CONSTRUCTION
· The three phase induction motor is the most widely used electrical motor.
· Almost 80% of the mechanical power used by industries is provided by three phase induction motors because of its simple and rugged construction, low cost, good operating characteristics, absence of commutator and good speed regulation.
· In three phase induction motor the power is transferred from stator to rotor winding through induction.
· The Induction motor is also called asynchronous motor as it runs at a speed other than the synchronous speed. Like any other electrical motor induction motor also have two main parts namely rotor and stator.
The Rotor Of Are Further Classified As
1. Squirrel cage rotor,
2. Slip ring rotor or wound rotor or phase wound rotor.
3. Type Of Rotor Construction Used 3 Phase Are Classified
ü Squirrel cage induction moto,
ü Slip ring induction motor or wound induction motor or phase wound induction motor.
The construction of stator for both the kinds of 3 phase remains the same and is discussed in brief in next paragraph. The other parts, which are required to complete the induction motor Shaft for transmitting the torque to the load. This shaft is made up of steel.
1. Bearings for supporting the rotating shaft.
2. One of the problems with electrical motor is the production of heat during its rotation. In order to overcome this problem we need fan for cooling.
3. For receiving external electrical connection Terminal box is needed.There is a small distance between rotor and stator which usually varies from 0.4 mm to 4 mm. Such a distance is called air gap.
· Stator of Three Phase Induction Motor
The stator of the 3-p induction motor consists of three main parts.
There are-
1. Stator frame
2. Stator core
3. Stator winding or field winding
1. Stator Frame
Ø outer most part of the three phase induction motor
Ø main function is to support the stator core and the field winding
2. Stator Core
Ø The main function of the stator core is to carry the alternating flux. In order to reduce the eddy current loss, the stator core is laminated.
3. Stator Winding or Field Winding
Ø The slots on the periphery of stator core of the three phase induction motor carries three phase windings.
Ø This three phase winding is supplied by three phase ac supply. The three phases of the winding are connected either in star or delta depending upon which type of starting method is used.
TROQUC SPEED CHARACTERCITIC
· The torque produced by depends upon the following three factors: Firstly the magnitude of rotor current, secondly the flux which interact with the rotor of three phase induction motor and is responsible for producing emf in the rotor part of induction motor, lastly the power factor of rotor of the three phase induction motor. Combining all these factors together we get the equation of torque as-
ü T is the torque produced by induction motor
ü φ is flux responsible for producing induced emf
ü I2 is rotor current
ü cosθ2 is the power factor of rotor circuit.
3.SINGLE PHASE SYNCHRONOUS MOTOR
OPERATION
· Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it.
· It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply.
· The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux.
· The rotor carrying DC supply also produces a constant flux.
· Considering the frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolution in 1 min or 50 revolutions in 1 sec.
· At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force.
· But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition.
· To overcome this inertia, rotor is initially fed some mechanical input which rotates it in same direction as magnetic filed to a speed very close to synchronous speed.
· After some time magnetic locking occurs and the synchronous motor rotates in synchronism with the frequency
CONSTRUCTION
· The Rotor Is Given Dc Supply, The Reason Of Which Is Explained Later
· The Stator Is Given Is Given Three Phase Supply And The Rotor Is Given Dc Supply.
Methods of Starting
1. Synchronous motors are mechanically coupled with another motor.
o It could be either 3 phase induction motor or DC shunt motor.
o DC excitation is not fed initially.
o It is rotated at speed very close to its synchronous speed and after that DC excitation is given.
o After some time when magnetic locking takes place supply to the external motor is cut off.
2. Damper winding
In Case, Synchronous Motor Is Of Salient Pole Type, Additional Winding Is Placed In Rotor Pole Face. Initially When Rotor Is Standstill, Relative Speed Between Damper Winding And Rotating Air Gap Flux In Large And An Emf Is Induced In It Which Produces The Required Starting Torque. As Speed Approaches Synchronous Speed , Emf And Torque Is Reduced And Finally When Magnetic Locking Takes Place, Torque Also Reduces To Zero. Hence In This Case Synchronous Is First Run As Three Phase Induction Motor Using Additional Winding And Finally It Is Synchronized With The Frequency.
4.THREE PHASE SQUIRREL CAGE INDUCTION
OPERATION
CONSTRUCTION
· Consists by stator, a rotor, and two end shields the bearings that support the rotor shaft
· The rotor windings a shorted to squirrel cage
· No commutater or slip rings to service
· There no brushes to replace
· The rotor bars may be made either of copper or aluminum.
· A very common structure uses die cast aluminum poured into the rotor after the laminations are stacked.
· Some larger motors have aluminum or copper bars which are welded or brazed to end-rings. Since the voltage developed in the squirrel cage winding is very low
· This kind of rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors, which are not wires, as we think, but thick, heavybars of copper or aluminium (aluminium) or its alloy
· The conductor bars are inserted from one end of the rotor and as one bar in each slot. There are end rings which are welded or electrically braced or even bolted at both ends of the rotor, thus maintaining electrical continuity. These end rings are short-circuited, after which they give a beautiful look similar to a squirrel thus the name.
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