Transcript Induction motor - KUET | Khulna University of Engineering

Course no: EE 3107
INDUCTION MOTOR
Md. Nur Kutubul Alam
Lecturer
Department of EEE
Khulna University of Engineering and Technology
Khulna-9203
Bangladesh
Like other motors, induction motor has two parts. One always remains stationary and
other part rotates when supply is given. The stationary part is called “Stator” and
rotating part is called “Rotor”.
Stator
Rotor
Both stator and rotor has an inductive circuit (Although there is some resistance,
it is very small in comparison with inductive reactance). But there is no electrical
conduction between these two circuits.
One of the motor element (either stator or rotor circuit) is energized from AC
source and energy transfer to the remaining element takes place
inductionally. That is why induction motor is called induction motor.
Inductive circuit is nothing but a current carrying coil that is used for producing magnetic
flux or magnetic field.
However, a permanent magnet also produces magnetic flux or magnetic field. So in this
sense an inductive circuit is equivalent to a magnet. Magnetic flux (by convention) is
assumed to flow from north pole to south pole.
Presence of magnetic field can be represented by a vector, magnitude of which indicates
the amount of flux (or flux per unit area) and its direction indicates the direction of flux flow
(north to south pole).
Primary requirement for operation of an induction motor is a “Rotating magnetic field”.
Rotating field is created by inductive circuit of the motor. To understand how to create,
first we need some basic idea of inductive circuit.
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If direction of current flow through an inductive coil is reversed, direction of
magnetic flux produced by the coil also reversed.
Now, if a coil is energized by an alternating source that changes its polarity
continuously, the direction of magnetic flux produced by the coil also changes
accordingly.
Also the amount of flux produced by the inductive coil is proportional to the product of
current through it and the number of turn of the coil. Since amount of current through
it changes with time as shown in current vs time curve hence amount of flux also
changes with time. (Here, top-left figure is actually an electromagnet, but its coil is not shown in
the figure. Only nature of flux produced by it when energized by AC source is shown.)
It is mentioned earlier that a magnetic field can be represented by a vector, where magnitude of vector
represents the amount of flux and direction indicates direction of flux(north to south). For simplicity of
analysis, we shall use vector representation.
Since flux produced by a coil or electromagnet energized by AC source changes its polarity after every
half cycle, hence direction of vector representing this flux also changes after every half cycle.
But alternating supply also changes its magnitude sinusoidaly with time, that means current through the
coil changes sinusoidaly and so the amount of flux. As amount of flux is proportional to current, it varies
sinusoidaly with time. So magnitude of vector also changes sinusoidaly with time.
So nature of the vector that representing the magnetic field (of previous slide) is shown in right figure.
Creation of Rotating field by stator circuit
Stator is made up of a
number of stampings which
are slotted to receive the
winding (coil).
Question is how to make
the winding and how does
it work?
We again emphasize that we want to create a rotating magnetic field. To create a rotating
magnatic field, polyphase action is needed. For a n-phase induction motor, n number of
coils are required. Each coil is wound inside the slots of stator as shown in left figure. For a
2-phase induction motor, 2 coils are required (and those are apart 90 degree electrical
angle). Their position in the stator is shown in right figure. Don’t be confused, there are two
coils, not four and it will be clarified in next slide.
Each coil is energized by one of the phase from 2-phase supply. Thus each coil
produces a pulsating field.
However, in a 2-phase supply, there is 90 degree phase displacement between two
phases. Thus when current from phase-1 comes to its peak current of phase-2
becomes zero and vice versa. As flux is proportional to current when flux produced
by one coil becomes maximum (highest vector length), other coil produces no flux
at all.
Resultant vector is found by taking vector sum of two pulsating vectors.
Resultant vector is a rotating vector having constant magnitude. As the direction of
vector indicates the direction of magnetic field and its magnitude indicates amount
of flux, the rotating vector indicates there is a rotating magnetic field within stator
having constant amount of flux. The speed at which flux rotates is called
synchronous speed.
But this is apparent that rotating field cuts each of the coils also during rotation. So
it induces a voltage into the coils called “back emf”. If resistance of stator circuit is
neglected, “back emf” will be out of phase from applied emf by 180 degree.
Rotor
There are two types of rotor namely ‘squirrel cage rotor’ and ‘wound rotor’. Depending on the
type of rotor used induction motor are classified into two groups.
