Ac Generator Construction 5u4k4z

AC Generator Construction - The Alternator Stator Introduction Alternating current generators or alternators as they are more popularly known are work on the principle of electromagnetic induction and consist of a stationery armature and a rotating field rotor. In the next couple of articles we will take a brief look at alternator construction studying its different parts one by one and learning about their construction starting with alternator stators First of all let us start by learning what are the advantages of a stationary armature system, rather than the other way round (as is normally the case in DC generators where armature rotates)   

Since the output current of the generator is generated in the armature, it is much easier to extract this current for external use from the stationary armature rather than from a rotating armature wherein the brushes come into the picture and the associated challenges of using brush s for transferring current. Secondly it is much easier to insulate the stationary armature winding for high voltages which are normally in the region of over 30 KV. Thirdly apart from insulation it is also easier to brace these windings to prevent any deformation.

The figures below show images related to stator assembly and the arrangement of coils etc

Stator Construction

Stator Frame and Rotor Construction The stator frame is used to hold the armature windings in alternators, and in case of larger diameter alternators (which are slow speed) the stator frame is cast out of sections and there are holes for ventilation in the casting itself. The recent trends towards such stator construction are more in favor of using mild steel plates which are welded together rather than using castings.

Another integral part of the stator is the stator core which is supposed by the stator frame. Actually you can see the combined diagram of both the stator core as well as the stator frame (first of the three diagrams above). The core is constructed in the form of laminations and the material used for the same is either magnetic iron or steel alloy. The main purpose of lamination is to prevent loss of energy in the form of eddy currents. There are different types of armature slots provided in the core to insert the conductors and the three various types are as follows   

Wide open type slots Semi closed type slots Close type slots

Whereas the wide open slots are easier from the winding and repair perspective, they are also having a disadvantage that improper distribution of air gap flux. The fully closed one on the other hand are good from this air gap flux perspective but obviously the initial construction and labor involved in such winding is substantially more in this case. The semi closed presents an optimum mix of both these but then the actual design used is decided by the design specialist based on combined effect of all factors involved.

PARALLEL OPERATION OF ALTERNATORS Alternators are connected in parallel to (1) increase the output capacity of a system beyond that of a single unit, (2) serve as additional reserve power for expected demands, or (3) permit shutting down one machine and cutting in a standby machine without interrupting power distribution. When alternators are of sufficient size, and are operating at different frequencies and terminal voltages, severe damage may result if they are suddenly connected to each other through a common bus. To avoid this, the machines must be synchronized as closely as possible before connecting them together. This may be accomplished by connecting one generator to the bus (referred to as bus generator), and then synchronizing the other(incoming generator) to it before closing the incoming generator’s main power or. The generators are synchronized when the following conditions are set:

1.

Equal terminal voltages. This is obtained by adjustment of the incoming generator’s field strength.

2.

Equal frequency. This is obtained by adjustment of the incoming generator’s prime-mover speed.

3.

Phase voltages in proper phase relation. The procedure for synchronizing generators is not discussed in this chapter. At this point, it is enough for you to know that the above must be accomplished to prevent damage to the machines.

Sources: http://www.brighthubengineering.com/commercial-electrical-applications/67627-ac -generatorconstruction-the-alternator-stator/#imgn_0 http://electriciantraining.tpub.com/14177/css/14177_78.htm

Synchronization of alternator means connecting an alternator into grid in parallel with many other alternators, that is in a live system of constant voltage and constant frequency. Many alternators and loads are connected into a grid, and all the alternators in grid are having same output voltage and frequency (whatever may be the power). It is also said that the alternator is connected to infinite bus-bar. A stationary alternator is never connected to live bus-bars, because it will result in short circuit in the stator winding (since there is no generated emf yet). Before connecting an alternator into grid, following conditions must be satisfied:

1.

Equal voltage: The terminal voltage of incoming alternator must be equal to the bus-bar voltage.

2.

Similar frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.

3.

Phase sequence: The phase sequence of the three phases of alternator must be similar to that of the grid or bus-bars.

