Saturday, August 27, 2016

Synchronous Generators Prime movers; Steam turbine; Hydraulic turbine & diesel engines

Synchronous machines construction depends upon type of prime mover used in machines. There are following types of prime movers used in power generation:-
1.       Steam turbines
2.       Hydraulic Turbines
3.       Diesel Engines
Let’s discuss them :-
1.     Steam turbine:-
These types of synchronous machines have high speed.  Generators driven by steam turbine are also called Turbogenerators. Maximum speed of Turbogenerators is 3000 RPM as per formula

Frequency= PN/ 120;
Where P is no. of poles
N is no. of revolutions
In 2 Pole machine at 50 HZ frequency Speed of Turbogenerator comes out to be= 50X120/ 2= 3000 RPM
With Such high speed lower value of armature diameter is to be designed. Lower diameter is selected to limit the centrifugal forces which have very much influence on the generator design.

Peripheral Speed of a Machine is given by formula as below:-
Peripheral Speed(V)= πDn  m/Sec
Where D is diameter of rotor in meters
n=speed of rotor in revolutions per second

As we have seen from above formula that Peripheral Speed increases with increase in diameter as both are directly proportion to each other, With increase in diameter centrifugal forces increases. So diameter should be kept low.
Due to this high speed diameter of 2 pole machine is limited to 1.2 meter which will give peripheral speed of about 175 m/s.

Also for turbogenerators cylindrical rotor construction is to be used instead of salient pole as in Salient pole construction is impractical in turbogenerators due to high mechanical forces.
In these alternators efficiency is very high. Turbo-generators are available upto rating of 1000 MW.
2.     Hydraulic Turbines:-
Power plants using hydraulic turbines are Hydro Power plants these power plants uses Synchronous generators driven by Hydraulic turbines. Hydraulic turbines are driven by water heads. There are 3 types of hydraulic turbines due to different levels of water head availability.
Types of Hydraulic turbine used according to head height are as below:-
(i)                  For Water heads of 400 Meter and above- Pelton Wheel design to be used
(ii)                For water heads upto 380 meter – Francis turbine design to be used
(iii)               For water heads upto 50 meter- Kaplan turbine design to be used
As water heads not so much high so speed of turbine is low varies from 50 to 500 rpm. So get such low rpm salient pole alternators are used for the same. No. of poles in these generators are 12 onwards.
Also further Pelton type turbine design can be Horizontal or vertical shaft type. Usually Horizontal Shaft design is more commonly used.  For high power low speed synchronous generators are usually installed for low water heads power plants are generally built with vertical shaft.
There are following fundamental types for vertical shaft Hydro generators:-
(i)                 Suspended Type:-
In this type thrust bearing is at upper bracket above alternator rotor
(ii)               Umbrella Type:-
In this type bearing is mounted on lower bracket on the turbine cover, Mostly umbrella type construction is used to reduce generator weight and height of power plant. As in suspended type where bearing is used on upper bracket it has to be bigger in size as it has to withstand both generator and turbine load, also it has to withstand water reaction. This will leads to higher generator weight and height of power plant.
These type of generators have rating upto 750 MW.

3.       Diesel Engines:-
These are most widely used in small commercial and industrial establishments also known as Diesel generators. They are usually of small rating. These are horizontal type,  Since these are slow speed machines so salient pole construction is used for the same. Torque generated during its operation is non-uniform so it makes the synchronous generator sensitive to torque variations.

Why Power isn't generated at Higher Frequency i.e. greater than 50/ 60 HZ??
There is more often a question arises while you are dealing with electrical systems.
As you know that for alternators (Synchronous generators)
Frequency=  No. of Pole X Speed of Machine
So increasing the frequency of power generation you have to either increase the no. of poles or increase the speed of alternator.
Now if you increase no. of poles than diameter of machine will increase considerably to accommodate increased no. of poles. Also if machine diameter is increased to accommodate no. of poles then you can't rotate the machine at higher speed as centrifugal forces and vibration forces will be very high. Mechanical strength requirements of machines will also be very high.

Now for increasing speed of alternator you have to provide higher input which will leads to very high maintenance as deterioration happens at such a higher input. So alternator should be mechanically very strong .