Characteristics of D.C. Generators

The properties of generators are analyzed with the aid of features which give the relations between fundamental quantities which is responsible for the operation of a generator. These include the voltage across the generator terminals V, the field or exciting current I_f, the armature current I_a, and the speed of rotation N.

The following three are the most important characteristics of D.C. generators:

Circuit Characteristics (O.C.C). It demonstrates the relation between the field or I_r(Exciting current) and the armature induced no lead generated e.m.f, E₀ at a fixed given speed. Whether sel-excited or separately excited, all generators possess similarly shaped curves. In simpler words, the material of the electromagnets is represented by these magnetization curves.

• This characteristic also known as performance characteristic is sometimes referred to as voltage regulating curve.

• External characteristic establishes the reaction between load current I and terminal voltage V.

• The placement of the curve is below the internal characteristic; it considers the drop in voltage over its armature circuit resistance. To obtain V one needs to subtract IaRa from E.

• This characteristic plays a big role in deciding the aptness of a generator for a specific purpose.

There are two ways to find the external characteristic:-

1. Make measurements using an ammeter and voltmeter on a loaded generator.\
2. It can be calculated from the OC.C providing the armature and field resistance. In addition to this, the demagnetizing power of the armature needs to be known.

5.1. Separately Excited Generator

• The links of a separately excited generator is shown in the above figure where a battery is designated as the source of the current, although one can use and other constant voltage source.

Fig. 29.Connection for a separately excited generator.

The field circuit is provided with adjustable struggle and would normally contain a field switch and an ammeter, these being omitted from the diagram for simplicity. The armature is connected through two poles main switch to the bus bars, between which the load is connected.

5.1.1. About No-load saturation characteristic (or O.C.C.):

• If the generator is run at constant speed with the main switch open, and the terminal voltage is noted at various values of exciting or field current then the O.C.C. shown in. Fig. 30 can be plotted. This is also referred to as the ‘magnetization curve’ since the same graph shows, to a suitably chosen scale, the amount of magnetic flux, there being a constant relationship (depending upon speed of rotation) between flux and induced voltage.

• It will be noticed that a small voltage is produced when the field current is zero, this being due to a small amount of permanent magnetism in the field poles. This is called residual magnetism and is usually sufficient to produce 2 or 3 per cent of normal terminal voltage, although in some special cases it is purposely increased to 10 per cent or more.

• The first part of the curve is nearly straight and shows that the flux produced is relational to the exciting current; but after a certain point, saturation of the iron becomes appreciable as the curve departs from straight line form.

5.1.2. Internal and external characteristics (or load characteristics):

We can see in the figure which shows the load characteristics for a separately excited generator. The most significant is the ‘external characteristic’ (or total characteristic), which shows the way in which the terminal voltage (V) diverges as the load current is enlarged from zero to its full load value, the speed of rotation and exciting current being persistent.

The voltage drop (drop of volts) at any particular load current, indicated by the vertical distance between the external characteristic and the no-load voltage is brought about by two causes:

(i) Armature reaction which has a demagnetizing effect upon the field.

(ii) Resistance drop, this being the product of the armature current and the total
armature-circuit resistance, consisting of the armature resistance, interpole resistance and brush contact resistance.

The ‘internal characteristic’ is found by computing the resistance drop for a few values of current and adding this to the voltage shown by the external characteristic. The upright space between the internal characteristic and no load voltage then signifies the effect of armature reaction.

When the resistance of load is R, then voltage across its terminals is V = IR, where I represents the current, so that if the values of V corresponding to various values of I are calculated, the values will all lie upon a straight line such as OL in Fig. 31. The load current and terminal voltage corresponding to this resistance is given by the inter-section of the line OL with the external characteristics.