Page 130 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Rotating field rotors are of two types, namely (i) salient pole type as shown in Fig 4 and (ii) smooth cylindrical type
or non-salient pole type, as shown in Fig 5.

Salient pole type: This type of rotor is used only for slow and medium speed alternators. This type is less
expensive, having more space for the field coils and vast heat dissipating area. This type is not suitable for high
speed alternators as the salient poles create a lot of noise while running in addition to the difficulty of obtaining
sufficient mechanical strength.
Fig 4 shows the salient pole type rotor in which the riveted steel laminations are fitted to the shaft fitting with the
help of a dovetailed joint. Pole faces are curved to have uniform distribution of the flux in the air gap leading to
production of sinusoidal wave form of the generated emf. These pole faces are also provided with slots to carry
the damper winding to prevent hunting.
Salient pole type alternators could be identified by their larger diameter, short axial length and low or medium
speed of operation.
Smooth cylindrical or non-salient pole type rotor: This type is used in very high speed alternators, driven by
steam turbines. To have good mechanical strength, the peripheral velocity is lowered by reducing the diameter
of the rotor and alternatively with the increased axial length. Such rotors have either two or four poles but run at
higher speeds.
To withstand such speeds, the rotor is made of solid steel forging with longitudinal slots cut as shown in Fig 5a
which shows a two-pole rotor with six slots. The winding is in the form of insulated copper strips, held securely in
the slots by proper wedges, and bound securely by steel bonds.
One part of the periphery of the rotor in which slots are not made is used as poles as shown in Fig 5b.

Smooth cylindrical pole type alternators could be identified by their shorter diameter, longer axial length and high
speed of operation.
Rating of alternators

An electrical machines is usually rated at the load, which is can carry without over heating and damage to
insulation. i.e the rating of electrical machine is governed by the temperature rise caused by internal losses of
the machine. The copper loss in the armature (I2R) depends upon the strength of the armature current and is
independent of power factor.

117

CITS : Power - Electrician & Wireman - Lesson 76-85