Electrical Machines · DC Machines

DC Generator Characteristics

Dr. Mithun Mondal BITS Pilani, Hyderabad Campus Electrical Machines

Demonstrative Video

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

OCC gen
Occ Gen
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 \(I\)

internal gen
Internal Gen
At no-load, prime mover drives armature at a certain
dc shunt
Dc Shunt

  • 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 \(V\)

\[E_{g}\uparrow\Rightarrow V\uparrow\Rightarrow I_{f}\uparrow\Rightarrow\Phi\uparrow\Rightarrow E_{g}\uparrow\Rightarrow V\uparrow\]
Flashing the field.
voltage build
Voltage Build

  • 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.

Critical
critical
Critical

  • 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

Ifis higher than, the generator fails to excite

  • 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.

shunt char
Shunt Char
series char
Series Char
Depending upon the number ofturns, Cumulative Gen
compound char
Compound Char

  • 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 \(I_a \Uparrow\).