The self excited induction generator has been made with capacitor as self excitation and voltage built up has been shown with experimentation.

Induction generators are increasingly being used in non conventional energy systems such as wind, micro\mini hydro, etc.

An induction machine is in generating mode for s<0 (negative slip). S being the synchronous speed. An induction generator is synchronous in nature because of which it is commonly used as windmill which runs at non-fixed speed.

The prime- mover must be provided with automatic control to increase the generator when it is required to meet the increased load.

• For prime-mover speed above synchronous speed, slip becomes negative. As a consequence, rotor emf and rotor current, torque and power all attains a negative values. This implies that for negative slip, electric torque developed is negative i.e., opposite to the prime-mover torque.

• Under this condition machine must act as a generator and delivers its generated power to the supply mains from which it was taking power when working as a 3- induction motor. In other words when rotor speed, slip becomes negative and 3- machine begins to operate as 3- induction generator.

Characteristics of induction motor at different slip

Figure 1.2 Characteristics of induction motor at different slip

EXTERNALLY-EXCITED INDUCTION GENERATOR

SELF-EXCITED INDUCTION GENERATOR

Equivalent Ckt. Diag. of Self Excited Induction Generaor

On the basis of rotor construction, induction generators are two types (i.e. the wound rotor induction generator and squirrel cage induction generator). Depending upon the prime movers used (constant speed or variable speed) and locations (near to the power network or at isolated places), generating schemes can be broadly classified as under:

Constant-speed constant frequency (CSCF) Variable-speed constant frequency(VSCF)Variable-speed variable –frequency(VSCF)

Performance comparison for different design need to be made using the corresponding capacitor connected across the terminals.

Due to the use of same capacitance value for different design there is considerable variations in the terminal voltage level. In the most of the cases peak of the generator output power lies outside the permitted voltage band. And therefore, in general the maximum power capability of the generator (Pm) can not be utilised. The maximum usable power of generator is then the power at which the terminal voltage falls to minimum permissible level. Since voltage level as defined with respect to alternator or dc generator is not applicable to SEIG, it is required to set proper criterion for its voltage regulation.

The equivalent circuit parameters of polyphase induction motors can be determined from no load test and stator winding D.C resistance.

The object of this test is to describe the method of determining the parameters of these tests.

The induction motor is made to run at no load at rated voltage and frequency per phase of applied stator voltage per unit Vnl , input current per unit Inl and input power per unit are recorded. From the instruments readings at no-load, stator no-load impedance,

Znl = Vnl / Inl And stator no load resistance

Rnl = Pnl / I2 nl

Xnl = ( Znl2 – R2

nl )1/2 .

The rotational losses Pr are usually assumed constant and can be obtained from the relation

Pr = M ( Pnl – I2nl * R1 )

Where M is the no of stator phases and R1 is the per phase stator resistance.

Induction motor equivalent circuit for no-load test.

Block rotor test, similar to the shot circuit test on a transformer, is performed on the induction motor to calculate its leakage impedance.

For performing this test, the rotor shaft is blocked by belt-pulley arrangement or by hand.

Now, balance polyphase voltages at rated frequency are applied to the stator terminals through a polyphase variac. This supply voltage is adjusted till rated current flows in the stator winding. Per phase values of applied voltage Vbr , input current Ibr and the input power Pbr are recorded.

From the instruments readings at block rotor test, the parameters can be obtained as under

Zbr = Vbr / Ibr

And stator no load resistance

Rbr = Pbr / I2 br

Xbr = ( Zbr2 – R2

br )1/2 .

Induction motor equivalent circuit for block rotor test

SATURATION CHARACTERISTICS OF SEIG

The result is proposed for the study of steady state performance of the SEIG system. Theoretical results are presented for the variety of operating conditions along with the experimental ones ; the steady state analysis is extended to determine the value of capacitor required for the starting of induction motor by hit and trial method.

The increasing importance of fuel saving has been responsible for the revival of interest in so-called alternative source of energy.

Thus, the drive towards the decentralization of power generation and increasing use of non-conventional energy sources such as wind energy, bio-gas, solar and hydro potential, etc. has become essential to adopt a low cost generating system capable of operating in the remote areas, and in conjunction with the variety of prime movers.

The analysis of steady-state performance is important for ensuring good quality power and assessing the suitability of the configuration for a particular application, while the analysis during transient conditions helps in determining the induction generator technology for electric power generation utilizing non-conventional energy resources.

The induction generators are being considered as an alternative choice to the well-developed synchronous generators because of their lower unit cost, inherent ruggedness, operational and maintenance simplicity.

The induction generator’s ability to generate power at varying speed facilitates its application in various modes such as self-excited stand-alone (isolated) mode; in parallel with synchronous generator to supplement the local load, and in grid-connected mode.