Induction motor works on the principle of electromagnetic induction.
When a three phase supply is given to the three phase stator winding, a
rotating magnetic field of constant magnitude is produced as discussed
earlier. The speed of this rotation magnetic field is synchronous speed Ns r.p.m.
This rotating field produces an effect of rotating poles around a
rotor. Let direction of rotation of this rotating magnetic field is
clockwise as shown in the Fig. 1(a).
instant rotor is stationary and stator flux R.M.F. is rotating. So its
obvious that there exists a relative motion between the R.M.F. and rotor
conductors. Now the R.M.F. gets cut by rotor conductors as R.M.F.
sweeps over rotor conductors. Whenever conductors cuts the flux, e.m.f.
gets induced in it. So e.m.f. gets induced in the rotor conductors
called rotor induced e.m.f. This is electro-magnetic induction. As rotor
forms closed circuit, induced e.m.f. circulates current through rotor
called rotor current as shown in the Fig.1(b). Let direction of this
current is going into the paper denoted by a cross as shown in the Fig.
current carrying conductor produces its own flux. So rotor produces its
flux called rotor flux. For assumed direction of rotor current, the
direction of rotor flux is clockwise as shown in the Fig. 1(c). This
direction can be easily determined using right hand thumb rule. Now
there are two fluxes, one R.M.F. and other rotor flux. Both the fluxes
interact with each as shown in the Fig. 1(d). On left of rotor
conductor, two fluxes cancel each other to produce low flux area. As
flux lines act as stretched rubber band, high flux density area exerts a
push on rotor conductor towards low flux density area. So rotor
conductor experience a force from left to right in this case, as shown
in the Fig. 1(d), due to interaction of the two fluxes.
As all the rotor conductors experience a force, the overall rotor
experiences a torque and starts rotating. So interaction of the two
fluxes is very essential for a motoring action. As seen from the Fig.
1(d), the direction of force experienced is same as that of rotating
magnetic field. Hence rotor starts rotating in the same direction as
that of rotating magnetic field.
|Fig 1 (d|
this can be explained as : according to Lenz’s law the direction of
induced current in the rotor is so as to oppose the cause producing it.
The cause of rotor current is the induced e.m.f. which is induced
because of relative motion present between the rotating magnetic field
and the rotor conductors. Hence to oppose the relative motion i.e. to
reduce the relative speed, the rotor experiences a torque in the same
direction as that of R.M.F. and tries to catch up the speed of the
rotating magnetic field.
If it catches the speed of the rotating magnetic field, the relative
motion between rotor and the rotating magnetic field will vanish ( Ns – N
= 0). In fact the relative motion is the main cause for the induced
e.m.f. in the rotor. So induced e.m.f. will vanish and hence there can
not be rotor current and the rotor flux which is essential to produce
the torque on the rotor. Eventually motor will stop. But immediately
there will exist a relative motion between rotor and rotating magnetic
field and it will start. But due to inertia of rotor, this does not
happen in practice and motor continues to rotate with a speed slightly
less than the synchronous speed of the rotating magnetic field in the
steady state. The induction motor never rotates at synchronous speed.
The speed at which it rotates is hence called subsynchronous speed and
motor sometimes called synchronous motor.
So it can be said that rotor slips behind the rotating magnetic field
produced by stator. The difference between the two is called slip speed
of the motor.
This speed decides the magnitude of the induction e.m.f. and the rotor
current, which in turn decides the torque produced. The torque produced
is as per the requirements of overcoming the friction and iron losses of
the motor along with the torque demanded by the load on the rotor.