Induction Motors
Construction
A
3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor
is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm,
depending on the power of the motor.
1. Stator
It
consists of a steel frame which encloses a hollow, cylindrical core made up of
thin laminations of silicon steel to reduce hysteresis and eddy current losses.
A number of evenly spaced slots are provided on the
inner
periphery of the laminations [See Fig.(1)]. The insulated connected to form a balanced
3-phase star or delta connected circuit. The 3-phase stator winding is wound
for a definite number of poles as per requirement of speed. Greater the number
of poles, lesser is the speed of the motor and vice-versa. When 3-phase supply
is given to the stator winding, a
rotating
magnetic field of constant magnitude is produced. This rotating field induces
currents in the rotor by electromagnetic induction.
Fig.(1)] |
2. Rotor
The
rotor, mounted on a shaft, is a hollow laminated core having slots on its outer
periphery. The winding placed in these slots (called rotor winding) may be one
of the following two types:
(i)
Squirrel cage type
(ii)
Wound type
(i)
Squirrel cage rotor. It consists of a laminated cylindrical core having parallel slots on
its outer periphery. One copper or aluminum bar is placed
in
each slot. All these bars are joined at each end by metal rings called end rings
[See Fig. .2)]. This forms a permanently short-circuited winding which is
indestructible. The entire construction (bars and end rings) resembles a
squirrel cage and hence the name. The rotor is not connected electrically to
the supply but has current induced in it by transformer action
from
the stator. Those induction motors which employ squirrel cage rotor are called squirrel
cage induction motors. Most of 3-phase induction motors use
squirrel
cage rotor as it has a remarkably simple and robust construction enabling it to
operate in the most adverse circumstances. However, it
suffers
from the disadvantage of a low starting torque. It is because the rotor bars
are permanently short-circuited and it is not possible to add any external
resistance to the rotor circuit to have a large starting torque.
Fig.( .2) |
(ii)
Wound rotor. It consists of a laminated cylindrical core and carries a 3- phase
winding, similar to the one on the stator [See Fig. (3)]. The rotor
winding
is uniformly distributed in the slots and is usually star-connected. The open
ends of the rotor winding are brought out and joined to three
insulated
slip rings mounted on the rotor shaft with one brush resting on each slip ring.
The three brushes are connected to a 3-phase star-connected rheostat as shown
in Fig. (.4). At starting, the external resistances are included in the rotor
circuit to give a large starting torque. These resistances are gradually
reduced to zero as the motor runs up to speed. The external resistances are
used during starting period only. When the motor
attains
normal speed, the three brushes are short-circuited so that the wound rotor
runs like a squirrel cage rotor.
Fig. (3) |
Fig. (.4). |
0 comments:
Post a Comment