bookpg.193–194$ syllabus6.17–6.20 - wordpress.com · 6 of 32 © boardworks ltd 2012 primary...

Book pg. 193 – 194 Syllabus 6.17 – 6.20

© www.cgrahamphysics.com

¡  Transformers -‐ YouTube [720p].mp4

Not me – a real

transformer


KEYWORDS: transformer, core, step up, Step down, primary, secondary, induced

Understand how transformers work

ALL – State the func,on of step up and step down transformers MOST – Explain how transformers work and perform transformer calcula,ons SOME – Explain why values from equa,ons are just an approxima,on


5 of 32 © Boardworks Ltd 2012

Transformers

Power can be transferred between circuits using two coils wound around a soft iron core. This is called a transformer.

The alternating current in the primary (input) coil produces an alternating magnetic field.

iron core

primary coil

secondary coil

This alternating magnetic field induces an alternating current in the secondary (output) coil.

This is the circuit symbol for a transformer:


Primary side – how it works

A transformer links two circuits together. To understand how it works, it is important to look at each side separately.

The primary side is simply an electromagnet. By passing an electric current through a coil of wire, it makes a magnetic field, just like the field around a bar magnet.

A direct current makes one end of the iron north, and the other end south. It also causes a potential difference between the ends of the coil.

N

S+

–

What would happen with an alternating current?

¡  Transformer Animation -‐ YouTube [720p].mp4



Secondary side – how it works

The secondary side coil is not connected directly to the primary side or any power supply.

The secondary side works using electromagnetic induction. When the magnetic field is perpendicular to the secondary coil and changes, a current is induced in the coil.

When there is an alternating current in the primary side, the magnetic field around the transformer alternates. This induces an alternating current in the secondary side coil, and a potential difference across it.

+

–


Parts of a transformer


Investigating transformers


Using a transformer to change voltage

The voltage induced in the secondary (output) coil depends on the number of turns on the primary and secondary coils.

A step-up transformer has more turns on the secondary coil and so

increases voltage.

A step-down transformer has fewer turns on the secondary coil and so decreases voltage.


Properties of transformers

Transformers transfer power between circuits. The design of a transformer determines the characteristics of the electricity flowing in its secondary circuit.

The frequency of the alternating current in the secondary circuit matches the primary circuit, but what about voltage?

In an ideal transformer, the voltage in each circuit is related to the number of coils on each side by the following formula:

primary voltage secondary voltage

primary turns secondary turns =

Vp

Vs

Np

Ns =


Step-up transformers

Step-up transformers increase the voltage.

× Vp Vs

NS

NP =

The secondary coil has more turns than the primary coil. Since there are more turns, a larger potential difference is generated across the secondary coil.

ratio greater than 1

+

–

+

–


Step-down transformers

Step-down transformers decrease the voltage.

× Vp Vs

NS

NP =

ratio less than 1

The secondary coil has fewer turns than the primary coil. Since there are fewer turns, a smaller potential difference is generated across the secondary coil. +

–

+

–


Example question

A transformer has 400 turns on the primary coil and 200 on the secondary. If the primary voltage is 300v, what will the secondary voltage be?

LO: Understand how transformers work

Given 𝑉↓𝑆 =?�𝑉↓𝑃 =300𝑉 𝑁↓𝑆 =200 𝑁↓𝑃 =400

Solution: 𝑉↓𝑆 = 200/400 ×300=150𝑉

Step down transformer


Example question

A transformer has 175 turns on the primary coil and 525 on the secondary. If the secondary voltage is 300v, what will the primary voltage be?


Given: 𝑉↓𝑆 = 300V �𝑉↓𝑃 =?�𝑁↓𝑆 =525 𝑁↓𝑃 =175

Step up transformer

Solution •  𝑉↓𝑃 /𝑉↓𝑆  = 𝑁↓𝑃 /𝑁↓𝑆   •  𝑉↓𝑃 = 𝑁↓𝑃 /𝑁↓𝑆  x 𝑉↓𝑆  •  𝑉↓𝑃 = 175/525 𝑥300=100𝑉


How transformers work

1.  The alternating current in the primary coil makes the iron core into an electromagnetic

2.  As the current is alternating, the magnetic field ‘moves’ and also changes direction

3.  This ‘moving’ magnetic field causes a current to be induced in the secondary coil

4.  If the number of coils on the secondary is higher, the potential difference will increase and it is a step-up transformer

5.  If the number of coils on the secondary is lower, the potential difference will decrease and it is a step-down transformer


Make a flow diagram of how a transformer

works in your book and draw a diagram to go

with each stage


Transformer power

Transformers not only change voltage; they change current.

For example, if the voltage increases, such as in a step-up transformer, the current decreases.

power in = power out

and:

power = voltage × current

Vp × Ip = Vs × Is

this gives:

This is because for any transformer:

+

–Vp +

–Vs

Ip

Is


Transformer power example

A transformer has a primary voltage of 1,000 V and a primary current of 0.5 A.

Vp × Ip = Vs × Is

If the secondary circuit has a current of 0.01 A flowing, what is the secondary voltage?

= 50,000 V

0.01 A

0.5 A 1,000 V × =

Is

Ip Vp × Vs =

+

–Vp +

–Vs

Ip

Is


Example question

A transformer has a primary voltage of 30v and a primary current of 2A. If the secondary voltage is 15v, what will the secondary current be?


Given: 𝑉↓𝑃 =30𝑉 𝑉↓𝑆 =15𝑉 𝐼↓𝑃 =2𝐴 𝐼↓𝑆 =?

More or less current?

Solution •  𝐼↓𝑆 = 𝑉↓𝑃 𝑥𝐼↓𝑃 /𝑉↓𝑆  

𝐼↓𝑆 = 30𝑥2/15 =4𝐴

More

current


Example question

A transformer has a primary current of 10A. If the secondary voltage is 2v and the secondary current is 20A, what will the primary voltage be?


Given: 𝑉↓𝑃 =? 𝑉↓𝑆 =2𝑉 𝐼↓𝑃 =10𝐴 𝐼↓𝑆 =20A

Step up or step down transformer?

Current increases à step-down transformer

Solution •  𝑉↓𝑃 = 𝑉↓𝑆 𝑥𝐼↓𝑆 /𝐼↓𝑃   •  𝑉↓𝑃 = 2×20/10 =4𝑉


Power loss in wires

power loss = current2 × resistance

l  power is measured in watts (W)

Wires have resistance, so there is a power lost in the form of heat when current flows through them:

P = I2 × R

l  current is measured in amps (A)

l  resistance is measured in ohms (Ω).

When transmitting electricity, how does a transformer help to minimize the power lost?


Transformers and the supply chain


Plenary: Why use high voltages for distribution?


Plenary - Calculating voltage

bookpg.193–194$ syllabus6.17–6.20 - wordpress.com · 6 of 32 © boardworks ltd 2012 primary...

Documents