# Basic Electrical Engineering (3110005)

BE | Semester-1   Winter-2019 | 11-01-2020

## Q4) (c)

#### Explain in detail the construction of an Alternator.

##### Construction of an Alternator (Synchronous Generator)
• Constructionwise, an alternator consists of field poles placed on the rotating fixture of the machine. i.e. rotor.
• The field poles are made to rotate at synchronous speed ????=(120×??)/?? for effective power generation. Where, f is the alternating current frequency and P is the number of poles.
• It is installed with a stationary armature winding and a rotating field.
• This modification is made to cope with the very high power of the order of few 100 MW.
• To accommodate such high power, the conductor weighs and dimensions naturally have to be increased for optimum performance.
• For this reason, it is beneficial to replace these high-power armature windings by low power field windings, which is also consequently of much lighter weight, thus reducing the centrifugal force required to turn the rotor and permitting higher speed limits.

##### Salient-Pole Rotor
• The term salient means projecting or protruding.
• The salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths.
• The poles are made of thick laminated steel sections riveted together and attached to a rotor with help of joint.
• To prevent the oscillations, the damper winding is provided on pole faces.
• The damper windings are generally copper bars short-circuited at both ends are placed in the holes made in pole axis.
• The term salient means projecting or protruding.
• The salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths.
• The term salient means projecting or protruding.
• The salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths.
• Salient pole alternators driven by water turbines are called hydro-alternators or hydro generators.

##### Cylindrical type Rotor
• The cylindrical type of rotor is generally used for high speed machines and employed in steam turbine driven alternators like turbo generators.
• The machines are built in a number of ratings from 10 MVA to over 1500 MVA.
• This type odd machine has a uniform length in all directions, giving a cylindrical shape to the rotor thus providing uniform flux cutting in all directions.
• The rotor consists of a smooth solid steel cylinder, having a number of slots along its outer periphery for hosting the field coils.
• The cylindrical rotor alternators are generally designed for 2-pole type.
• The cylindrical rotor type machine gives better balance and quieter operation along with lesser windage losses.