Basic Motor Theory (13)

Generator Characteristics

No Load Saturation Curve
A typical no load saturation curve is shown in Figure 23. This is similar to the magnetization curve mentioned previously except that it represents the entire magnetic circuit of a machine rather than one particular magnetic material.

Also, it has generator output voltage plotted against field current rather than flux density against magnetizing force. This can be done since generator voltage is directly proportional to the field flux and the number of turns is fixed. There is a different saturation curve for each speed. The lower straight line portion of the curve represents the air gap because the magnetic parts are not saturated. When the magnetic parts start to saturate, the curve bends over until complete saturation is reached. Then the curve becomes a straight line again.

Figure 23. No Load Saturation Curve


Figure 23.1 No Load Saturation Curve


Figure 23.2 No Load Saturation Curve

Generator Build Up
Generator build up usually refers to the gradual rise in voltage at the armature terminals when the machine is self-excited and operated at normal speed. This is illustrated in Figure 25 by referring to the field resistance line which shows how the field current varies as field voltage is varied. The slope of this line is the field resistance at a constant temperature. The voltage rise starts with the residual magnetism of the field iron. This provides a small voltage output E1 that is fed back to the field as 1. 1 increases the flux providing a slightly larger voltage, E2 . E2 causes 2 to flow. This process continues until the machine starts to saturate and stops at the point where the field resistance line intersects the saturation curve. If the speed of the machine is reduced so that the saturation curve becomes tangent to field resistance curve, the voltage will not build up. This is known as the critical speed. Also, at any given speed, if the field resistance is increased by addition of external resistance, a critical resistance can be reached.


Figure 25. DC Motor Curves

Voltage Output The voltage equation has been expressed as:
E = K1 S.

However, this is the generated voltage and part of it must be used to overcome the IR drops in the machine, which are caused by the resistance’s of the armature, series field, interpoles, brushes, etc. If these resistance’s are combined together and called armature resistance, then the voltage output at the generator terminals can be expressed as:
V = E – Ia Ra – K1 Flux S – Ia Ra

where:
E = generated voltage
Ia = armature current
Ra = armature circuit resistance
K1 = machine constants
Flux = flux per pole
S = speed.

External Characteristics
The curve showing the relationship between output voltage and output current is known as the external characteristic. Shown in Figure 24 are the external characteristic curves for generators with various types of excitation. If a generator, which is separately excited, is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current as shown. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. However, the demagnetizing component of armature reactions tends to decrease the flux, thus adding an additional factor, which decreases the output voltage.


Figure 24. DC Generator Curves

In a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. This is due to the fact that, since the output voltage is reduced because of the armature reaction effect and armature IR drop, the field voltage is also reduced which further reduces the flux. It can also be seen that beyond a certain critical value, the shunt generator shows a reversal in trend of current values with decreasing voltages. This point of maximum current output is known as the breakdown point. At the short circuit condition, the only flux available to produce current is the residual magnetism of the armature.
To build up the voltage on a series generator, the external circuit must be connected and its resistance reduced to a comparatively low value. Since the armature is in series with the field, load current must be flowing to obtain flux in the field. As the voltage and current rise the load resistance may be increased to its normal value. As the external characteristic curve shows, the voltage output starts at zero, reaches a peak, and then falls back to zero.
The combination of a shunt field and a series field gives the best external characteristic as illustrated in Figure 24. The voltage drop, which occurs in the shunt machine, is compensated for by the voltage rise, which occurs in the series machine. The addition of a sufficient number of series turns offsets the armature IR drop and armature reaction effect, resulting in a flat-compound generator, which has a nearly constant voltage. If more series turns are added, the voltage may rise with load and the machine is known as an over-compound generator.

Voltage Regulation
Voltage Regulation is the change in terminal voltage with the change in load current at constant speed. A generator has good regulation if the change in voltage between no load and full load is small. If the change is large, the regulation is poor. Expressed in equation form:
Percent Voltage Regulation = (ENL – EFL ) / EFL x 100 or for some compound machines, Percent Voltage Regulation = (EFL – ENL ) / EFL x 100
Figure 24 shows that the regulation of a separately excited machine is better than that of a shunt machine. However, the best regulation is obtained with a compound machine. The series machine has practically no regulation at all and, therefore, has little practical application.

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