Induction machine
Introduction
Induction machine gain its name because energy transfer by induction not conducion
Induction machine invented by tesla
It is very widely used as motor and little used as agenerator
Advantage of induction machine
1_small in size
2_rugged
3_cheap
4_require little maintenance
Construction of induction machine
1_STATOR
it is simple consist of three phase winding placed in aslots
This slots is skewed to
1_reduce magnitec hum
2_reduce motor locking tendency
2_ROTOR
Induction machine may classified with respect to its rotor into two categories
1_wound rotor
The rotor is simply consist of athree phase winding exactly like stator
2_squiral cage motor
In this type rotor consist of solid bars shorted at both ends
Theory of operation
Three phase supply applied to the stator so amagnetic flux produced in the air gab
Between stator and rotor and by faraday’s law
Emf induced in the rotor this EMF circulate current in the rotor as aresult
Of this current flux produced
Due to interaction between stator’sflux and rotor’s flux atorque produced
Cause rotor to rotate in the same direction of synchronous speed.
Rotor speed must be less than synchronous speed whn operate as amotor…………………………..ehy????
because
If rotor rotate at the same speed of synchronous speed
1_novoltage induced
2_no current flow
3_torque induced equal to zero
Starting of inuction machine
Induction machine draw avery large current at starting (5-8 times like full load current)
So to reduce this current we may use the following method
1_in case of wound rotor we may connect resistance series with rotor
At starting and then short circuit this resistance at full load.
2_using autotransformer between line and the motor to
Reduce line voltage at starting and then at full load
Full line voltage applied to the machine
Speed control of induction machine
Speed control of induction machine is very difficult
But we may could control it dy
1_PAM: pole amplitude modulation
2_consequent pole
3_changing frequency
Inter view questions and its answers:
1_what do you mean by synchoronous speed?
The speed at which the flux produced by 3-PHASe stator winding
2_why is the field producing winding of a 3_PHASE induction machine made stationary?
making the field winding stationary element eliminates the use of slip rings and brushes and ,there for,will result in avery trouble free construction
3_why does the rotor of a 3-phase induction motor rotate in the same direction as the rotating field??
this can be explained by lenz’s law the direction of rotor current will be such so as to oppose the cause producing them.now the cause producing the rotor
Current is relative speed etween the rotating field and stationary rotor .hence to reduce this relative speed the rotor starts running in the same direction as the stator field and tries to catch it.
4_the air gab between the rotor and stator is made as smallas possible ??
in order that
1_mutual flux may be produced with aminimum exciting current
2_leakage reactance are as small as possible
5_why is the power factor low at no load ??
Because of the air gab ,the reluctance of the magnetic circuit is very high.
6_what are the advantage of skewed slots in the rotor??
Because this arrangement offer the following advantage
1_it reduce motor noise and variation
2_it increase the starting torque and reduce the starting current
موضوع: شرح الآلات الحثية 
الثلاثاء فبراير 05, 2008 11:21 am
