Generators and Transformers

• Electrical Machines
• Dynamic Machines
• Generators

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• Motors

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• Static Machines
• Transformers

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• electrical power is generated, transmitted and distributed as AC because of economical considerations.
• Although, usage in most electronic devices is DC

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• Batteries
• Disposable
• AAA,AA,C & D sizes

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• Single Use

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• Rechargeable
• NiCd, NiMH, Li-ion and Pb-H2SO4

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• Can be used several times

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• First DC cell was invented by Alessandro Volta
• Cell converts electrical energy into chemical energy

Battery lasts

• depends on the charge it stores
• And on the rate at which current is drawn from it
• 10Ah battery can supply 10 A current for 1 hr
• Or 5A current for 2hrs

DC power supply

• Power is mainly available as AC
• AC to DC convertor

DC Generator

• What Is a DC Generator?

A DC generator is an electrical machine whose main function is to convert mechanical energy into electricity. When the conductor slashes magnetic flux, an emf will be generated based on the electromagnetic induction principle of Faraday’s Laws. This electromotive force can cause a flow of current when the conductor circuit is closed.

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Parts of a DC Generator�

• A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.
• Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
• Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.
• Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.

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• Armature core: Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed (fixed) to the shaft.
• Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.
• Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed (or fixed) to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

• Working Principle Of A DC Generator:
• According to Faraday’s laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the conductor.
• The magnitude of induced emf can be calculated from the emf equation of dc generator. If the conductor is provided with a closed path, the induced current will circulate within the path. In a DC generator, field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus, an electromagnetically induced emf is generated in the armature conductors. The direction of induced current is given by Fleming’s right hand rule.

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• According to Fleming’s right hand rule, the direction of induced current changes whenever the direction of motion of the conductor changes. Let’s consider an armature rotating clockwise and a conductor at the left is moving upward. When the armature completes a half rotation, the direction of motion of that particular conductor will be reversed to downward. Hence, the direction of current in every armature conductor will be alternating. If you look at the above figure, you will know how the direction of the induced current is alternating in an armature conductor. But with a split ring commutator, connections of the armature conductors also gets reversed when the current reversal occurs. And therefore, we get unidirectional current at the terminals.

• E.M.F Equation of DC generator

The emf equation of the DC generator is given by the equation:

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E=P Φ Z (N/60)A

where

Φ = Flux produced by each pole in weber (Wb)

Z is the total number of armature conductor

P is the number of poles in a generator

A is the number of parallel lanes/psths within the armature

N is the rotation of armature in r.p.m

E is the induced e.m.f in any parallel lane within the armature

N/60 is the number of turns per second

Time taken to complete one revolution=60/N sec.

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AC Generator

• What is an AC Generator?
• A machine that turns mechanical energy into electrical energy in the form of alternate EMF is known as an AC generator. Faraday’s Law of Electromagnetic Induction governs the operation of a simple AC generator. It’s made out of a wire coil that spins in a magnetic field.

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• Working Principle of AC Generator
• The Working Principle of AC Generators is that they are frequently referred to as alternators and operate on the principle of Faraday’s Law of Electromagnetic Induction. The magnetic flux associated with the coil changes when a conductor moves in a uniform magnetic field, causing an EMF.

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• The Function of AC Generator in Details
• To create a strong magnetic field, the coil is rotated in the magnetic field. An EMF is induced in one direction as a coil goes up through the magnetic field on one side. An EMF is induced in the reverse direction as the coil rotates and this side of the coil moves down and another side of the coil moves up. The direction of the induced EMF is determined using Fleming’s right-hand rule. Every cycle, this process is repeated, and the EMF generated is of the alternating type.

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• A graph depicting the output of an AC generator is displayed above. The description of the letters is as follows:
• A – When the coil is at 0 degrees, it moves parallel to the magnetic field’s direction and so produces no EMF.
• B – The coil moves at 90 degrees to the magnetic field and hence induces the most EMF when it is at 90 degrees.
• C – When the coil is rotated 180 degrees, it moves parallel to the magnetic field again, causing no EMF to be generated.
• D – When the coil is at 270 degrees, it goes back to 90 degrees to the magnetic field, inducing the maximum EMF. The induced EMF in this case is the polar opposite of B’s.
• A – The coil has completed one rotation when it reaches 360 degrees when it moves parallel to the magnetic field and produces zero EMF.

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• Consider a rectangular coil with ‘N’ turns rotating in a homogeneous magnetic field ‘B’ with an angular velocity of ‘’. At every time ‘t’, the angle between the magnetic field ‘B’ and the normal to the coil is given by, θ = ωt.
• The magnetic flux is perpendicular to the plane of a coil in this location, and it is given by B Cos ωt. The magnetic flux associated with a coil of N turns is equal to ɸ = B Cos ωt A, where A is the coil’s area. Faraday’s Laws of Electromagnetic Induction determine the induced EMF in the coil.

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Flux linked with single turn

ϕ` =B.A= BA Cos(ωt)

Flux linked with N turn

ϕ =NBA Cos(ωt)

E=dϕ/ dt​=−d(NBAcosωt)/dt​=NBAωsin(ωt)

When the coil rotates through 90 degrees, the value of sine approaches 1 and the induced EMF reaches its maximum, reducing the preceding equation to:

 E0​=NBm​Aω=NBm​A2πf

Where

The maximal flux density in Wb/m² is denoted by Bm,

The area of a coil in m² is denoted by the letter ‘A’,

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By Substituting the second equation in the first one,

E=E0​sin(ωt)

The term “induced alternating current” refers to the current that is generated when a device is turned on.

I=E/R​=E0​sinωt​/R

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