EE 205: Introduction to Electrical Systems
Lecture 24 : DC Generator Characteristics
DC Shunt Generator - OCC
• Plot between Ea and If, with Ia = zero and wm= constant
more convenient to be run as separately
excited to get saturation curve
• Obtained by running the shunt generator as a separately excited
generator and hence the characteristics is similar to the separately
excited generator.
DC Shunt Generator – Load Characteristic
• Plot between Vt and If, with Ia and wm = constant
• The load characteristics is same as that of separately excited generator.
The slight difference is due to the different armature currents. Shunt
generator: Ia = IL + If ; Separately excited generator: Ia = IL
• Different armature currents result in different armature reaction,
giving slightly different voltage drops in the two cases.
DC Shunt Generator – External characteristics
• Plot between Vt and IL, with If and wm = constant
The drop in terminal voltage is due to (a) armature resistance drop, (b)
reduction in main field flux due to armature reaction and (c) further reduction
in field current since the terminal voltage has fallen because of the first two
effects.
Voltage Build-up in Shunt Generators
• Straight line Oa is the graphical plot of
Ohm’s law for the field circuit.
• Stable point n is the intersection of OCC
and Oa.
• Beyond the point n, the generated emf
given by the OCC is less than that required
to maintain the corresponding field
current.
• If the voltage build-up from Oc to np fails,
then reverse the shunt field terminals to
obtain the voltage build up.
Critical Field Resistance and Critical Speed
𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑦𝑡
𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑠𝑝𝑒𝑒𝑑 𝑛! =
×𝑛
𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑦𝑠 "
• The field resistance represented by
line OB is called the critical field
resistance at a given speed.
• If shunt field resistance is more than
the critical field resistance, there will
be no voltage build-up.
• The critical field resistance at a given
speed is obtained by drawing a line
through origin O and touching the
maximum straight line portion of the
magnetization curve.
• Critical speed is the speed at which
the dc shunt generator just fails to
build up with no external resistance in
field circuit.
Reasons for Failure to Build-up Voltage
• No residual magnetism
The process of connecting the shunt field winding to a
separate dc source for creating the necessary residual
magnetism in the main poles is called field flashing.
• Field connections reversed
• High field circuit resistance
• Speed less than critical speed