REGULATION OF ALTERNATORS


When an alternator is subjected to a varying load, the voltage at the armature terminals varies to certain extent, the amount of this variation determines the regulation of the machine.Phasor diagram of an alternator at lagging power factor is as follows.


Regulation is found by the following expression


Where V is the terminal voltage and E0  is the induced voltage. E0 is estimated by the following methods.
1) EMF Method
2) MMF Method
3) ZPF Method
4) ASA Method
5) Slip Test

1) EMF method (Synchronous impedance method)

Conduct tests to find I) OCC (up to 125% of rated voltage)   II) SCC (for rated current)             III) Armature resistance (per phase)


V = rated phase voltage
Isc = short circuit current corresponding to the field current producing the rated voltage
Synchronous impedance per phase,

For any load current I and phase angle Φ, find E0 as the vector sum of V, IRa and IXs

For lagging power factor

For unity power factor


2) MMF method (Ampere turns method)

Conduct tests to find I) OCC (up to 125% of rated voltage) II) SCC (for rated current)


STEPS:
1.  By suitable tests plot OCC and SCC
2.  From the OCC find the field current If1 to produce rated voltage, V.
3.  From SCC find the magnitude of field current If2 to produce the required   
     armature current.
4. Draw If2 at angle (90+Φ) from If1, where Φ is the phase angle of current from
    voltage. If current is leading, take the angle of If2 as (90-Φ).
5. Find the resultant field current, If and mark its magnitude on the field current  
    axis.
6. From OCC. find the voltage corresponding to If, which will be E0.
3) ZPF method (Potier method)
Conduct tests to find I) OCC (upto 125% of rated voltage) II) SCC (for rated current) III) ZPF (for rated current and rated voltage) IV) Armature Resistance (if required)



Steps:
1. By suitable tests plot OCC and SCC
2. Draw tangent to OCC (air gap line)
3. Conduct ZPF test at full load for rated voltage and fix the point B.
4. Draw the line BH with length equal to field current required to produce full load
    current at short circuit.
5. Draw HD parallel to the air gap line so as to touch the OCC.
6. Draw DE parallel to voltage axis. Now, DE represents voltage drop IXL and BE
    represents the field current required to overcome the effect of armature reaction.
Triangle BDE is called Potier triangle and XL is the Potier reactance

7. Find E from V, IXL and Φ. Consider Ra also if required. The expression to use is
8. Find field current corresponding to E.
9. Draw FG with magnitude equal to BE at angle (90+Ψ) from field current axis,
    where Ψ is the phase angle of current from voltage vector E (internal phase angle).
10. The resultant field current is given by OG. Mark this length on field current axis.
11. From OCC find the corresponding E0.


CONCLUSION:
The above mentioned methods are indirect methods. Each has it’s own features.
            EMF method is a pessimistic method because Zs is more than the actual impedance and  hence the regulation is more than actual values.
            MMF method gives the voltage regulation less than the actual value. Therefore it is called Optimistic method. The voltage regulation is less compared to actual because, the excitation to overcome armature reaction is determined on unsaturated part of the saturation curve.
            ZPF method gives the voltage regulation close to the actual value, therefore this is the accurate method.
            ASA method gives the actual voltage regulation because, drop due to saturation effect is also considered so that it is most accurate method.
Above all the methods for non salient pole machines and the slip test is for salient pole machines.


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