VOLTAGE BUILD-UP IN DC SHUNT GENERATOR

 

                                                                                Figure of Shunt Generator

For developing a voltage in a shunt generator we require field current and field current in Shunt Generator is derived from Armature Current. For the flow of Armature Current we need Induced emf across the generator windings. So, the voltage build-up in Shunt Generator requires some residual flux and emf for voltage build-up.

• Due to residual flux, emf oc is induced in Armature.
• Due to this induced emf a field current od flows in the circuit.
• This cause an emf de to be induced as per Open Circuit Characteristics.
• This further causes a current df to flow in field winding.

This effect goes on till the Field Resistance Line (a) intersects (at 'n') the Open Circuit Characteristics (occ).

Necessary Conditions for Voltage Build-up

• There must be residual flux.
• Correct polarity of field winding with respect to armature winding, so that field flux adds residual flux for a given direction of rotation, so that net flux increases and induced emf also increases.

• Field resistance Rf  must be less than critical value. For Voltage to build-up to a steady state value there must be an intersection of field line with the Open Circuit Characteristics. In case, the field line is tangential to Open Circuit Characteristics the field resistance is called as Critical Resistance. Oa, OA and OB are field resistance lines.

           Rf  < Rf (cr)

           On air gap line

           Ea = KⲫꞶm = K (KfIf) Ꞷm = K Kf Ꞷm If

           Rf (cr) = K Kf Ꞷm  ∝ Ꞷm

• Speed of rotation should be more than critical value for given field resistance Rf. As speed is reduced the OCC moves downwards and at some speed it is tangential to the field line. Such a speed is called as Critical Speed.