The balanced threephase winding of the stator is supplied with a
balanced three phase voltage. As shown in the previous lesson (#29), the
current in the stator winding produces a rotating magnetic field, the
magnitude of which remains constant. The axis of the magnetic field
rotates at a synchronous speed ( n s = (2 ⋅ f ) / p ), a function of the
supply frequency (f), and number of poles (p) in the stator winding.
The magnetic flux lines in the air gap cut both stator and rotor (being
stationary, as the motor speed is zero) conductors at the same speed.
The emfs in both stator and rotor conductors are induced at the same
frequency, i.e. line or supply frequency, with No. of poles for both
stator and rotor windings (assuming wound one) being same. The stator
conductors are always stationary, with the frequency in the stator
winding being same as line frequency.
As the rotor winding is
shortcircuited at the sliprings, current flows in the rotor windings.
The electromagnetic torque in the motor is in the same direction as that
of the rotating magnetic field, due to the interaction between the
rotating flux produced in the air gap by the current in the stator winding, and the current in the rotor winding. This is as per Lenz’s
law, as the developed torque is in such direction that it will oppose
the cause, which results in the current flowing in the rotor winding.
This is irrespective of the rotor type used − cage or wound one, with
the cage rotor, with the bars shortcircuited by two endrings, is
considered equivalent to a wound one The current in the rotor bars
interacts with the airgap flux to develop the torque, irrespective of
the no. of poles for which the winding in the stator is designed. Thus,
the cage rotor may be termed as universal one. The induced emf and the
current in the rotor are due to the relative velocity between the rotor
conductors and the rotating flux in the airgap, which is maximum, when
the rotor is stationary ( nr = 0.0 ).As the rotor starts rotating in the same direction, as that of the
rotating magnetic field due to production of the torque as stated
earlier, the relative velocity decreases, along with lower values of
induced emf and current in the rotor. If the rotor speed is equal that
of the rotating magnetic field, which is termed as synchronous speed,
and also in the same direction, the relative velocity is zero, which
causes both the induced emf and current in the rotor to be reduced to
zero. Under this condition, torque will not be produced. So, for
production of positive (motoring) torque, the rotor speed must always be
lower than the synchronous speed.
The rotor speed is never equal to the
synchronous speed in an IM. The rotor speed is determined by the
mechanical load on the shaft and the total rotor losses, mainly
comprising of copper loss. The difference between the synchronous speed
and rotor speed, expressed as a ratio of the synchronous speed, is
termed as ‘slip’ An alternative explanation for the production of torque
in a threephase induction motor is given here, using two rules (right
hand and left hand) of Fleming. The stator and rotor, along with airgap,
is shown in Both stator and rotor is shown there as
surfaces, but without the slots as given in Also shown is the
path of the flux in the air gap. This is for a section, which is under
North pole, as the flux lines move from stator to rotor. The rotor
conductor shown in the figure is at rest, i.e., zero speed (stand
still). The rotating magnetic field moves past the conductor at
synchronous speed in the clockwise direction.
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