The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor
conductors, which are not wires but heavy bars of copper or aluminium or alloys. These bars are
short circuited by heavy end ring or cage in squirrel cage rotor.
Wound rotor is provided with 3-phase, double layer, distributed winding. These 3-phases are
connected in ‘y-connection’ internally. Then three lines of rotor circuit is brought out through
slip ring. These 3 lines are connected by resistance during starting of motor to provide higher
starting torque.
What ever the type of rotor, it is placed inside the stator as shown in pictures.
Direction of rotation
of field
Direction of rotation
of rotor conductors
with respect to field
N
.
Direction of
induced voltage
S
Relative spped of rotor
with respect to field
When stator is fed from polyphase supply it creates a rotating magnetic field. In figure rotating lines
representing flux lines of the field. We assume here the field rotates in clockwise direction.
This rotating field also passes through the rotor and during rotation it cuts the rotor conductors. So
according to Faradays law, a voltage induces into the rotor conductors.
To understand this more easily, lets consider the magnetic field is stationary as like in DC generator
and directed from left to right. Relative velocity of rotor (and its conductor) with respect to stationary
field, is obviously opposite to the rotation of field i,e counter clockwise. Under this circumstance,
direction of induced voltage in one of the conductor of rotor is shown in figure (red arrow), which is
found from right hand rule.(Direction of induced voltage in the rotor conductor which is laid exactly at
the opposite side of the conductor marked by the red arrow, would be opposite to the red arrow).
Direction of rotation
of field
Direction of rotation
of rotor conductors
with respect to field
N
.
Direction of
induced voltage
S
Relative spped of rotor
with respect to field
When stator is fed from polyphase supply it creates a rotating magnetic field. In figure rotating lines
representing flux lines of the field. We assume here the field rotates in clockwise direction.
This rotating field also passes through the rotor and during rotation it cuts the rotor conductors. So
according to Faradays law, a voltage induces into the rotor conductors.
To understand this more easily, lets consider the magnetic field is stationary as like in DC generator
and directed from left to right. Relative velocity of rotor (and its conductor) with respect to stationary
field, is obviously opposite to the rotation of field i,e counter clockwise. Under this circumstance,
direction of induced voltage in one of the conductor of rotor is shown in figure (red arrow), which is
found from right hand rule.(Direction of induced voltage in the rotor conductor which is laid exactly at
the opposite side of the conductor marked by the red arrow, would be opposite to the red arrow).
End ring
N
.
S
Now, we know that there is a voltage in each rotor conductor and direction of
induced voltage depends upon their position at the rotor surface.
And, the conductors are short circuited by the “end ring”. So, a current would
flow through each conductor and their direction will be determined by the rotor
voltage.
Hence, now the arrows indicating the direction of current flow through the rotor
conductor
Direction of force
force
N
S
.
force
Direction of current
through the rotor
conductor
Relative spped of rotor
with respect to field
So, we have current through each rotor conductor. Let think about one of those, current through
which is flowing to the direction shown by red arrow.
But these current carrying conductors are located in a magnetic field. So each of the conductor will
experience a force given by F = BiL . Direction of force can be found by left hand rule.
Now, it is worth verifying that, two conductors laid 180 degree apart from each other feels oppositely
directed force. And this two oppositely directed forces are separated by rotor diameter. So together
they produce a torque which causes the rotor to rotate.
Cause of rotation of rotor can be described more simply
That is, when stator is energized from supply main, a rotating magnetic field is set
up. This rotating field flux sweeps over rotor surface and so cuts the rotor
conductors and that is why an emf is induced in them.
Since the rotor bars or conductors are short circuited, a current flows through them.
But, according to Lenz’s law, this current will oppose the very cause producing it.
And here the cause is relative speed between rotor conductor and rotating field.
Hence to reduce the relative speed the rotor starts running in the same direction as
that of the flux and tries to catch up with the rotating flux.
We know rotating field rotates at synchronous speed and rotor tries to catch up the
field. But practically it can never catch up the field, that is rotor rotes a little bit slower
than synchronous speed (this is the reason for calling induction motor as
asynchronous motor).
Because, if rotor starts rotating in synchronous speed, relative speed of it with
respect to field will be zero. That means there will remain no induced voltage as well
as current in rotor conductor. And no current in rotor conductor means no torque is
present. In that case, due to friction and windage loss rotor will slow down. Practically
it will never goes to synchronism even for once because produced torque decreases
as rotor approaches to synchronous speed.