Turbo Alternator Construction -Tutorial
Turbo Alternator - Introduction:
High speed alternators are called as Turbo alternators.
- To provide high speed of rotation, the rotor diameter is reduced and the axial length is increased.
- Two or four poles are normally used in the turbo alternators.
- Mostly steam turbines are used as prime movers.
Turbo Alternator Stator Construction:
The stators of turbo alternators are almost similar to the low-speed alternators except few differences in the external appearance. The differences are:
- Smaller air gap diameter.
- Larger axial length.
- Greater depth of stampings behind the teeth.
- To increase the cooling effect, axial ventilating holes are provided. Air or hydrogen is passed through these holes for the purpose of cooling.
- Stator stampings are made up of silicon alloy steel sheets.
- By increasing the percentage of silicon, the iron loss can be reduced.But this leads to increasing brittleness.
- Stampings are prepared in segments and they are assembled for the required size.
- The stator conductors used in the turbo alternator may be in the form of bars or strips.
- These strips are placed in the slots and connected in parallel.
- They are twisted/transposed in the slot to nullify the effect of local differences in the magnetic flux density. Because of this, the eddy current losses in the conductors are greatly reduced.
Rotor of Turbo alternator
- Smooth cylindrical(non-salient pole) type rotor is used in the turbo alternator.
- Owing to the high-speed, they have either two poles or four poles. In order to prevent the peripheral speed from exceeding the safe limit, the diameter of the rotor is kept down.
- The core of cylindrical rotor and shaft generally forged in one piece. Radial slots are cut around the outer periphery to receive the field windings.
- Remember that rotor poles are the non-salient type.
- It means they do not project out from the surface of the rotor. Over certain regions, the slots are omitted to form the surface of the rotor.
At this moment it is good to refresh the pros of non-salient type rotor over salient type rotor.
Advantages of Cylindrical rotor over salient pole rotor
- The rotor winding ( field winding) is very easily provided firmly in its place.
- The air gap length is uniform throughout the periphery of the rotor.
- A better mechanical balancing of the rotor is obtained.
- Noise less running is possible as the surface of the rotor is smooth cylindrical shape.
- The winding provided in the rotor is a distributed one like DC armature. Hence a better cooling surface is available.
Comparison of salient pole rotor and cylindrical rotor(non-salient pole rotor)
|Salient pole rotor||Cylindrical rotor|
|Large diameter and short axial length||Small diameter and long axial length|
|used for low-speed alternators||Used for high-speed turbo alternators|
|Has no projecting poles||Has no projecting poles|
|Needs damper windings||Do not need damper windings|
|Windage loss is more||Windage loss is less|
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