A 75 kW, three-phase star-connected 50 Hz, 440 V cylindrical rotor alternator operates at the rated condition with 0.8 power factor leading. The alternator efficiency excluding field and starter losses is 95% and Xs = 2.5 Ω. Calculate the mechanical power developed.

Concept :

Non- Salient Pole Rotor Or Cylindrical Rotor

• In this type of rotor, there are no projected poles, but the poles are formed by the current flowing through the rotor exciting winding. Cylindrical rotors are made from solid forgings of high-grade nickel chrome-molybdenum steel. It has a comparatively small diameter and long axial length.

• They are useful in high-speed machines. The cylindrical rotor type alternator has two or four poles on the rotor. Such a construction provides greater mechanical strength and permits more accurate dynamic balancing. The smooth rotor of the machine makes fewer windage losses and the operation is less noisy because of the uniform air gap.

The figure below shows the end view of the 2 poles and 4 pole cylindrical rotors

They are driven by steam or gas turbines. Cylindrical synchronous rotor synchronous generators are called turbo-alternators and turbo generators. The machines are built in ratings from 10 MVA to over 1500 MVA. The biggest size used in India has a rating of 500 MVA installed in the super thermal power plant.

Non-salient pole-type rotors have the following parts. They are as follows

Rotor Core

The rotor core is made of silicon steel stampings. It is placed on the shaft. At the outer periphery, slots are cut in which exciting coils are placed.

Rotor Winding Or Exciting Winding

It is placed on the rotor slots, and the current is passed through the winding in such a way that the poles are formed according to the requirement.

Slip Rings

Slip rings provide DC supply to the rotor windings.

Brushes

Brushes are made of carbon, and they slip over the slip rings. A DC supply is given to the brushes. Current flows from the brushes to the slip rings and then to the exciting windings.

Bearings

Bearings are provided between the shaft and the outer stationary body to reduce friction. They are made of high-carbon steel.

Shaft

The shaft is made of mild steel. Mechanical power is taken or given to the machine through the shaft.

Calculations :

Efficiency = P_o / P_i

Given η = 95 %

P₀  = 75 kW

∴ P_i = P₀ / η

= 75000/0.95

= 78950 W