Working Principle of an Induction Motor
The motor which works on the principle of
electromagnetic induction is understood because the induction motor. The
electromagnetic induction is that the phenomenon during which the voltage
induces across the electrical conductor when it's placed during a rotating
magnetic flux.
The stator and rotor are two essential parts of the
motor. The stator is that the stationary part, and it carries the overlapping
windings while the rotor carries the most or field coil. The windings of the
stator are equally displaced from one another by an angle of 120°.
The induction motor is that the single excited
motor, i.e., the availability is applied only to the one part, i.e., stator.
The term excitation means the method of inducing the magnetic flux on the parts
of the motor.
When the three phase supply is given to the stator,
the rotating magnetic flux produced there on. The figure below shows the
rotating magnetic flux found out within the stator.
Consider that the rotating magnetic flux induces
within the anticlockwise direction. The rotating magnetic flux has the moving
polarities. The polarities of the magnetic flux vary by concerning the positive
and negative half cycle of the availability. The change in polarities makes the
magnetic flux rotates.
The conductors of the rotor are stationary. This
stationary conductor cut the rotating magnetic flux of the stator, and since of
the electromagnetic induction, the EMF induces within the rotor. This EMF is
understood because the rotor induced EMF, and it's due to the electromagnetic
induction phenomenon.
The conductors of the rotor are short-circuited
either by the top rings or by the assistance of the external resistance. The
relative motion between the rotating magnetic flux and therefore the rotor
conductor induces the present within the rotor conductors. Because the current
flows through the conductor, the flux induces thereon . The direction of rotor
flux is same as that of the rotor current.
Now we've two fluxes one due to the rotor and
another due to the stator. These fluxes interact one another. On one end of the
conductor the fluxes cancel one another, and on the opposite end, the density
of the flux is extremely high. Thus, the high-density flux tries to push the
conductor of rotor towards the low-density flux region. This phenomenon induces
the torque on the conductor, and this torque is understood because the
electromagnetic torque.
The direction of electromagnetic torque and rotating
magnetic flux is same. Thus, the rotor starts rotating within the same
direction as that of the rotating magnetic flux.
The speed of the rotor is usually but the rotating
magnetic flux or synchronous speed. The rotor tries to the run at the speed of
the rotor, but it always slips away. Thus, the motor never runs at the speed of
the rotating magnetic flux, and this is often the rationale due to which the
induction motor is additionally referred to as the asynchronous motor.
An induction motor consists essentially of two main
parts:
(a) a stator and (b) a rotor
(a)Stator
The stator of an induction motor is, in theory, an
equivalent as that of a electric motor or generator. it's made from variety of
stamping, which are slotted to receive the windings. The stator carries a
3-phase wining and is fed from a 3-phase supply. it's wound for a particular
number of poles, lesser the speed and the other way around . It'll be there in
the stator windings, when furnished with 3-phase currents, produce a magnetic
flux, which is of constant magnitude but which revolves at synchronous speed.
This revolving magnetic flux induces an e.m.f. within the rotor by mutual
induction.
(b)Rotor
1. Squirrel-cage rotor: Motor employing this sort of
rotor are referred to as squirrel-cage induction motors.
2. Phase-wound or wound rotor: Motors employing this
sort of rotor are variously referred to as ‘phase-wound’ motors or ‘wound’
motors or as ‘slip-ring’ motors.
Squirrel-cage
Rotor
Almost 90 percent of induction motors are
squirrel-cage type, because this sort of rotor has the only and most rugged
construction imaginable and is nearly indestructible. The rotor consists of a
cylindrical laminated core with parallel slots for carrying the rotor
conductors which, it should be noted clearly, aren't wires but contains heavy
bars of copper, aluminum or alloys. One bar is placed in each slots, rather the
bars are inserted from the top when semi-closed slots are used. The rotor bars
are brazed or electrically welder or bolted to 2 heavy and stout
short-circuiting end-rings, thus giving us, what's so picturesquely called, a
squirrel-case construction.
It should be noted that the rotor bars are
permanently short-circuited on themselves, hence it's impossible to feature any
external resistance serial with the rotor circuit for starting purposes.
The rotor slots are usually almost parallel to the
shaft but are purposely given a small skew. This is often useful in two ways.
1. It helps to form the motor run quietly by
reducing the magnetic hum and
2. It helps in reducing the locking tendency of the
rotor i.e. the tendency of the rotor tech to stay under the stator teeth thanks
to direct magnetism between the two.
In small motors, another method of construction is
employed. It consists of placing the whole rotor core during a mold and
casting all the bars and end-rings in one piece. The metal commonly used is an
aluminum alloy.
Another sort of rotor consists of a solid cylinder
of steel with none conductors or slots in the least. The motor operation
depends upon the assembly of eddy currents within the steel rotor.
