Day 4: Transformers

• Definition of a Transformer

• Transformer Windings

• Transformer Operation using Faraday’s Law

• Step-up & Step-down Transformers

• Turns Ratio

• Examples of Transformer Use

Definition of a Transformer

• A transformer is a device for increasing

or decreasing AC voltage

• A transformer consists of two

independent coils wrapped around

a common ferrite core.

• The input winding is called the primary winding, and the output winding is called the secondary winding

Transformer Operation• A transformer is designed so that a change in magnetic

flux in the primary coil, passes through the secondary coil. Energy losses through the core are minimal

• An AC voltage applied to the primary coil, produces a ΔΦB and induces an AC voltage of the same frequency in the secondary coil

• The voltage in the secondary coil is dependent upon the ratio of the turns in the coils in the transformer

P

SPS

P

S

P

S

BSS

BPP

N

NVVor

N

N

V

Vdt

dNV

LawFaradaydt

dNV

'

Step-up & Step-down Transformers• If the secondary coil has more turns (loops) than the

primary, it is called a step-up transformer

• If the secondary coil has less turns (loops) than the primary, it is called a step-down transformer

• Energy conservation requires that (neglecting core losses) that the primary power equals the secondary power.

S

P

P

S

SSPP

N

N

I

I

or

IVIV

Transformer Operation• Transformers only work with AC current. A DC

current in the primary coil does not produce a changing magnetic flux, therefore it does not induce an EMF in the secondary coil.

• A voltage step applied to the primary coil, will only induce a voltage spike in the secondary coil.

Examples of Transformer Use• Cell phone chargers• Car ignition coil• Power company HV Transformers

Examples of Transformer Use