Characteristics of DC generators

1. Magnetic or no-load saturation characteristic or open-circuit characteristic (O.C.C) :-
            

       This curve shows the relation between the generated emf at no-load (E₀ ) and the field current (I_f) at constant speed. The shape of O.C.C  curve is practically the same for all types generators , whether separately or self-excited.

2. Internal or total characteristic (E/I_a) :-

      This curve shows the relation between the generated emf on load (E) and armature current (I_a). The emf E will be less than E₀ because of the effects of armature reaction. So this curve will stay below the open circuit characteristics.

3. External characteristic (V/I_L) :-

      This curve shows the relation between the terminal voltage (V) and load current (I_L).

The terminal voltage V will be less than E because of voltage drop in the armature circuit.

So this curve will stay below the internal characteristic.  

  

_{Characteristics of Separately Excited DC Generator :-}

O.C.C :-

• In separately excited D.C generator, the field winding is separately excited by external D.C source.
• The generator is driven by a prime mover at rated speed, say n rps. With switch S in opened condition, field is excited via a potential divider connection from a separate d.c source.
• So field current is gradually increased and then field flux increases.
• The voltmeter V connected across the armature terminals of the machine will record the generated emf (E_G)
  
• So if the field current is increased then E_G will increase and speed is constant. E_G versus I_f plot at constant speed n is shown in figure (b)
• It may be noted that even when there is no field current, a small voltage (OD) is generated due to residual flux. If field current is increased, φ increases linearly initially and O.C.C follows a straight line.
  

Internal characteristic & External characteristic :-

            The operation considered here assumes that the armature is driven at constant speed and the field excitation is adjusted to give rated voltage at no-load . If there were no armature reaction and no armature voltage drop ,then this voltage would have remained constant as show in fig (c) by the dotted horizontal line (I). But when the generator is loaded , the voltage falls due to these two causes. As a result giving slightly bending characteristic. If we subtract from E₀, the values of voltage drop due to armature reaction for different loads, then we get the value of E the emf actually induced in the armature under load conditions. Therefore curve (II) is plotted and is known as the internal characteristic. 

The straight line (oa) represents the (I_aR_a) drops corresponding to different armature currents. If we subtract from E , the armature drop (I_aR_a), so we get terminal voltage (V_L). Therefore curve (III) represents the external characteristic and is obtained by subtracting ordinates of line (oa) from those the curve (II)

                                                       V= E-I_aR_a
             

Characteristics of DC Shunt Generator :- 

O.C.C :- 
            Let us first assume that the field is unexcited that is switch s is open ,with the field open. If the generator is run at rated voltage , at rated speed initially the field current is 0 (zero) and the generated voltage will be equal to the residual voltage(od). And if field is current flows then the generated voltage will follow the O.C.C of the separately excited machine. As soon as the switch is closed there is a current I_f that will flow in the field circuit its magnitude will be equal to the generated terminal voltage which is same as M and op is field resistance characteristics.(refer to figure : e)

             
External characteristic & Internal characteristic :-
          The shunt generator is first excited on no-load so that it gives its full open circuit voltage (oa). Then, the load is gradually applied and, at suitable intervals, the terminal voltage(V_L) (as read by the voltmeter) and the load current (I_L)(as read by the ammeter A₂) are noted . The field current as recorded by ammeter (A₁), is kept constant by a rheostat. By plotting these reading then we get External characteristic fig (f).
 The portion (ab) is working part of this curve. Over this part, if the load resistance is decreased (due to more load resistances getting connect in parallel) therefore load current is increased as usual, although this results in a comparatively small additional drop in voltage. These condition hold good till point (b) is reached. This point is known as break-down 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.Internal characteristic is obtained from the external characteristic by adding I_aR_a.

Characteristics of DC Series Generator :-

Characteristics :-

• No load test is conducted by exciting the series field separately from a low voltage source. It is the saturation curve V_OC vs If = Ia drawn in (Fig. h) curve 1. 
• Internal characteristic ,It is the induced emf E_a which is less than V_OC by the armature reaction voltage drop. The voltage drop sharply increases with saturation and E_a beings to reduce; curve 2. 
• External characteristic lies below lies below curve 2 by I_a (R_a + R_se) voltage drop, curve 3.

Characteristics of DC Compound-wound Generator :-

Characteristics:- 

       A shunt generator is unsuitable where constancy of terminal voltage is essential, because its terminal voltage decreases as the load on it increases. This decrease in V is particularly objectionable for lighting circuit where even slight change in the voltage makes an appreciable change in the candle power of the incandescent lamps. A shunt generator may be made to supply substantially constant voltage (or even a rise in voltage as the load increases) by adding to it a few turns joined in series with either the armature or the load (Fig. j). These turns are so connected as to aid to shunt turns when the generator supplies load. As the load current increases, the current through the series windings also increase thereby increasing the flux. Due to the increase in flux, induced e.m.f. is also increased. By adjusting the number of series turns (or Compound-wound generator series amp-turns), this increase in e.m.f. can be made to balance the combined voltage drop in the generator due to armature reaction and the armature drop. Hence, V remains practically constant which means that field current is also almost unchanged. We have already discussed the three causes which decrease the terminal voltage of a shunt generator . Out of these three, the first two are neutralized by the series field amp-turns and the third one, therefore, does not occur.

        If the series field amp-turns are such as to produce the same voltage at rated load as at no-load, then the generator is flat-compounded. It should be noted, however, that even in the case of a flat-compounded generator, the voltage is not constant from no-load to rated- load. At half the load, the voltage is actually greater than the rated voltage as seen from Fig. j.

If the series field amp-turns are such that the rated-load voltage is greater than the no-load voltage, then generator is over-compounded. If rated-load voltage is less than the no-load voltage, then the generator is under-compounded but such generators are seldom used.

For short distances such as in hotels and office buildings, flat-compound generators are used because the loss of voltage over small lengths of the feeder is negligible. But when it is necessary to maintain a constant voltage then an over-compounded generator, which combines the functions of a Fig. j generator and a booster, is invariably used.