4.

Phase angle: The phase angle between the generated voltage and the voltage of grid must be zero.

The first condition of voltage equality can be satisfied by a voltmeter. To satisfy the conditions of equal frequency and identical phases, one of the following two methods can be used: (i) Synchronization using incandescent lamp (ii) Synchronization using synchroscope.

Synchronization of Alternator Using Incandescent Lamp Let, alternator 2 is to be synchronized in a grid and the alternator 1 is already in the grid as shown in above figure. The alternator 2 is connected to grid through three synchronizing lamps (L1, L2 and L3) as shown in above figure. If the speed of the alternator 2 is not such that the frequency of output voltage is equal to the frequency of the grid, there will also be a phase difference in the voltages, and in this case the lamps will flicker. Three lamps are connected asymmetrically, because if they were connected symmetrically, they would glow or dark out simultaneously (if the phase rotation is same as that of busbars). Asymmetrically connected lamps indicate whether the incoming machine is running slower or faster. If the alternator 2 is running slower, the phase rotation of alternator 2 will appear to be clockwise relative to the phase rotation of the grid and the lamps will light up in the order 3, 2, 1; 3, 2, 1.... If the alternator 2 is running faster, the phase rotation of alternator 2 will appear to be anticlockwise relative to the phase rotation of the grid and the lamps will light up in the order 1, 2, 3; 1, 2, 3.... When the speed of the alternator 2 reaches so that, the frequency and phase rotation of output voltage is similar to that of the grid voltage, lamp L1 will go dark and lamps L2 and L3 will dimly but equally glow (as they are connected between different phases and due to this there will be phase difference of 120 degree). The synchronization is done at this very moment. This method of synchronization is sometimes also known as 'two bright and one dark method'. Drawbacks of 'synchronization using incandescent lamps' method are: 

Synchronization by using incandescent lamps depends on the correct judgment of the operator.



This method does not tell how slow or fast the machine is.



To use this method for high voltage alternators, extra step down transformers need to be added as ratings of lamps are normally low.

Synchronization of Alternator Using Synchroscope

A synchroscope is a device which shows the correct instant of closing the synchronizing switch.Synchroscope has a pointer which rotates on the dial. The pointer rotates anticlockwise if the machine is running slower or it rotates clockwise if the machine is running fast. The correct instant of closing synchronizing switch is when the pointer is straight upwards.

Advantages of Parallel Operation of Alternator or Generators When a large number of alternators or synchronous generators are connected in parallel to an infinite bus bar system having a constant terminal voltage, constant bus bar frequency and very small synchronous impedance, then this kind of connection is known as parallel operation of alternators. Parallel operation of alternators is often called as synchronizing. In this post we shall be discussing some of the advantages of the parallel operation of alternators. 1.

Continuity of Supply and Maintenance: This kind of connection is quite feasible when the repair and periodical maintenance work is concerned. It is easier for maintenance when smaller individual generating units are used, since we can schedule the maintenance of each unit one after the other without affecting the continuity of power generation. While if we are using a single unit then the whole system has to be shut down for its maintenance work, this would definitely affect the power demand during that duration.

2.

Efficiency: You might have seen the efficiency versus load current curve of the generator, according to that the generator unit must run on full load, since efficiency is maximum when the load is 100%. So if we are using a single large unit then it is uneconomical to use it on lower loads. While if we use smaller individual units we can add up or switch off the generator units as per the load requirements in order to meet the maximum efficiency.

3.

Expansion Plans: Suppose the current capacity of the power plant is 500 MW, if after 2 year it plans to expand the capacity to 700 MW then it will be much costlier to replace the whole single generator setup and to buy a bigger unit. Rather it is economical to buy small individual alternator units which can be added in parallel to the bus bar system in order to reduce the initial capital investment.

4.

Size of Alternators: More the rating of the generator, huge is the size of the setup. It is very difficult to manage a single large alternator which may range around 1000 MVA or more. Rather if we are using small individual units connected in parallel, it will be easier to manage considering its size.