INDUCTION MOTORS: An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.
The induction motor with a wrapped rotor was invented by Nikola Tesla Nikola
Tesla in 1882 in France but the initial patent was issued in 1888 after Tesla had moved to the theUnited States. In his scientific work, Tesla laid the foundations for understanding the waythe motor operates. The induction motor with a cage was invented by Mikhail
Dolivo-Dobrovolsky about a year later in Europe. Technological development in the field
has improved to where a 100 hp (74.6 kW) motor from 1976 takes the same volume as a
7.5 hp (5.5 kW) motor did in 1897. Currently, the most common induction motor is the cagerotor motor.
An electric motor converts electrical power to mechanical power in its rotor
(rotating part). There are several ways to supply power to the rotor. In a DC motor, this power is supplied to the armature directly from a DC source, while in an induction motor thispower is induced in the rotating device. An induction motor is sometimes called a
rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrialdrives.
Induction motors are now the preferred choice for industrial motors due to their rugged construction, the absence of brushes (which are required in most DC motors) and the abilityto control the speed of the motor.
CONSTRUCTION:
The induction motor with a wrapped rotor was invented by Nikola Tesla Nikola
Tesla in 1882 in France but the initial patent was issued in 1888 after Tesla had moved to the theUnited States. In his scientific work, Tesla laid the foundations for understanding the waythe motor operates. The induction motor with a cage was invented by Mikhail
Dolivo-Dobrovolsky about a year later in Europe. Technological development in the field
has improved to where a 100 hp (74.6 kW) motor from 1976 takes the same volume as a
7.5 hp (5.5 kW) motor did in 1897. Currently, the most common induction motor is the cagerotor motor.
An electric motor converts electrical power to mechanical power in its rotor
(rotating part). There are several ways to supply power to the rotor. In a DC motor, this power is supplied to the armature directly from a DC source, while in an induction motor thispower is induced in the rotating device. An induction motor is sometimes called a
rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrialdrives.
Induction motors are now the preferred choice for industrial motors due to their rugged construction, the absence of brushes (which are required in most DC motors) and the abilityto control the speed of the motor.
CONSTRUCTION:
A typical motor consists of two parts namely stator and rotor like another type of motors.
2. An inside rotor attached to the output shaft that is given a torque by the rotating field.
.
Type of rotors
The rotor is of two different types.
1. Squirrel cage rotor
2. Wound rotor
Squirrel-Cage Rotor
In the squirrel-cage rotor, the rotor winding consists of single copper or aluminumbars placed in the slots and short-circuited by end-rings on both sides of the
rotor. Most of single phase induction motors have Squirrel-Cage rotor. One or 2 fans are attachedto the shaft in the sides of the rotor to cool the circuit.
Wound Rotor
In the wound rotor, an insulated 3-phase winding similar to the stator winding wound for the same number of poles as a stator, is placed in the rotor slots. The ends of the star-connected rotor winding are brought to three slip rings on the shaft so that a connection can be made to it for starting or speed control. It is usually for large 3 phase induction motors.
Rotor has a winding the same as a stator and the end of each phase is connected to a
slip ring.
Compared to squirrel cage rotors, wound rotor motors are expensive and require
maintenance of the slip rings and brushes, so it is not so common in the industry
applications.
PRINCIPLE OF OPERATION
An AC current is applied in the stator armature which generates a flux in the
stator magnetic circuit.
This flux induces an emf in the conducting bars other f rotor as they are “cut” by the
flux while the magnet is being moved (E = BVL (Faraday’s Law))
A current flows in the rotor circuit due to the induced emf, which in term
produces a force, (F = BIL) can be changed to the torque as the output
In a 3-phase induction motor, the three-phase currents Ia, ib, and ic, each of equal magnitude, but differing in phase by 120°. Each phase current produces a magnetic flux and there is physical 120 °shift between each flux. The total flux in the machine is the sum of the three fluxes. The summation of the three ac fluxes results in a rotating flux, which turns with constant speed and has a constant amplitude. Such a magnetic flux
produced by balanced three phase currents flowing in thee-phase windings is called a
rotating magnetic flux or rotating magnetic field (RMF).RMF rotates with a constant
speed (Synchronous Speed). The existence of a RFM is an essential condition for the
operation of an induction motor.
