Electrical generators - principle of operation and construction briefly explained.

in stemng •  3 months ago
Let me ask you a question, do you rely on electricity as much as you rely on food and water? I guess your answer is yes. What would life be like without electricity to power your television set, charge your mobile phones, cook your foods and even the lights you read by at night? Think about it… without electricity, you wouldn't be able to watch the world cup going on in Russia right now, you wouldn't even be on steemit right now! What a horrible thought! But don't worry. Since electricity was discovered by Benjamin Franklin in 1752, it has become very important in all human endeavors. So for this reason there is need for continuous generation of electricity in order to meet its high demand. To generate electricity you need a synchronous electrical generator or better still called alternator.

800px-STM_Stirling_Generator_set.jpg

Generator Wikimedia commons - public domain license


Introduction

The synchronous electrical generator is also called an alternator and it belongs to the family of electric rotating machines. Other members are the direct current generator or motor, the induction motor or generator, and a host of others derived from these three. The synchronous electrical generator converts mechanical energy to electrical energy. It operates on the fundamental principle of electromagnetic induction.

This principle states that whenever a magnetic flux linking a circuit changes, an electromotive force is always induced in the circuit and the magnitude of the induced electromotive force is equal to the rate of change of the flux linkages. In simple terms, this principle states that if you place a conductor in a varying magnetic field, electricity is induced in that conductor and this induced electricity depends on the rate of varying the magnetic field.

Principle of operation

The rotor winding is energized by a direct current supplied to the rotor winding by a set of brushes and slip rings and an alternate North and South poles is developed on the rotor. When the rotor is rotated in anticlockwise direction by a prime mover of rotating machines in power generating stations, the armature winding in the stator are cut by the magnetic field developed by the rotor poles. Consequently, three phase electromotive force is induced in the armature winding according to electromagnetic induction. The induced electromotive force is alternating since the north and south poles alternately cuts the the armature winding. The direction of the electromotive force induced is given by the Fleming’s right hand rule

Fleming’s right hand rule states that the thumb represents the direction of the movement of the conductor, fore-finger represents direction of the generated magnetic field, then the middle finger represents direction of the induced current when the thumb, fore finger and the middle finger of the right hand are placed perpendicular to each other.

and the frequency is given by

F = PN/120 Hertz

where N is the rotor speed in rpm
and P is the number of poles

The magnitude of the electromotive force induced in each phase is dependent on the rotor magnetic field, the speed of the rotor, the excitation current and the number and position of windings in each phase. The magnitude of the electromotive force induced in each phase of the armature windings is the same but differs by 120 electrical degree.

Details of construction

The armature winding is usually mounted in slot on a stationary element called stator and the magnetic field winding is mounted on a rotating element called rotor. Alternators consists basically of two parts which are stator and rotor. So we'll begin our tour on the parts of the alternator from the rotor;

The rotor consists of a coil of conductor usually copper, wound around an iron core. Current through the conductor coil called excitation current produces a magnetic field around the iron core. The strength of the excitation current determines the strength of the magnetic field generated. The excitation current is a direct current and it is generated by a direct current generator called exciter. it is supplied to the conductor coil by a set of brushes and slip rings. Because the magnetic field produced are alternately North and South, they induce an alternating electromotive force in the stator conductors. The rotor shaft is connected to, and rotated by prime movers of rotating machines in power generating stations.

Two types of rotors are used in alternators, these are the salient-pole type and smooth-cylindrical type.

Salient-pole rotor Wikimedia commons-public domain license

The salient-pole type is used in low and medium speed alternators ranging from 125 rpm to 500 rpm. It has a large number of projecting poles mounted on a heavy magnetic wheel of cast iron or a steel of good magnetic quality. Such generators that uses salient-pole are characterised by their large diameters and short axial lengths. The poles are laminated to minimize heating due to eddy currents. Salient-pole rotors usually need damper windings.

The damper winding is useful in preventing momentary speed fluctuations in generators and are needed to provide the starting torque. Let me state here that under normal running conditions, the damper winding does not carry any current because the rotor runs at synchronous speed. The damper winding also tends to maintain balanced three phase voltage under unbalanced load conditions.

smooth-cylindrical rotor Wikimedia commons-public domain license

The smooth-cylindrical type is used for steam turbine-driven alternators, which runs at very high speeds ranging from 1500 rpm to 3600 rpm. The rotor consists of a smooth solid steel cylinder, having a number of slots milled out at intervals along the outer periphery and parallel to the shaft for accommodation of the field coils. The smooth-cylindrical rotors are designed mostly for 2-pole or 4-pole alternators because of their very high speeds. It should be noted that they do not have projecting poles. They are characterised by their small diameter and long axial length. The smooth-cylindrical rotors have quieter-operation and less winding losses.
Now, let's talk about the stator;

alternator stator Wikimedia commons-public domain license

The stator consists of the stator frame and the stator core.
The stator frame is used for holding the armature windings in position. Low-speed large diameter alternators generally have frames which are cast as sections because of the ease of manufacture. There's need for proper ventilation and this is maintained with the help of holes cast in the frame itself.
The stator core is supported by the stator frame and it is built up of laminations of special magnetic iron or steel alloy insulated from each other with varnish or paper. To minimize loss due to eddy currents, the core is laminated. The slot for holding the armature conductors lie along the inner periphery of the core.

The armature winding is the current carrying winding in which the electromotive force is induced. In alternators the armature windings are open that is there's no closed path for the armature currents in winding itself. The two types of armature windings used in three phase alternators are the single layer winding and the double layer winding.


Conclusion

In electricity generation, the synchronous electrical generator converts mechanical energy to electrical energy for residential, commercial and industrial purposes. The source of mechanical energy may vary from an internal combustion engine to the prime mover of a hydraulic turbine. The conditions necessary to induce a voltage in a conductor are the magnetic field, the conductor and the relative motion between the two. As the rotor rotates, magnetic field is generated. This magnetic field cuts the stator armature and thus a voltage is induced in it. Rotation speed of the rotor influences the induced voltage magnitude and frequency. If an electrical load is connected to the loop terminals, a current will be established in the circuit. The current generated by an electrical generator is alternating current.


References

  1. mokslai
  2. explain that stuff
  3. study electrical
  4. electrical4u
  5. electrical easy

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