Phasor Diagram of an Induction Motor Assignment Help

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Phasor Diagram of an Induction Motor:

Induction motor is analogous to the transformer. For the stator winding

V1  =- E1  + I1 (R1  + j X1 )

where  V1 = Supply voltage per phase,

I1 = Stator current per phase,

R1 = Stator resistance per phase,

X1 = Stator leakage reactance per phase, and

E1 = Voltage induced per phase in the stator.

and also,

E1 = 4.44 φ f  N1 kw1

Where φ = Mutual flux per pole,

f = Stator frequency,

N1 = Stator winding turns per phase, and

kw1 = Winding factor of stator.

We know that rotor winding is short circuited at the ends

 V2  = 0 = E2 s  - I2  ( R2  + j X 2 s )

where  I2 = Rotor current per phase,

R2 = Rotor resistance per phase, and

X2s = Leakage reactance of rotor at slip frequency per phase.

X 2s  = 2π f2  L2 ;

f2  = S f 

X 2 s  = 2π S f2  L2

X 2 s  = S X 2

where  X2 = Leakage reactance of rotor as stand fill per phase, and

E2s = Induced emf per phase in the rotor of induction motor.

E2 s  = 4.44 φ f2  N2  Kw2

= S E2

where  E2 = Induced emf in rotor per phase at stand still,

N2 = Rotor winding turns per phase, and

Kw2 = Winding factor of rotor.

From the Eqs. (39) and (41), we have

E1/ E2 = N1 Kw1 /N2 Kw2 =  N1′ / N2′ = a

where, N1′ and N2′ are the effective number of stator and rotor turns respectively and "a" is called the effective turns ratio of an induction motor.

| V1 | ≈ | E1 |

Since the value of R1 and X1 are small.

The exciting current in the induction motor is specified by

0  = I¯i  + ¯Im

Where Im  is the per phase value of the magnetising component of the exciting current and I¯i is the per phase value of the loss component of the exciting current.

The no load current in induction motor is more than that of transformer due to the air gap among the stator and rotor winding.

From the current and voltage equations discussed above, we may develop the phasor diagram.

The rotor power factor angle at any slip, S is

θ  = tan -1  (S X 2  / R2         

2109_Phasor Diagram of an Induction Motor.jpeg

Figure: Phasor Diagram of an Induction Motor for a > 1 and S < 1


I2′ = (N2' /N1′ )I2  = (1/a) I2


1 = I¯0  + I¯2

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