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Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) PDF Download

3-Phase Induction Machine

  • Basically an induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.
  • 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 cage rotor motor.
  • In 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 industrial drives.
  • Induction motors are now the preferred choice for industrial motors due to their rugged construction, absence of brushes (which are required in most DC motors) and the ability to control the speed of the motor.
  • It is a single excited AC machine. Its stator winding is directly connected to AC source, whereas its rotor winding receives its energy from f stator by means of induction (i.e., transformer action).

Type of rotors Rotor 

  • Squirrel cage rotor
  • Wound rotor

Squirrel-Cage Rotor
In the squirrel-cage rotor, the rotor winding consists of single copper or aluminium bars 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 attached to the shaft in the sides of rotor to cool the circuit.
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Wound Rotor
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

  • In the wound rotor, an insulated 3-phase winding similar to the stator winding wound for the same number of poles as 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.
  • 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 industry applications.
  • Rotor has a winding the same as stator and the end of each phase is connected to a slip ring.

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 of 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.
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

  • 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 summation of the three ac fluxes results in a rotating flux, which turns with constant speed and has 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). Existence of a RFM is an essential condition for the operation of an induction motor. If stator is energized by an ac current, RMF is generated due to the applied current to the stator winding.
  • This flux produces 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.

POWER FLOW
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Per phase induced emf In stator winding, E1 = 4.44 Nlf1φkω1 volt
In rotor winding, E2 = 4.44 N2f2φkω2 volt
where kω1 and kω2 = Winding factors of stator and rotor winding
N1 = Number of turns in stator winding
N2 = Number of turns in rotor winding
f1 and f2 = Frequencies of supply in stator and rotor windings respectively.
Slip: The difference between the synchronous speed (Ns) and the actual rotor speed (Nr).
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
where, Ns = Synchronous speed
Nr = Rotor speed

Equivalent Circuit of an Induction Motor:
The energy is transferred from primary (stator) winding to secondary (rotor) winding entirely by induction therefore, induction motor is essentially a transformer. At standstill, the induction motor is actually a static transformer having its secondary (rotor) winding short-circuited.
Here, stator emf per phase Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
where, N1 = Number of stator turns per phase
φ = Flux per pole
kω1 = Stator winding factor
Rotor emf at standstill Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
where, Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)= Effective stator turns per phase = Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) = Effective rotor turns per phase = Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
a = Reduction factor
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
sE2 = I2R2 + jI2sX2 or Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Rotor equivalent circuit
Study Notes For Three-Phase Induction Machines - 1 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Rotor Torque: The torque developed by the rotor of an induction motor is directly proportional to (a) rotor current l2 (b) stator flux per pole φ and (c) power factor of the rotor circuit cos φ2
∵ T ∝ φl2 cos φ2
But E ∝ φ
T ∝ E2l2 cos φ2
or T = kE2l2 cos φ2 where k is constant.
Rotor Frequency: In rotor the frequency of current and voltage must be same as the supply frequency
fr = sf
where, f = Supply frequency.

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FAQs on Study Notes For Three-Phase Induction Machines - 1 - Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

1. What is a three-phase induction machine?
Ans. A three-phase induction machine, also known as an induction motor, is a type of electric motor that operates on the principle of electromagnetic induction. It converts electrical energy into mechanical energy by inducing a rotating magnetic field in its stator windings, which interacts with the rotor to produce motion.
2. How does a three-phase induction machine work?
Ans. A three-phase induction machine operates through the interaction of a rotating magnetic field produced by the stator windings and the conductive rotor. When three-phase alternating current is supplied to the stator windings, it creates a rotating magnetic field. This magnetic field induces currents in the rotor bars, producing another magnetic field. The interaction between the stator and rotor magnetic fields results in the generation of torque, causing the rotor to rotate.
3. What are the advantages of three-phase induction machines?
Ans. Three-phase induction machines have several advantages. Firstly, they have a simple and robust construction, making them reliable and cost-effective. They also have a high starting torque and can operate under varying load conditions. Additionally, they do not require brushes or commutators, eliminating the need for maintenance associated with these components. Furthermore, they have a high efficiency and can operate at high speeds.
4. What are the applications of three-phase induction machines?
Ans. Three-phase induction machines find widespread applications in various industries. They are commonly used in pumps, fans, compressors, and conveyors, where continuous and efficient operation is required. They are also extensively utilized in the industrial and manufacturing sectors for driving machinery, such as machine tools and production lines. Additionally, they are employed in HVAC systems, electric vehicles, and renewable energy systems.
5. How can the efficiency of a three-phase induction machine be improved?
Ans. The efficiency of a three-phase induction machine can be improved through various measures. One approach is to optimize the design of the machine, such as selecting appropriate materials and dimensions for the stator and rotor. Another method is to improve the power factor by using power factor correction techniques. Additionally, reducing losses in the machine, such as copper and iron losses, can enhance its efficiency. Regular maintenance, including proper lubrication and alignment, also contributes to improving efficiency.
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