Determination of Voltage Regulation

DETERMINATION OF VOLTAGE REGULATION

In the case of small machines, the regulation may be found by direct loading. The procedure is as follows:

The alternator is driven at synchronous speed and the terminal voltage is adjusted to its rated value V. The load is varied upto the rated values at desired p.f. Then the entire load is removed while the speed and field excitation are kept constant. The open‒circuit or no‒load voltage E₀ is read. Hence, regulation can be found from,

% regulation up = [ (E₀‒V) / V ] × 100

In the case of large machines, the cost of finding the regulation by direct loading becomes prohibitive. Hence we move on to indirect method. The indirect methods are given in section 4.14.2.

Requirements of Indirect Method

These are the tests required for the indirect method

1. Open circuit / No‒load test

2. Short‒circuit test (but zero power factor lagging characteristics for potier method)

3. Armature resistance (R_a)

1. Open Circuit Test

Procedure to conduct open circuit test is as follows:

(i) The prime mover is started and adjusted to make the alternator run at the synchronous speed.

(ii) Keeping rheostat in the field circuit maximum, the DC supply is switched on.

(iii) The T.P.S.T switch in the armature circuit is to be kept open.

(iv) With the help of rheostat, field current is varied from its minimum value to the rated value. Due to this, flux increases, increasing the induced emf. Hence voltmeter reading, which is measuring line value of open circuit voltage increases. For various values of field current, these voltmeter readings are observed and OCC curve is drawn.

2. Short Circuit Test

After completing the open circuit test observations, the field rheostat is brought to maximum position, reducing field current to a minimum value. The TPST switch is closed. As ammeter has negligible resistance, the armature gets short circuited. Then the field excitation is gradually increased till full load current is obtained through armature winding. This can be observed on the ammeter connected in the armature circuit.

The graph of short circuit armature current against field current is plotted from the observation table of short circuit test. This graph is called Short Circuit Characteristics, SCC.

3. Armature Resistance (R_a)

Armature resistance (R_a) per phase can be measured directly by voltmeter and Ammeter method or by using whetstone bridge method. However, under working conditions, the effective value of R_a is increased due to 'skin effect'. Generally, the value of armature resistance (R_a) 1.6 times the DC value is taken.

Types of Indirect Method

1. Synchronous Impedance or E.M.F method

2. The Ampere turn or M.M.F method

3. Zero Power Factor or Potier method

4. American Standard Association (ASA) method

Here, we shall discuss about only E.M.F and M.M.F method.

1. Synchronous Impedance Method or EMF Method

This method involves the following four steps,

(i) As per the given data, plot the open‒circuit characteristic (OCC) as shown in fall Fig. 4.10.

(ii) Plot the short‒circuit characteristic (SCC) from the data given by short‒circuit test. Both these curves are drawn on a common field ‒ current base.

Consider a field current If. The open circuit voltage corresponding to this field current is If. When winding is short‒ circuited, the terminal voltage is zero. Hence, it may be assumed that the whole of this voltage E₁ is being used to circulate the armature short circuit current I₁ against the synchronous impedance Z_S.

 E₁ = I₁Z_S

Here, the synchronous impedance is given by,

 Z_S = Open circuit voltage (E₁) / Short‒circuit current (I₁)      . ……..(4.7)

(iii) Find the Synchronous reactance.

We know that, Z_S = √[R_a²+X_S²]               ……..(4.8)

Therefore, X_S = √[Z_S²‒R_a²]        ………. (4.9)

(iv) After finding R_a and X_S vector diagrams for any load and any power factor can be drawn. The vector diagram as shown in Fig. 4.11.

From the vector diagram,

 OD=E₀

 ∴ E₀=√[OB²+BD²]

∴ E₀ = √[OB²+BD²]

OB = V cos ϕ + IR_a

BD = V sin ϕ + IX_a

Therefore, E₀ = √ [ (Vcosϕ + IR_a)² + (Vsinϕ + IX_s)² ]

% regulation 'up' = [ (E₀‒V) / V ] × 100

2. Ampere Turn or MMF Method

This method also utilises OC and SC data, but it is the converse of the emf method in the sense that the change in potential drop on load is entirely due to armature reaction. This is shown in Fig. 4.12.

Now, field Ampere turn required to produce a voltage of V on full‒load is the vector sum of the following terms.

(i) Field A.T required to produce V + IR_acosϕ on no load. This can be found from OCC and

(ii) Field A.T required to overcome the demagnetising effect of armature reaction on full load. This value is found from short circuit test.

From OC test field current I_f' is determined to give rated voltage V on no‒load, neglecting armature resistance drop and I_f" is determined to cause short‒circuit current, equal to full‒load current, on short‒circuit.

Cases of Power Factor

Now, let us consider a general case when the alternator supplies full‒load current at a power factor of cos ϕ. The regulation for any load power factor can be found out as follows.

(i) Draw OL representing I_f' to give full‒load rated voltage, V. (or more exactly equal to V+ IR_acosϕ) Ref Fig. 4.13 (a).

(ii) Draw LM at an angle (90° ± ϕ) representing I_f" to give full‒load current on short circuit. (Ref Fig. 4.13 (b) and (c).

( +ve sign for lagging p.f and ‒ve sign for leading p.f)

(iii) Find field current I_f, measuring OM, which will give open circuit emf E₀ which can be determined from open circuit characteristic (OCC).

(iv) The percentage regulation can be obtained from the following relation.

% regulation up = [ (E₀‒V) / V ] × 100

Regulation given by this method is much lower than that given by the synchronous impedance method. But, it is nearer the correct value. This method is called then "Optimistic method.".