Generator characteristic:Voltage Build up of a Shunt Generator

Voltage Build up of a Shunt Generator

Before loading a shunt generator, it is allowed to build up its voltage. Usually, there is always present some residual magnetism in the poles, hence a small e.m.f. is produced initially.

This e.m.f. circulates a small current in the field circuit which increases the pole flux (provided field circuit is properly connected to armature, otherwise this current may wipe off the residual magnetism). When flux is increased, generated e.m.f. is increased which further increases the flux and so on. As shown in Fig. 28.17, Oa is the induced e.m.f. due to residual magnetism which appears across the field circuit and causes a field current Ob to flow. This current aids residual flux and hence produces, a larger induced e.m.f. Oc. In turn, this increased e.m.f. Oc causes an even larger current Od which creates more flux for a still larger e.m.f. and so on.

Now, the generated e.m.f. in the armature has

(a) to supply the ohmic drop If Rsh in the winding and (b) to overcome the opposing self-induced e.m.f. in the field coil i.e. L. (d If / dt)because field coils have appreciable self-inductance.

If (and so long as), the generated e.m.f. is in excess of the ohmic drop If Rsh, energy would continue being stored in the pole fields. For example, as shown in Fig. 28.17, corresponding to field current OA, the generated e.m.f. is AC. Out of this, AB goes to supply ohmic drop If Rsh and BC goes to overcome self-induced e.m.f. in the coil. Corresponding to If = OF, whole of the generated e.m.f. is used to overcome the ohmic drop. None is left to overcome L.dIf /dt. Hence no energy is stored in the pole fields. Consequently, there is no further increase in pole flux and the generated e.m.f. With the given shunt field resistance represented by line OP, the maximum voltage to which the machine will build up is OE. If resistance is de- creased, it will built up to a some- what higher voltage. OR represents the resistance known as critical resistance. If shunt field resistance is greater than this value, the generator will fail to excite.

Conditions for Build-up of a Shunt Generator

We may summarize the conditions necessary for the build-up of a (self-excited) short generator as follows :

1. There must be some residual magnetism in the generator poles.

2. For the given direction of rotation, the shunt field coils should be correctly connected to the armature i.e. they should be so connected that the induced current reinforces the e.m.f. produced initially due to residual magnetism.

3. If excited on open circuit, its shunt field resistance should be less than the critical resistance (which can be found from its O.C.C.)

4. If excited on load, then its shunt field resistance should be more than a certain minimum value of resistance which is given by internal characteristic (Art 28.11).

Other Factors Affecting Voltage Building of a DC Generator

In addition to the factors mentioned above, there are some other factors which affect the voltage building of a self-excited d.c. generator. These factors are (i) reversed shunt field connection (ii) reversed rotation and (iii) reversed residual magnetism. These adverse effects would be explained with the help of Fig. 28.18 and the right-hand rule for finding the direction of the coil flux. For the sake of simplicity, only one field pole has been shown in the Fig. 28.18.

Fig. 28.18 (a) represents the normal operation, the prime mover rotation is clockwise and both the residual flux FR and the field flux FF are directed to the left.

Fig. 28.18 (b) shows reversed connection of the field circuit which causes FF to oppose FR. Consequently, the generator voltage builds down from its original residual value.

In Fig. 28.18 (c), reversed armature rotation causes the reversal of the voltage produced by the residual magnetism. Even though the field coil connections are correct, the reversed field current flow causes FF to oppose FR so that the voltage builds down from its original residual value.

Fig. 28.18 (d) shows the case when due to some reason the residual magnetism gets reversed. Hence, the armature voltage is also reversed which further reverses the field current. Consequently, both FF and FR are reversed but are directed to the right as shown. Under this condition, the voltage buildup is in the reversed direction. Obviously, the generator will operate at rated voltage but with reversed polarity.

If desired, the reversed polarity can be corrected by using an external d.c. source to remagnetise the field poles in the correct direction. This procedure is known as field flashing.

reaction, the shunt field current is given by Ish (eff.) = Ish - 0.003 Ia. Armature resistance, includ- ing brush contact resistance, is 0.1 W. What will be the p.d. on open circuit at the same speed ?

Solution. As shown in Fig. 28.19, the O.C.C. has been plot- ted from the given data.

Voltage drop due to armature resistance = 300 ´ 0.1 = 30 V.

Reduction of field current due to armature reaction = 0.003 ´ 300 = 0.9 A.

Any point A is taken on the O.C.C. A vertical distance AB = 30 V is taken and then the horizontal line BC = 0.9 A is drawn thus completing triangle ABC which is known as drop reaction triangle. Then, point C lies on the 300 ampere load saturation curve. This curve can be drawn by finding such similar points like C¢ etc.

From point L representing 600 V, a horizontal line is drawn cutting the load saturation curve at D. Join OD. Current corresponding to point D is 9.4 A. The slope of If the line OD gives the value of shunt resistance to give 600 V with 300 amperes of armature current.

External Characteristic

After becoming familiar with the no-load characteristic of a shunt generator, we will now proceed to find its external characteristic (V/I) when loaded. It is found that if after building up, a shunt generator is loaded, then its terminal voltage V drops with increase in load current. Such a drop in voltage is undesirable especially when the generator is supplying current for light and power for which purpose it is desirable that V should remain practically constant and independent of the load. This condition of constant voltage is almost impossible to be fulfilled with a shunt generator unless the field current is being automatically adjusted by an automatic regulator. Without such regulation terminal voltage drops considerably as the load on the generator is increased. These are three main reasons for the drop in terminal voltage of a shunt generator when under load.

