5.1. Separately Excited Generator
- Fig. 29 shows the connections of a separately excited generator, a battery being indicated as the source of exciting current, although any other constant voltage source could be used.
Fig. 29. Connection for a separately excited generator.
- The field circuit is provided with a variable resistance and would normally contain a field switch and an ammeter, these being omitted from the diagram for simplicity. The armature is connected through 2-pole main switch to the bus bars, between which the load is connected.
5.1.1. 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 ‘magnetisation 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 approximately straight and shows that the flux produced is proportional to the exciting current; but after a certain point, saturation of the iron becomes perceptible as the curve departs from straight line form.
Fig. 31. Load characteristics of a separately excited generator.
5.1.2. Internal and external characteristics (or load characteristics) :
- Load characteristics for a separately excited generator are shown in Fig. 31. The most important is the ‘external characteristic’ (or total characteristic), which indicates the way in which the terminal voltage (V) varies as the load current is increased from zero to its full load value, the speed of rotation and exciting current being constant.
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 demagnetising 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 obtained by calculating the resistance drop for a few values of current and adding this to the voltage shown by the external characteristic: The vertical distance between the internal characteristic and no load voltage then represents the effect of armature reaction alone.
- 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 are given by the inter-section of the line OL with the external characteristics.
Note. The great advantage of separate excitation over all other forms of excitation is that the current is entirely independent of the load current in the armature. It is however, rather inconvenient to have to depend upon a separate source of supply and, therefore, the method is used only in special cases, where the generator has to operate over a wide range of terminal voltage.