DC Generator Characteristics

No-load Characteristics $\left(E_0/I_f\right)$

• Magnetic or Open-circuit Characteristic (O.C.C)

• Relation between $E_0$ and $I_f$ at a given fixed speed

• Magnetization curve for the material of the electromagnets

• Shape is practically the same for all generators

Internal Characteristics $\left(E/I_a\right)$

• Relation between $E$ actually induces in the armature (after demagnetization effect) and $I_a$

• Characteristics is of mainly interest to the designer

External Characteristics $\left(V/I\right)$

• Performance characteristics or voltage-regulating curve

• Relation between load $V$ and $I$

• great importance in judging the suitability of a generator for a practical purpose

Separately Excited Generator

No-load Characteristics

Internal and External Characteristics

• No armature reaction generated voltage is straight line

• Voltage drop $\Delta V_{AR}$ because of armature reaction

• Operating Point $P$, intersection between generator external and load characteristics by the relation $V_L=I_LR_L$

• $P$ gives operating value of terminal $V$ and $I$

At no-load, prime mover drives armature at a certain $N$ Desired terminal voltage is buildup Residual flux present in the field poles is responsible for the voltage buildup A small voltage (1-2 volts) is generated $E_g=K\Phi_{res}N$

• This voltage causes $I_f=V/R_f$ to flow in the field winding

• The flux is increased by mmf produced by $I_f$

• As a result $E_g$ increases, which further increase $V$

There must be a sufficient $\Phi_{res}$ in the field poles. The field terminals should be connected in such a way that the $I_f$ increases $\Phi$ in the direction of $\Phi_{res}$ $R_f$ should be less than the $R_c$

If $\Phi_{res} = 0$ disconnect the field and apply a DC voltage to the field winding.This process is called Flashing the field.

• A decrease in $R_f$ reduces the slope of $R_f$ line resulting in a higher voltage, and vice-versa

• If $R_f$ is increased to $R_c$, the $R_f$ line becomes tangent to the initial part of the magnetization curve

• If $R_f$ is higher than $R_c$, the generator fails to excite.

• At $N_c$ the $R_f$ line becomes tangential to the magnetization curve.

• Below $N_c$ the voltage will not build up.

Depending upon the number of $R_{se}$ turns, Cumulative Gen. can be Over, Flat, and Under Compounded If series winding $NI$ are adjusted such that $I_L \Uparrow$ $V_T \Uparrow$ $\Rightarrow$ over compounded $V_T$ remains constant $\Rightarrow$ flat compounded

• If $N_{se}$ is lesser than required to be flat compounded, then the generator is called to be under compounded.

• In Differential Compounded $V_T \Downarrow\Downarrow$ with $I_a \Uparrow$.