If the stator is energized by an ac current, RMF is generated due to the applied current to the stator winding. This flux produces a magnetic field and the field revolves in the air gap between stator and rotor. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars. The interaction of the rotating flux and the rotor current generates a force that drives the motor and a torque is developed consequently. The torque is proportional with the flux density and the rotor If the stator is energized by an ac current, RMF is generated due to the applied current to the stator winding. This flux produces a magnetic field and the field revolves in the air gap between stator and rotor. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars. The interaction of the rotating flux and the rotor current generates a force that drives the motor and a torque is developed consequently. The torque is proportional with the flux density and the rotor bar current (F=BLI). The motor speed is less than the synchronous speed. The direction of the rotation of the rotor is the same as the direction of the rotation of the revolving magnetic
field in the air gap.
However, for these currents to be induced, the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before.
This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor the (slip speed) to the speed of the rotating stator field. Due to this an induction motor is
sometimes referred to as an asynchronous machine.
SLIP
The relationship between the supply frequency, f, the number of poles, p, and the
synchronous speed (speed of rotating field), as is given by
n=120f/P
The stator magnetic field (rotating magnetic field) rotates at a speed, ns, the
synchronous speed. If, an n= speed of the rotor, the slip, s for an induction motor is defined
as
S= N-n/N
At standstill, rotor does not rotate , n = 0, so s = 1.
At synchronous speed, n= nS, s = 0
The mechanical speed of the rotor, in terms of slip and synchronous speed, is given by,
n=(1-s) ns
The frequency of Rotor Current and Voltage
With the rotor at stand-still, the frequency of the induced voltages and currents is the sameas that of the stator (supply) frequency, the.
If the rotor rotates at speed of n, then the relative speed is the slip speed:
nslip=ns-n
nslip is responsible for induction.
Hence, the frequency of the induced voltages and currents in the rotor is, fr= sfe
EQUIVALENT CIRCUIT
The induction motor consists of two magnetically connected systems namely,
stator and rotor. This is similar to a transformer that also has two magnetically connected systems namely primary and secondary windings. Also, the induction motor operates on thesame principle as the transformer. Hence, the induction motor is also called as
rotating transformer
The stator is supplied by a balanced three-phase voltage that drives a three-phase
current through the winding. This current induces a voltage in the rotor. The applied
voltage (V1) across phase A is equal to the sum of the
–induced voltage (E1).
–the voltage drop across the stator resistance (I1R1).
–voltage drop across the stator leakage reactance (I1 j X1).
Let
I1 = stator current/phase
R1 = stator winding resistance/phase
X1 = stator winding reactance/phase
RR = stator winding resistance/phase
XR = stator winding reactance/phase
IR = rotor current
V1 = applied voltage to the stator/phase
Io = Ic+Im (Im-magnetising component, Ic-core loss component).
1. An outside stationary stator having coils supplied with AC current to produce a
rotating magnetic field,
.
Stator construction
The stator of an induction motor is laminated iron core with slots similar to a
stator of a synchronous machine. Coils are placed in the slots to form a three or single
phase winding
Type of rotors
The rotor is of two different types.
1. Squirrel cage rotor
2. Wound rotor
Squirrel-Cage Rotor
In the squirrel-cage rotor, the rotor winding consists of single copper or aluminumbars placed in the slots and short-circuited by end-rings on both sides of the
rotor. Most of single phase induction motors have Squirrel-Cage rotor. One or 2 fans are attachedto the shaft in the sides of the rotor to cool the circuit.
Wound Rotor
In the wound rotor, an insulated 3-phase winding similar to the stator winding wound for the same number of poles as a stator, is placed in the rotor slots. The ends of the star-connected rotor winding are brought to three slip rings on the shaft so that a connection can be made to it for starting or speed control. It is usually for large 3 phase induction motors.
Rotor has a winding the same as a stator and the end of each phase is connected to a
slip ring.
