TRANFORMERS & DC MACHINES BY: PRATIK SINGH ( ) MANISH K ROY ( ) GAGANDEEP SINGH PANESAR ( ) PARSE SANDEEP ( ) SHUBHAM SINGH ( )

TRANSFORMERS A transformer is a device for increasing or decreasing an a.c. voltage.

STRUCTURE OF TANSFORMER

Circuit Symbol for Transformer

How Transformer works ? Laminated soft iron core Input voltage (a.c.) Primary coil Output voltage (a.c.) Secondary coil

All transformers have three parts : Primary coil – the incoming voltage V p (voltage across primary coil) is connected across this coil. Secondary coil – this provides the output voltage V s (voltage across the secondary coil) to the external circuit. Laminated iron core – this links the two coils magnetically. Notice: that there is no electrical connection between the two coils, which are constructed using insulated wire

Two Types of Transformer A step-up transformer increases the voltage - there are more turns on the secondary than on the primary. A step-down transformer decreases the voltage - there are fewer turns on the secondary than on the primary. To step up the voltage by a factor of 10, there must be 10 times as many turns on the secondary coil as on the primary. The turns ratio tells us the factor by which the voltage will be changed.

Formula For Transformer Where V p = primary voltage Vs = secondary voltage N p = Number of turns in primary coil N s = Number of turns in a secondary coil.

Worked example No. 1 The diagram shows a transformer. Calculate the voltage across the secondary coil of this transformer. Step-up transformer!

Solution :

Worked example No. 2 A transformer which has 1380 turns in its primary coil is to be used to convert the mains voltage of 230 V to operate a 6 V bulb. How many turns should the secondary coil of this transformer have? V P = 230 V N P = 1380 V S = 6 V N S = ? Obviously, a Step-down transformer!!

Solution :

DC GENERATOR & DC MOTOR

DC GENERATOR

Introduction In this we consider various forms of rotating electrical machines These can be divided into: ◦ generators – which convert mechanical energy into electrical energy. ◦ motors – which convert electrical energy into mechanical energy. Both types operate through the interaction between a magnetic field and a set of windings.

Wires connected to the rotating coil would get twisted Therefore we use circular slip rings with sliding contacts called brushes.

A Simple DC Generator The alternating signal from the earlier AC generator could be converted to DC using a rectifier A more efficient approach is to replace the two slip rings with a single split slip ring called a commutator ◦ this is arranged so that connections to the coil are reversed as the voltage from the coil changes polarity ◦ hence the voltage across the brushes is of a single polarity ◦ adding additional coils produces a more constant output

Use of a commutator

The ripple can be further reduced by the use of a cylindrical iron core and by shaping the pole pieces this produces an approximately uniform field in the narrow air gap the arrangement of coils and core is known as the armature

DC Generators or Dynamos Practical DC generators or dynamos can take a number of forms depending on how the magnetic field is produced can use a permanent magnet ◦ more often it is generated electrically using field coils ◦ current in the field coils can come from an external supply ◦ this is known as a separately excited generator ◦ but usually the field coils are driven from the generator output ◦ this is called a self-excited generator ◦ often use multiple poles held in place by a steel tube called the stator

DC MOTOR

Motors Everywhere! The fan over the stove and in the microwave oven The dispose-all under the sink The blender The can opener The washer The electric screwdriver The vacuum cleaner The electric toothbrush The hair dryer

DC Electric Motors Electric Motors or Motors convert electrical energy to mechanical motion Motors are powered by a source of electricity – either AC or DC. DC Electric Motors use Direct Current (DC) sources of electricity: Batteries DC Power supply Principle of How Motors Work: Electrical current flowing in a loop of wire will produce a magnetic field across the loop. When this loop is surrounded by the field of another magnet, the loop will turn, producing a force (called torque) that results in mechanical motion.

Motor Basics Motors are powered by electricity, but rely on principles of magnetism to produce mechanical motion. Inside a motor we find: ◦ Permanent magnets, ◦ Electro-magnets Or a combination of the two.