coil is 500 turns.

**[2 marks]**

**Question 3:** Describe the effect of eddy effects in a transformer.

**[2 marks]**

Eddy currents are hard to prevent. They form in the iron core and **produce an electric current opposite the motion that caused them**. They **therefore increase power loss and decrease the efficiency of a transformer**.

## Transformers Worksheet and Example Questions

### Transformers Questions

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