Performance prediction using design values.
Based on the design data of the
stator and rotor of DC Machine, performance of the machine has to be evaluated.
The parameters for performance evaluation are
load power factor,
Based on the values of these
parameters design values of stator and rotor can be justified. Iron losses:
Iron losses are occurring in all the iron parts due to the varying magnetic
field of the machine. Iron loss has two components, hysteresis and eddy current
losses occurring in the iron parts depend upon the frequency of the applied
voltage. The frequency of the induced voltage in rotor is equal to the slip
frequency which is very low and hence the iron losses occurring in the rotor is
negligibly small. Hence the iron losses occurring in the induction motor is
mainly due to the losses in the stator alone. Iron losses occurring in the
stator can be computed as given below.
a. A 150hp, 500V, 6pole, 450rpm, dc
shunt motor has the following data. Armature diameter = 54cm, length of
armature core = 24.5cm, average flux density in the air gap = 0.55T, number of
ducts = 2, width of each duct = 1.0cm, stacking factor = 0.92. Obtain the
number of armature slots and work the details of a suitable armature winding.
Also determine the dimensions of the slot. The flux density in the tooth at one
third height from the root should not exceed 2.1T.
b. For a preliminary design of a
1500kW, 275V, 300rpm, dc shunt generator determine the number of poles,
armature diameter and core length, number of slots and number of conductors per
slot. Assume: Average flux density over the pole arc as 0.85T, Output
coefficient 276, Efficiency 0.91.Slot loading should not exceed 1500A.
c. Calculate the armature diameter and
core length for a 7.5kW, 4pole, 1000rpm, and 220V shunt motor. Assume: Full
load efficiency = 0.83, field current is 2.5% of rated current. The maximum
efficiency occurs at full load.
d. For a preliminary design of a 50hp,
230V, 1400 rpm dc motor, calculate the armature diameter and core length,
number of poles and peripheral speed. Assume specific magnetic loading 0.5T,
specific electric loading 25000 ampere- conductors per meter, efficiency 0.9.
e. Determine the diameter and length of
the armature core for a 55kW, 110V, 1000rpm, and 4pole dc shunt generator.
Assume: Specific magnetic loading 0.5T, Specific electric loading 13000 ampere
‚Äď turns, Pole arc 70% of pole pitch and length of core about 1.1 times the pole
arc, Allow 10A for field current and a voltage drop of 4V for the armature
f. Determine also the number of
armature conductors and slots. A design is required for a 50kW,4pole,600rpm,
and 220V dc shunt generator. The average flux density in the air gap and
specific electric loading are respectively 0.57T and 30000 ampere- conductors
per metre. Calculate suitable dimensions of armature core to lead to a square
pole face. Assume that full load armature drop is 3% of the rated voltage and
the field current is 1% of rated full load current. Ratio pole arc to pole
pitch is 0.67.
g. Determine the main dimensions of the
armature core, number of conductors, and commutator segments for a 350kW, 500V,
450 rpm, 6pole shunt generator assuming a square pole face with pole arc 70% of
the pole pitch. Assume the mean flux density to be 0.7T and ampere conductors
per cm to be 280.
h. Determine the number of poles,
armature diameter and core length for the preliminary design of a 500kW, 400V,
600 rpm, dc shunt generator assuming an average flux density in the air gap of
0.7 T and specific electric loading of 38400 ampere- conductors per metre.
Assume core length/ pole arc = 1.1. Apply suitable checks