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The slip ring induction motor has two distinctly separate parts, one is the
stator and other is the rotor. The stator circuit is rated as same in the
squirrel cage motor, but the rotor is rated in frame voltage or short circuit
current. A slip ring (in electrical engineering terms) is a method of making
an electrical connection through a rotating assembly. Slip rings, also called
rotary electrical interfaces, rotating electrical connectors, collectors,
swivels or electrical rotary joints, are commonly found in electrical generators
for AC systems and alternators and in packaging machinery, cable reels, and wind
turbines. A slip ring consists of a conductive circle or band mounted on a
shaft and insulated from it. Electrical connections from the rotating part of
the system, such as the rotor of a generator, are made to the ring. Fixed
contacts or brushes run in contact with the ring, transferring electrical power
or signals to the exterior, static part of the system.
STATOR
The stator consists of 3-ph winding forms wound 'poles' that carry the
supply current to induce a magnetic field that penetrates the rotor. In a very
simple motor, there would be a single projecting piece of the stator (a
salient pole ) for each pole, with windings around it; in fact, to
optimize the distribution of the magnetic field, the windings are distributed in
many slots located around the stator, but the magnetic field still has the same
number of north-south alternations. The number of 'poles' can vary between motor
types but the poles are always in pairs
SLIP RING ROTOR
The slip ring induction motors usually have “Phase-Wound” rotor. This type of
rotor is provided with a 3-phase, double-layer, distributed winding consisting
of coils used in alternators. The rotor core is made up of steel laminations
which has slots to accommodate formed 3-single phase windings. These windings
are placed 120 degrees electrically apart. The rotor is wound for as many poles
as the number of poles in the stator and is always 3-phase even though the
stator is wound for 2-phase. These three windings are “starred” internally and
other end of these three windings are brought out and connected to three
insulated slip-rings mounted on the rotor shaft itself. The three terminal ends
touch these three slip rings with the help of carbon brushes which are held
against the rings with the help of spring assembly These three carbon brushes
are further connected externally to a 3-phase star connected rheostat Thus these
slip ring and external rheostat makes the slip ring induction motors possible to
add external resistance to the rotor circuit, thus enabling them to have a
higher resistance during starting and thus higher starting torque.
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