Compared to squirrel cage rotors, wound rotor motors are expensive and require
maintenance of the slip rings and brushes, so it is not so common in the industry
applications.
PRINCIPLE OF OPERATION
An AC current is applied in the stator armature which generates a flux in the
stator magnetic circuit.
This flux induces an emf in the conducting bars other f rotor as they are “cut” by the
flux while the magnet is being moved (E = BVL (Faraday’s Law))
A current flows in the rotor circuit due to the induced emf, which in term
produces a force, (F = BIL) can be changed to the torque as the output
In a 3-phase induction motor, the three-phase currents Ia, ib, and ic, each of equal magnitude, but differing in phase by 120°. Each phase current produces a magnetic flux and there is physical 120 °shift between each flux. The total flux in the machine is the sum of the three fluxes. The summation of the three ac fluxes results in a rotating flux, which turns with constant speed and has a constant amplitude. Such a magnetic flux
produced by balanced three phase currents flowing in thee-phase windings is called a
rotating magnetic flux or rotating magnetic field (RMF).RMF rotates with a constant
speed (Synchronous Speed). The existence of a RFM is an essential condition for the
operation of an induction motor.
If the stator is energized by an ac current, RMF is generated due to the applied current to the stator winding. This flux produces a magnetic field and the field revolves in the air gap between stator and rotor. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars. The interaction of the rotating flux and the rotor current generates a force that drives the motor and a torque is developed consequently. The torque is proportional with the flux density and the rotor If the stator is energized by an ac current, RMF is generated due to the applied current to the stator winding. This flux produces a magnetic field and the field revolves in the air gap between stator and rotor. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars. The interaction of the rotating flux and the rotor current generates a force that drives the motor and a torque is developed consequently. The torque is proportional with the flux density and the rotor bar current (F=BLI). The motor speed is less than the synchronous speed. The direction of the rotation of the rotor is the same as the direction of the rotation of the revolving magnetic
field in the air gap.
However, for these currents to be induced, the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before.
This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor the (slip speed) to the speed of the rotating stator field. Due to this an induction motor is
sometimes referred to as an asynchronous machine.
SLIP
The relationship between the supply frequency, f, the number of poles, p, and the
synchronous speed (speed of rotating field), as is given by
n=120f/P
The stator magnetic field (rotating magnetic field) rotates at a speed, ns, the
synchronous speed. If, an n= speed of the rotor, the slip, s for an induction motor is defined
as
S= N-n/N
At standstill, rotor does not rotate , n = 0, so s = 1.
At synchronous speed, n= nS, s = 0
The mechanical speed of the rotor, in terms of slip and synchronous speed, is given by,
n=(1-s) ns
The frequency of Rotor Current and Voltage
With the rotor at stand-still, the frequency of the induced voltages and currents is the sameas that of the stator (supply) frequency, the.
If the rotor rotates at speed of n, then the relative speed is the slip speed:
nslip=ns-n
nslip is responsible for induction.
Hence, the frequency of the induced voltages and currents in the rotor is, fr= sfe
EQUIVALENT CIRCUIT
The induction motor consists of two magnetically connected systems namely,
stator and rotor. This is similar to a transformer that also has two magnetically connected systems namely primary and secondary windings. Also, the induction motor operates on thesame principle as the transformer. Hence, the induction motor is also called as
rotating transformer
The stator is supplied by a balanced three-phase voltage that drives a three-phase
current through the winding. This current induces a voltage in the rotor. The applied
voltage (V1) across phase A is equal to the sum of the
–induced voltage (E1).
–the voltage drop across the stator resistance (I1R1).
–voltage drop across the stator leakage reactance (I1 j X1).
Let
I1 = stator current/phase
R1 = stator winding resistance/phase
X1 = stator winding reactance/phase
RR = stator winding resistance/phase
XR = stator winding reactance/phase
IR = rotor current
V1 = applied voltage to the stator/phase
Io = Ic+Im (Im-magnetising component, Ic-core loss component).
Working Of Inducation Motors
Reviewed by Unknown
on
June 13, 2018
Rating:
Reviewed by Unknown
on
June 13, 2018
Rating:
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