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 \(V\)

\[E_{g}\uparrow\Rightarrow V\uparrow\Rightarrow I_{f}\uparrow\Rightarrow\Phi\uparrow\Rightarrow E_{g}\uparrow\Rightarrow V\uparrow\]

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 \(I_a \Uparrow\).