01․ A frog leg winding is used in a DC machine to eliminate
Frog leg winding composed of a multiplex wave winding in the same slots with a simplex or multiplex lap winding. Both have the same no. of parallel path and are connected to commutator. Thus the current distribution is equal at brush and thereby eliminate the equalizer in lap winding.
02․ A 4 pole DC shunt generator having a wave winding supplies 50 lamps, each of 60 W at 120 V. The armature and field resistance are 0.2 Ω and 60 Ω respectively. The current in each armature conductor is
Total load current IL = 0.5 X 50 = 25 A. Armature current Ia = IL + Ish = 25 + 2 = 27 A. A wave winding has two parallel paths. So current in each armature is equal to 27 / 2 = 13.5 A.
03․ A 6 pole lap connected DC generator with 480 conductors has armature resistance of 0.06 Ω. If the conductors are reconnected to from a wave winding other things remaining unchanged, the value of armature resistance will be
Where Z = number of conductor, P = number of pole, L = length of conductor and A = Cross section area of conductor. ⇒ Rwave = Rlap X 9 = 0.06 X 9 = 0.54 Ω.
04․ The sparking at the the brushes, in a DC machines is due to
The self induced emf in the coil under going commutation is called reactance voltage. Sparking at brushes is due to inability of the current in the short circuit coil to reverse completely by the end of short circuit period.
05․ In a DC machine, for the same value of Ãâ€ , Z and N; which one of the following statements is correct?
In case of lap winding A = P and wave winding A = 3. So the emf generated in wave wound armature is greater than lap wound armature.
06․ An ideal DC generated is one that has _____________ voltage regulation.
An ideal DC generator has no armature resistance and no field winding resistance. Thus there are no voltage drop due to armature winding and field winding resistance. Hence the generated emf is equal to terminal voltage. Thereby an ideal DC generated has zero voltage regulation.
07․ Series booster is a
Series booster is a series wound DC generator, which is inserted in a circuit to add a certain voltage. So that the excessive voltage drop in the feeder is compensated for.
08․ If residual magnetism of a shunt generator is destroyed accidentally it may be restored by connecting its shunt field
In case the generator residual magnetism is destroyed accidentally it may be restored by connecting the field winding to a DC source for short while to magnetize the poles.
09․ A commutator with a diameter of 50 cm rotating at 1000 rpm. For a brush width of 1.5 cm, find the time of commutation in ms.
10․ The coil spam
The coil span may or may not be exactly equal to pole pitch. Pole pitch is the number of conductor per pole.