eqckt indmotor
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8/6/2013 PRB/EEE Dept/SCE 1 Phasor diagram of induction motor (Induction motor as a Generalized Transformer)
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induction motor
Transcript of eqckt indmotor
8/6/2013 PRB/EEE Dept/SCE 1
Phasor diagram of induction motor
(Induction motor as a Generalized Transformer)
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Induction motor as a generalized transformer
In a 3-phase induction motor ,the stator winding is
connected to 3-phase ac supply and rotor winding is
short circuited.
The energy is magnetically transferred (mutual
induction principle) from the stator winding to the
short circuited rotor winding.
Therefore, an induction motor may be considered to
be a transformer with a rotating secondary (Short
circuited)
Stator winding ---transformer’s primary winding
Rotor winding ----- transformer’s secondary winding
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Equivalent circuit of an induction motor in per phase quantity
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Stator circuit
V1 = applied voltage per phase to the stator
R1, X1 = stator resistance and stator leakage reactance
per phase respectively
V1→ flux, φ→ links stator and rotor winding → self
induced emf E1 and mutually induced emf
Er =SE2=SKE1
Where, K = transformation ratio
K = (E2/E1) = (T2/T1)
In induction motor
(I1/I2) ≠ K because of distributed winding
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I1 causes voltage drop in R1 and X1
V1 = E1 + I1R1+jX1 ---------------------- phasor sum
When the motor is at no load, the stator winding
draws a current Io
Io = Iw + Im ------------ phasor sum
Iw = working component (no load loss)
Im – magnetizing component (setup flux in the core)
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Rotor circuit
R2, X2 = rotor resistance and reactance
SX2= rotor reactance at any slip
Er→ set up I2 in the rotor
Er = I2R2 + jI2SX2 ---------------phasor sum
The rotor current I2 is reflected as I’2 in the stator
I’2 = KI2
I1 = Io + I’2 ------------ phasor sum
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Phasor diagram of 3-phase induction motor
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V1
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V1
Ф
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V1
Ф
E1
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V1
Ф
E1
Im
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V1
Ф
E1
Im
Iw
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V1
Ф
E1
Im
Iw Io
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V1
Ф
E1
Im
Iw Io
Er
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V1
Ф
E1
Im
Iw Io
Er
I2
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V1
Ф
E1
Im
Iw Io
Er
I2
I’2
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V1
Ф
E1
Im
Iw Io
Er
I2
I’2 I1
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V1
Ф
E1
Im
Iw Io
Er
I2
I’2 I1
I1R1
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V1
Ф
E1
Im
Iw Io
Er
I2
I’2 I1
I1R1
I1X1
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V1
Ф
E1
Im
Iw Io
Er
I2
I’2 I1
I1R1
I1X1
I2R2
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V1
Ф
E1
Im
Iw Io
Er
I2
I’2 I1
I1R1
I1X1
I2R2
I2SX2
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Ф2
Ф1
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Equivalent circuit of
three-phase induction
motor
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The machine parameters can be
represented in an electric circuit is
known as equivalent circuit.
Equivalent circuit
PurposeThe purpose of deriving an equivalent
circuit for a 3-phase induction motor is to
represent it by a simple electric circuit for
easy analysis.8/6/2013 PRB/EEE Dept/SCE
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Step 2 : Equivalent circuit to represent stator core
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Equivalent circuit of the rotor
In a transformer, secondary is connected to electrical load. But, in a motor, the
rotor drives the mechanical load. Therefore mechanical load of the motor is
replaced by the equivalent electrical load.
Equivalent circuit per phase of the rotor at slip S
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Step 3 : Convert actual rotating circuit into an equivalent stationary rotor circuit
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Exact equivalent circuit
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Approximate equivalent circuit
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Step 6 Approximate equivalent circuit referred to stator
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Drawbacks
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