Characteristics of DC compound generator

•     The external characteristics of dc shunt generator are drooping and that of series generators are rising.

•     Therefore in a dc cumulative compound generator the drop in terminal voltage can be compensated either in full or in part by appropriately selecting the number of turns in the series field winding.

•     If the number of turns in the series field winding are chosen such that the generator delivers an almost constant terminal voltage from no load to full load.

•     The curve b is called as level or flat compounded.

•     In such generators at half load, the terminal voltage may be higher than that of the full load or at no load since the magnetic circuit will be in unsaturated region.

•     If the number of turns in the series field winding is increased further the terminal voltage rises with increase in load as shown by the curve a and such generators are called as over compounded generator.

•     They are used to compensate the voltage drop in the long feeders. If the number of turns in the series field winding is reduced it is possible to get an external characteristic as shown by the curve c where the terminal voltage decreases when the generators loaded. Such generators are called under compounded generator.

•     In differential compound generators the series field flux opposes the shunt field flux.

•     Therefore when the generator is loaded the terminal voltage of the generator falls of rapidly to zero.