Explains, in detail, how a 3-Phase motor works. The ppt explains through diagrams, the basic functioning of a 3-phase motor operations.

1. Fundamental of
motors
S.O.Katiyar
Consultant

2. ROTOR
R
Y
B

18. How does a- 3 phase motor works?
• 3 phase AC voltage
will create rotating
magnetic field , which
will sweep the rotor
conductor and a force
=BIL Sin Q shall be
experienced by rotor.
Rotor shall start
rotating in the
direction of magnetic
flux.

19. How does a 3- phase motor works
• At the moment of start
because rotor is
stationary therefore field
created by rotor current
shall be stationary hence
no emf shall be induced
in stator winding and no
back emf, therefore full
starting current limited by
stator impedance shall
flow through the motor
stator winding.

20. What happens when motor picks up speed?
• As rotor start rotating due to
torque experienced by rotor
bar due to sweeping flux
,magnetic field created by rotor
current shall also rotate along
with rotor and this rotating field
will cause a back emf in the
stator winding. The magnitude
of the back emf shall be directly
proportional to speed of rotor.
That is why as rotor picks up
speed stator current start
diminishing due to rising back
emf.
Back EMF
TIME
CURRENT

21. What happens when motor is near to synchronous speed
• When motor is near to
synchronous speed ,back
e.m.f will limit the stator
current to no load current.
This no load current is
used to meet frictional and
wind losses plus current
required to create
magnetic flux in iron of
stator winding.

22. What happens when motor is exposed to load
• When load is applied to motor ,
it will slow down the rotor hence
field created by rotor shall also
slow down and hence back emf
shall reduce and this will cause
stator current to increase and
that in turn will increase rotor
current and hence torque
applied to rotor. thus as for
every action there is opposite
reaction as per Newton's law.
So increase in load will cause
more current to flow through
stator winding.

23. What happens when excessive load is applied?
• As the load grows so does
the stator current. So
motor can not limit stator
current at it’s own. if no
protection is there the
excessive current either
may damage the rotor
coupling. If rotor coupling
is strong then it will burn
the stator winding/rotor
bars due to excessive
current.

24. How to prevent damages to motor system
• We can protect the motor
by two means.
1. Free the coupling if
applied torque increases
beyond a certain level.
Fusible plug in hydraulic
coupling.
2. Disconnect the power to
motor when stator current
rises beyond thermal
stability of the motor. By
overload relaying.

25. Induction generator
• If the rotor of generator is
turned at a higher speed
then synchronous speed
then it will work as
induction generator.
• Stator magnetic field will
induce rotor current due to
relative motion between
stator magnetic field and
rotor,
• Rotor current magnetic
field will develop EMF in
stator coils
Wind
turbine

26. What happens when phase sequence is changed?
• As the phase
sequence of applied
voltage is changed
direction of rotation of
magnetic flux shall
reverses , hence so
does the force applied
on the rotor bar,
hence rotor starts
rotating in reverse
direction.

27. INDUCTION MOTOR
• Induction motor is a generalized transformer. Difference is
that transformer is an alternating flux machine while induction
motor is rotating flux machine.
• Rotating flux is only possible when 3 phase voltage (or poly
phase) which is 120 degree apart in time is applied to a three
phase winding (or poly phase winding) 120 degree apart in
space then a three phase rotating magnetic flux is produced
whose magnitude is constant but direction keeps changing.
• In transformer the flux produced is time alternating and not
rotating.
• There is no air gap between primary and secondary of
transformer where as there is a distinct air gap between
stator and rotor of motor which gives mechanical movability
to motor.
• Because of higher reluctance ( or low permeability) of air
gap the magnetizing current required in motor is 25-40% of
rated current of motor where as in transformer it is only 2 -5
% of rated primary current.

28. INDUCTION MOTOR
• Induction motor is a generalized transformer. Difference is
that transformer is an alternating flux machine while induction
motor is rotating flux machine.
• Rotating flux is only possible when 3 phase voltage (or poly
phase) which is 120 degree apart in time is applied to a three
phase winding (or poly phase winding) 120 degree apart in
space then a three phase rotating magnetic flux is produced
whose magnitude is constant but direction keeps changing.
• In transformer the flux produced is time alternating and not
rotating.
• There is no air gap between primary and secondary of
transformer where as there is a distinct air gap between
stator and rotor of motor which gives mechanical movability
to motor.
• Because of higher reluctance ( or low permeability) of air
gap the magnetizing current required in motor is 25-40% of
rated current of motor where as in transformer it is only 2 -5
% of rated primary current.

29. Star and delta connected motors
• In star, phase current is same as line current. But phase voltage is
1/1.732 times line voltage. So insulation required in case of HT(star)
motor is less.
• The starting current for motors is 6 to 7 times full load current. So
start-up power will be large if HT motors are delta connected. It may
cause instability (voltage dip) in case small Power system.
• In star connected HT motor starting current will be less compared to
delta connected motor. So starting power is reduced. Starting torque
will also be reduced.
• Also as current is less copper (Cu) required for winding will be less.
• LT motors are delta connected.
– Insulation will not be problem as voltage level is less.
– Starting current will not be problem as starting power in all will be
less. So no problem of voltage dips.
– Starting torque should be large, as motors are of small capacity.

32. Thanks