Constructional Features of Synchronous Machines

Synchronous machines have important role in electrical engineering, so its essential to know the constructional details of synchronous machines.
We have already discussed the types of synchronous machines in my recent articles Introduction to Synchronous Machines. Normally Synchronous machines are used for power generation purpose as a generator. So I'll discuss the constructional features of all Synchronous Generators in details.
The synchronous generators are of three main types. These are :

• Hydro Generators
• Turbo Alternators
• Industrial Generators

Out of these three first two viz. Hydro-Generators and Turbo-Alternators are very important. Before discussing these generators two concepts or features are essential to discuss about synchronous machines or specially about synchronous generators. These are

• Prime Mover
• Run-Away Speed

Prime Movers for Synchronous Generators

The type of construction used for synchronous generators depends upon the type of prime mover. The following types of prime movers are used, generally :

• Steam Turbine
• Hydraulic Turbine
• Diesel Engine

Steam Turbine

The efficiency of steam turbines is high at large speed and therefore synchronous machines driven by steam turbines are high speed machines. The synchronous generators driven by steam turbines are known as " Turbo-Alternators. " The maximum speed of turbo-alternator is 3000 rpm corresponding to 2 poles and 50 Hz frequency.

Hydraulic Turbine

The synchronous generators driven by water turbines are called water wheel generators. The hydraulic turbines are of different types. The type of water turbine depends upon the water head used. Normally used water turbines are :

Water Head	Turbine
400 mtrs and above	Pelton Wheel
Upto 380 mtrs	Francis Turbine
Upto 50 mtrs	Kaplan Turbine

Diesel Engine

There are used as prime movers for synchronous generators of small ratings.

Run-Away Speed

The run-away speed is defined as the which which the prime mover would have, if it is suddenly unloaded when working at its rated load. When the prime mover is working at full load it receives its feed ( water, steam or diesel ) corresponding to full load conditions and therefore when it is suddenly unloaded it tries to race. This is because there is no load on the prime mover while it is receiving its input corresponding to full load. Steam turbine are equipped with a quick acting over speed governor set to trip at 1.1 times the rated speed and therefore the operation of the governor is reliable.