Characteristics of DC Generators

CHARACTERISTICS OF DC GENERATORS

The following are the three most important characteristics of a DC generator.

1. Open Circuit Characteristic (OCC) (or) Magnetisation characteristic (or) No load characteristic (E₀/I_f).

It shows the relation between the no load generated emf in armature, E₀ and field or exciting current I_f at a given fixed speed. Its shape is practically the same for all generators whether separately or self excited.

2. Internal or Total characteristic (E/I_a)

It gives the relation between the generated emf on load (E) and the armature current (I_a). The emf E is less than E₀ due to demagnetising effect of armature reaction. Therefore, this curve will lie below the open circuit characteristic (OCC). The internal characteristic is of interest chiefly to the designer. It cannot be obtained directly by experiment. It is because a voltmeter cannot read the emf generated on load due to the voltage drop in armature resistance. The internal characteristic can be obtained from external characteristic if winding resistance are known because armature reaction effect is included in both characteristics.

3. External Characteristic (V/I_L)

It gives the relation between the terminal voltage (V) and load current (I_L). The terminal voltage V will be less than E due to voltage drop in the armature circuit. Therefore this curve will lie below the internal characteristic. This characteristic is very important in determining the suitability of a generator for a given purpose. It can be obtained by making simultaneous measurements of terminal voltage and load current (with voltmeter and ammeter) of a loaded generator.

Characteristics of Separately ‒ Excited Generator

1. Open Circuit Characteristics (OCC) (or) Magnetisation characteristics (or) No Load characteristics. (E₀/I_ƒ)

The arrangement for obtaining the necessary data to plot this curve is shown in Fig. 2.18 (a).

It shows the variation of no load generated emf (E₀) with the field current (I_f) at constant speed.

The armature is driven at a constant speed by a prime mover with no load. The field winding is excited from a separate DC source to ensure independent flux control.

The voltage equation of a d.c. generator is,

 E_g = (ϕΖΝ / 60) × (P/A) Volt

Since speed is constant, the above relation becomes E_g ∝ ϕ

When the field current is zero, a small residual flux is present in the magnetic poles and therefore a small voltage is generated (residual emf) even when field current is zero, which is appearing across the armature can be taken as no load voltage. (OA ‒ Residual emf)

The field current is increased in steps using rheostat, the flux increases proportionately and no load voltage increase with I_f till the poles are unsaturated.

On plotting the relation between E₀ and I_f, we get the open circuit characteristics as shown in Fig. 2.18 (b).

We can observe that the major portion of this characteristics (B‒C) is linear. There are two non linear portions A ‒ B and C ‒ D. Beyond the operating point D, we can say that the poles are saturated and so there will not be any appreciable increase in the generated voltage even for large increase in field current.

The build up of voltage of DC shunt generator under open circuited condition depends on the following factors.

(i) Residual flux

(ii) Reverse connection of shunt field

(iii) Shunt field circuit resistance and

(iv) Speed of armature.

• If there is no residual flux, no emf is induced. As a result there is no further induced emf. As a result there is no further increase in field flux and the induced emf is zero.

• The residual flux and the flux produced by the shunt field winding must aid each other. So that the net air gap flux increases which in turn develops voltage other wise, the residual flux may get wiped off and hence induced emf drops to zero. So the field winding terminals should be properly connected.

• The field circuit resistance must be equal to or less than the critical field resistance R_C, otherwise the generator will fail to build up voltage.

• The speed of generator must be equal to or greater than the critical speed N_c of the generator, otherwise, the generator will fail to build up voltage.

2. Internal and External characteristics

The external characteristics is the graph of the terminal voltage V, against load current Į_L.

The internal characteristics is the graph of the generated induced emf, E against the armature current, l_a.

While plotting both the characteristics, the speed of the generator is maintained constant.

These characteristics is also known as load characteristics of generator.

To plot the load characteristics the circuit arrangement is shown in Fig. 2.19.

The generator is made to run at its rated speed by prime mover. Field current is adjusted using rheostat to generate the rated voltage at no load. If there were no armature reaction and armature voltage drop, then this voltage would have remain constant as shown in Fig. 2.20 by the horizontal line I.

Now by varying the load the terminal voltage V and load current I_L are noted from voltmeter and ammeter respectively. The readings are plotted as shown in Fig. 2.20. The terminal voltage of the generator decreases on loading. This is because of,

(i) Voltage drop due to armature reaction,

(ii) Voltage drop due to armature resistance, R_a

If we subtract from E₀ the values of voltage drops due to armature reaction for different loads, then we get the value of E ‒ the emf actually induced in the armature under load conditions. Curve II is plotted in this way and is known as the internal characteristics [∴ E/I_a]. These two quantities are internal quantities for the generator and E cannot be measured directly, but it can be estimated. Hence the name Internal characteristics.

If we subtract from E the armature drop, I_aR_a we get terminal voltage, V Curve III represent the external characteristics [i.e., V/I_L]. These quantities two are external quantities and they can be measured directly. Hence the name external characteristics.

(i) Critical Field Resistance (Rc)

It is the maximum value of resistance in the field circuit with which the generator will just build up voltage. Beyond this value of resistance the machine will fail to build up voltage.

(ii) Critical Speed (Nc)

It is the minimum speed at which the generator will just build up voltage. Below this critical speed it will fail to build up voltage.

The critical speed (Nc) and the critical field resistance Rc of generator can be obtained from its OCC at rated speed as discussed as follows.

Draw the OCC at rated speed (N_R). Draw the tangent to the initial portion of the OCC. The slope of the tangent gives,

Rc = AC/OC

Now, measure the value R_sh of the generator and draw the line representing field circuit resistance. Draw any ordinate which cuts the Rc line, R_sh line and X axis at A, B and C.

The critical speed is calculated as

Nc = (BC/AC) × Rated speed (N_R)

Characteristics of DC Shunt Generator

To plot the characteristics of DC shunt generator the circuit diagram is shown in Fig. 2.22.

1. OCC

The armature is driven at rated speed by a prime mover, with no load. The field current is increased in steps using rheostat, and the Voltmeter reading E₀ and Ammeter reading I_f are noted. From the reading OCC are drawn as shown in Fig. 2.23.

2. Load characteristics

The connection diagram for conducting the load test is shown in Fig. 2.22.

The generator is made to run at its rated speed by prime mover. Field current is adjusted using rheostat to generate rated voltage at no load. The load is increased step by step up to rated current of the generator, while increasing the load the voltmeter reading (V) and ammeter reading (I_L) are noted and these reading are plotted as shown in Fig. 2.24.

Curve (1) Shows the Internal Characteristics (EV_SI_a)

When the generator is loaded, the flux per pole is reduced due to armature reaction, Therefore emf generated on load is less than the emf generated at no load. So internal characteristics lies below the open circuit voltage curve.

Curve (2) Shows the External characteristic of a shunt generator

It gives the relation between terminal voltage V and load current I_L.

 V = E‒l_aR_a = E ‒ (I_L+I_sh)R_a

Therefore External characteristic curve will lie below the internal characteristic curve by an amount equal to drop in the armature circuit [i.e., (I_L+I_sh)R_a] as shown in Fig. 2.24.