Three Phase Induction:
An Induction motor consists of mainly two parts:
(a) Stator
(b) Rotor Stator:
Rotor: (i) Stator core is made of laminated steel stampings and has slots and teeth on its inner periphery to house stator windings. The stampings are 04.to 0.5 mm thick.
(ii) Stator carries a 3-phase winding having space displacement of 120° electrical
(iii) The 3-phase winding is either star or delta connected and is fed from 3-phase supply
(iv) The radial ventilating ducts are provided along the length of the stator core Stator Stamping
i. Rotor comprises a cylindrical laminated iron core, with slots on outer periphery
ii. Like stator, rotor lamination are punched in one piece for small Machine
iii. In larger machine the lamination are segmented
iv. If there are ventilating ducts on the stator core, an equal number of such ducts is provided on rotor core According to windings rotor are of two types:
a) Squirrel cage rotor
b) Slip ring or wound rotor
Squirrel Cage Rotor:
This rotor consists of a cylindrical laminated core with parallel slots Rotor slots are usually not quite parallel to the shaft but for reducing the magnetic hum and locking tendency rotor slots are slight skew In rotor slots heavy copper, aluminum or alloy bars are housed.
Rotor bars are permanently short circuited at the ends.
This limits that no external resistance insertion is possible
Advantages of squirrel cage induction rotor-
Its construction is very simple and rugged.
As there are no brushes and slip ring, these motors requires less maintenance.
Applications:
Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc
Slip Ring or Wound Rotor:
The rotor is wound for the same number of poles and number of phase as that of stator Rotor
winding is either star or delta but star connection is preferred
The three star terminals are connected to three brass slip ring mounted on rotor shaft These slip rings are insulated from rotor shaft Slip rings connected with brushes and three brushes can further be connected externally to 3variable rheostats This makes possible introduction to additional resistance in the rotor circuit during starting period.
Advantages of slip ring induction motor
- It has high starting torque and low starting current.
Possibility of adding additional resistance to control speed.
Application: Slip ring induction motor are used where high starting torque is required i.e. in hoists, cranes, elevator etc.
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10 янв 2019