(i) Armature resistance drop :

As the load current increases, more and more voltage is consumed in the ohmic resistance of the armature circuit. Hence, the terminal voltage V = E - Ia Ra is decreased where E is the induced e.m.f. in the armature under load condition.

(ii) Armature reaction drop

Due to the demagnetising effect of armature reaction, pole flux is weakened and so the induced

e.m.f. in the armature is decreased.

(iii) The drop in terminal voltage V due to (i) and (ii) results in a decreased field current If which further reduces the induced e.m.f.

For obtaining the relation between the terminal voltage and load current, the generator is connected as shown in Fig. 28.20 (a).

The shunt generator is first excited on no-load so that it gives its full open circuit voltage = Oa [Fig. 28.20 (b)]. Then, the load is gradually applied and, at suitable intervals, the terminal voltage V (as read by the voltmeter) and the load current I (as read by the ammeter A2) are noted. The field current as recorded by ammeter A1 is kept constant by a rheostat (because during the test, due to heating, shunt field resistance is increased). By plotting these readings, the external characteristic of Fig. 28.20 (b) is obtained. The portion ab is the working part of this curve. Over this part, if the load resistance is decreased, load current is increased as usual, although this results in a comparatively small additional drop in voltage. These conditions hold good till point b is reached. This point is known as breakdown point. It is found that beyond this point (where load is maximum = OB) any effort to increase load current by further decreasing load resistance results in decreased load current (like OA) due to a very rapid decrease in terminal voltage.

We will discuss the reason for this unusual behaviour of the generator in more details. Over the earlier portion ab [Fig. 28.20 (b)] where the load current is comparatively small, when external load resistance is decreased, it results in increased load current as might be expected keeping Ohm’s law in mind. It should not, however, be forgotten that due to increase in load current, V is also decreased somewhat due to the cause (iii) given above. But over the portion ab, the effect of decrease in load resistance predominates the effect of decrease in V because load current is relatively small.

At point b, generator is delivering a very large current i.e. current which is many times greater than its normal current. If load resistance is decreased at this point so as to be able to draw a load current greater than OB, the current is increased momentarily. But due to the severe armature reaction for this heavy current and increased Ia Ra drop, the terminal voltage V is drastically reduced. The effect of this drastic reduction in V results in less load current ( = OA). In other words, over the portion bdc of the curve, the terminal voltage V decreases more rapidly than the load resistance. Hence, any, further decrease in load resistance actually causes a decrease in load current (it may seem to contravene Ohm’s law but this law is not applicable, here since V is not constant). As load resistance is decreased beyond point b, the curve turns back till when the generator is actually short- circuited, it cuts the current axis at point c. Here, terminal voltage V is reduced to zero, though there would be some value of E due to residual magnetism (Fig. 28.22).

Voltage Regulation

By voltage regulation of a generator is meant the change in its terminal voltage with the change in load current when it is running at a constant speed. If the change in voltage between no-load and full load is small, then the generator is said to have good regulation but if the change in voltage is large, then it has poor regulation. The voltage regulation of a d.c. generator is the change in voltage when the load is reduced from rated value to zero, expressed as percentage of the rated load voltage.

If no-load voltage of a certain generator is 240 V and rated-load voltage is 220 V, then, regn. = (240 - 220)/220 = 0.091 or 9.1 %

Internal or Total Characteristic

As defined before, internal characteristic gives the relation between E and Ia. Now in a shunt generator

Hence, E/Ia curve can be obtained from V/I curve as shown in Fig. 28.21. In this figure, ab represents the external characteristic as discussed above. The field resistance line OB is drawn as usual. The horizontal distances from OY line to the line OB give the values of field currents for different terminal voltages. If we add these distances horizontally to the external characteristic ab, then we get the curve for the total armature current i.e. dotted curve ac. For example, point d on ac is obtained by making gd = ef. The armature resistance drop line Or is then plotted as usual. If brush contact resistance is assumed constant, then armature voltage drop is proportional to the armature current. For any armature current = OK, armature voltage drop IaRa = mK. If we add these drops to the ordinates of curve ac, we get the internal characteristic. For example, St = mK. The point t lies on the internal characteristic. Other points like t can be found similarly at different armature currents as the total characteristic can be drawn.

It may be noted here, in passing, that product EIa gives the total power developed within the armature. Some of this power goes to meet I2 R losses in armature and shunt field windings and the rest appears as output.

As explained in Art 28.10 if load resistance is decreased, the armature current increases up to a certain load current value. After that, any decrease in load resistance is not accompanied by increase in load current. Rather, it is decreased and the curve turns back as shown in Fig. 28.22. If the load resistance is too small, then the generator is short-circuited and there is no generated e.m.f. due to heavy demagnetisation of main poles.

Line OP is tangential to the internal characteristic MB and its slope gives the value of the mini- mum resistance with which the generator will excite if excited on load.

In this generator, because field windings are in series with the armature [Fig. 28.23 (a)], they carry full armature current Ia. As Ia is increased, flux and hence generated e.m.f. is also increased as shown by the curve. Curve Oa is the O.C.C. The extra exciting current necessary to neutralize the weakening effect of armature reaction at full load is given by the horizontal distance ab. Hence, point b is on the internal characteristic. If the ordinate bc = gh = armature voltage drop, then point c lies on the external characteristic [Fig. 28.23 (b)].

It will be noticed that a series generator has rising voltage characteristic i.e. with   increase in load, its voltage is also increased. But it is seen that at high loads, the voltage starts decreasing due to excessive demagnetising effects of armature reaction. In fact, terminal voltage starts decreasing as load current is increased as shown by the dotted curve. For a load current OC¢, the terminal voltage is reduced to zero as shown.