Question & Answers About Electrical Machines-II (part-3)

21. Upon what factors does the load angle depend?

The magnitude of load angle   increases with increase in load. Further the load angle is positive during generator operation and negative during motor operation.

22. An Alternator is found to have its terminal voltage on load condition more than that on no load. What is the nature of the load connected?

The nature of the load is of leading power factor , load consisting of resistance and capacitive reactance.

23. Define the term voltage regulation of Alternator.

The voltage regulation of an Alternator is defined as the change in terminal voltage from no-load to load condition expressed as a fraction or percentage of terminal voltage at load condition ; the speed and excitation conditions remaining same.

Voltage regulation in percentage , URP = [(|E|-|U|)/|U| ]x i00


24. What is the necessity for predetermination of voltage regulation?

Most of the Alternators are manufactured with large power rating , hundreds of kW or MW, and also with large voltage rating upto 33kV. For Alternators of such power and voltage ratings conducting load test is not possible. Hence other indirect methods of testing are used and the performance like voltage regulation then can be predetermined at any desired load currents and power factors.

25. Name the various methods for predetermining the voltage regulation of 3-phase
Alternator.


The following are the three methods which are used to predetermine the voltage regulation of smooth cylindrical type Alternators
•    Synchronous impedance / EMF method
•    Ampere-turn / MMF method
•    Potier / ZPF method


26. How synchronous impedance is calculated from OCC and SCC?

Synchronous impedance is calculated from OCC and SCC as
|Zs| = E0/Isc(for same If)
A compromised value of Zs is normally estimated by taking the ratio of (E0/Isc)
at normal field current Ifn. A normal field current Ifn is one which gives rated voltage
Ur on open circuit.
|Zs| = Ur/Iscn


27. What are the advantages and disadvantages of estimating the voltage regulation of
an Alternator by EMF method?



Advantages:


•    Simple no load tests (for obtaining OCC and SCC) are to be conducted
•    Calculation procedure is much simpler

Disadvantages:

•    The value of voltage regulation obtained by this method is always higher than the actual value

28. Why is the synchronous impedance method of estimating voltage regulation considered as pessimistic method?

Compared to other methods, the value of voltage regulation obtained by the synchronous impedance method is always higher than the actual value and therefore this method is called the pessimistic method.

29. In what way does the ampere-turn method differ from synchronous impedance method?

The ampere-turn /MMF method is the converse of the EMF method in the sense that instead of having the phasor addition of various voltage drops/EMFs, here the phasor addition of MMF required for the voltage drops are carried out. Further the effect of saturation is also taken care of.

30. What are the test data required for predetermining the voltage regulation of an
Alternator by MMF method?

Data required for MMF method are :
•    Effective resistance per phase of the 3-phase winding R
•    Open circuit characteristic (OCC) at rated speed/frequency
•    Short circuit characteristic (SCC) at rated speed/frequency

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