Phase-wound
Rotor
This type of rotor is given 3-phase, double-layer,
distributed winding consisting of coils as utilized in alternators. The rotor
is wound for as many poles because the number of stator poles and is usually
wound 3-phase even when the stator is wound two-phase.
The three phases are starred internally. The
opposite three winding terminals are brought out and connected to 3 insulated
slip-rings mounted on the shaft with brushes resting on them. These three
brushes are further externally connected to a 3-phase star-connected rheostat.
This makes possible the introduction of additional resistance within the rotor
circuit during the starting period for increasing the starting torque of the
motor, and for changing its speed-torque/current characteristics. When running
under normal condition, the slip-rings are automatically short-circuited by
means of a metal collar, which is pushed along the shaft and connects all the
rings together. Next, the brushes are automatically lifted from the slip-rings
to scale back the friction losses and therefore the wear and tear. Hence, it's
seen that under normal running conditions, the wound rotor is short-circuited
on itself a bit like the squirrel-cage rotor.
The longitudinal section of a slip-ring motor, whose
structural details are as under:
1.
Frame. Made from close-grained alloy forged iron.
2.
Stator and Rotor Core. Built from high-quality low-loss
silicon steel laminations and flash enameled on each side.
3.
Stator and Rotor Wining. Have moisture proof tropical
insulation embodying mica and top quality varnishes. Are carefully spaced for
simpler air circulation and are rigidly braced to face up to centrifugal forces
and any short-circuit stresses.
4.
Air-gap. The stator rabbets and bore are machined carefully
to make sure uniformity of air-gap.
5.
Shafts and Bearing. Ball and ball bearing are wont to suit
heavy duty, trouble-free running and for enhanced service life.
6.
Fans. Light aluminum fans are used for adequate
circulation of cooling air and securely keyed onto the rotor head.
7.
Sip-rings and Slip-ring Enclosures. Slip-ring are made
from top quality phosphor-bronze and are molded construction.
Induction
Motors in Practice
What
controls the speed of an AC motor?
In synchronous AC motors, the rotor turns at
precisely the same speed because the rotating magnetic field; in an induction
motor, the rotor always turns at a lower speed than the sector , making it an
example of what is called an asynchronous AC motor. The theoretical speed of
the rotor in an induction motor depends on the frequency of the AC supply and
therefore the number of coils that structure the stator and, with no load on
the motor, comes on the brink of the speed of the rotating magnetic flux. In
practice, the load on the motor (whatever it's driving) also plays a
part-tending to slow the rotor down. The greater the load, the greater the
"slip" between the speed of the rotating magnetic flux and therefore the
actual speed of the rotor. To regulate the speed of an AC motor (make it go
faster or slower), you've got to extend or decrease the frequency of the AC
supply using what's called a variable-frequency drive. So once you adjust the
speed of something sort of a factory machine, powered by an AC induction motor,
you're really controlling a circuit that's turning the frequency of the present
that drives the motor either up or down.
What's
the "phase" of an AC motor?
We don't necessarily need to drive the rotor with
four coils (two opposing pairs), as illustrated here. It's possible to create
induction motors with all types of other arrangements of coils. The more coils
you've got , the more smoothly the motor will run. the amount of separate
electric currents energizing the coils independently, out of step, is understood
because the phase of the motor, therefore the design shown above may be a
two-phase motor (with two currents energizing four coils that operate out of
step in two pairs). during a three-phase motor, we could have three coils
arranged round the stator during a triangle, six evenly spaced coils (three
pairs), or maybe 12 coils (three sets of 4 coils), with either one, two, or
four coils switched on and off together by three separate, out of phase
current.
Advantages
of an induction motor (IM):
1. It's rock bottom in cost to match the opposite
motors.
2. It's a highly efficient motor. The efficiency of
IM is varying from 85 to 95%.
3. The brushes aren't utilized in an induction
motor. So, there are not any sparks within the motor and it are often utilized in
polluted and hazards environment.
4. The upkeep of IM is extremely less compared to the
DC motor and electric motor.
5. 3 phase induction motor is that the self-starting
motor. So, any special starting arrangement or extra starter isn't required.
However, single-phase induction motors don't have self-starting torque, and it
uses some auxiliaries to rotate.
6. During this motor, just one AC source requires to
work. It doesn't require DC excitation sort of a electric motor.
7. The speed variation from no-load to rated load is
extremely less.
Disadvantages
of Induction Motor:
1. The facility factor of the motor is extremely low
during the sunshine load condition.
2. The three-phase induction motor is constant speed
motor. The change in speed of the motor is extremely low during different
loading conditions. So, the speed control of IM is difficult.
3. During light load condition, it operates at very
low power factor. Due to this, it draws higher current. Which ends up in higher
copper loss and fewer efficiency.
4. Single-phase induction motor isn't self-starting.
It requires some auxiliary for stating.
5. The motor cannot use in such applications where
high starting torque is important like traction and lifting weight.
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