3-Phase Induction Machine
Type of rotors 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.
Wound Rotor
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.
POWER FLOW
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).
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
where, N1 = Number of stator turns per phase
φ = Flux per pole
kω1 = Stator winding factor
Rotor emf at standstill
where, = Effective stator turns per phase =
= Effective rotor turns per phase =
a = Reduction factor
sE2 = I2R2 + jI2sX2 or
Rotor equivalent circuit
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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1. What is a three-phase induction machine? |
2. How does a three-phase induction machine work? |
3. What are the advantages of three-phase induction machines? |
4. What are the applications of three-phase induction machines? |
5. How can the efficiency of a three-phase induction machine be improved? |
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