Why are Alternators rated in kVA and not in kW?

The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class.

Apart from the constant loss incurred in Alternators is the copper loss, occurring in the 3 –phase winding which depends on I2R, the square of the current delivered by the generator. As the current is directly related to apparent – power delivered by the generator , the Alternators have only their apparent power in VA/kVA/MVA as their power rating.

Read More

Which type of Synchronous generators are used in Hydro-electric plants and why?

As the speed of operation is low for hydro turbines use din Hydro-electric plants, salient pole type Synchronous generators are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor.

Read More

Why do cylindrical Alternators operate with steam turbines?

Steam turbines are found to operate at fairly good efficiency only at high speeds. The high speed operation of rotors tends to increase mechanical losses and so the rotors should have a smooth external surface. Hence, smooth cylindrical type rotors with less diameter and large axial length are used for Synchronous generators driven by steam turbines with either 2 or 4 poles.

Read More

Why almost all large size Synchronous machines are constructed with rotating field system type?

The following are the principal advantages of the rotating field system type
construction of Synchronous machines:
· The relatively small amount of power, about 2%, required
for field system via slip-rings and brushes.
· For the same air gap dimensions, which is normally decided
by the kVA rating, more space is available in the stator part
of the machine for providing more insulation to the system
of conductors, especially for machines rated for 11kV or
above.
· Insulation to stationary system of conductors is not
subjected to mechanical stresses due to centrifugal action.
· Stationary system of conductors can easily be braced to
prevent deformation.
· It is easy to provide cooling arrangement for a stationary
system of conductors.
· Firm stationary connection between external circuit and
system of conductors enable he machine to handle large
amount of volt-ampere as high as 500MVA.

Read More