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Electrical Machines (EE256)
1. What is the function of carbon brush used in D.C generator?
The function of carbon brush is to collect current from the commutator and supply is to the external load circuit and to the field circuit.
2. Distinguish between lap winding and wave winding used in dc machine
 Lap winding is designed for high current and low voltage machines.
 Wave winding is designed for low current and high voltages.
3. Write the number of parallel paths in a lap and wave connected windings
In a lap wound machine, the number of parallel paths is equal to the number of poles. But in wave wound machine, the number of parallel paths is always two irrespective of number of poles.
4. Name the three things required for the generation of emf
o Presence of armature conductors
o Presence of magnetic field
o Relative motion between conductor and magnetic field
5. What are the different types of dc generator
6. What is meant by self excited and separately excited dc generator
• Self-excited generator are those whose field magnets are energized by the current produced by the generator themselves
• Separately excited generator are those whose field magnets are energized from an independent external source of dc current
7. What is the basic difference between dc generator and dc motor
Generator converts mechanical energy into electrical energy. Motor converts electrical energy into mechanical energy. But there is no constructional difference between the two.
8. Write down the emf equation of dc generator. Give the meaning of each symbol
Emf induced E=ФZNP/60A
Ф=flux per pole in Weber
Z=total number of armature conductors
N=speed of armature in rpm
P=number of poles
A=number of parallel paths
9.what is pole pitch?
The periphery of the armature is divided for a number of poles of the generator. the center to center distance between two adjacent poles is called pole pitch
It is also equal to the number of armature slots or armature conductors per pole.
10.How can the voltage in a DC generator be increased?
Increasing the main field flux and the speed of the armature can increase the voltage in a DC generator.
11.What is critical resistance of a DC shunt generator?
The value of resistance of shunt field winding beyond which the shunt generator fails to build up its voltage is known as “ critical resistance “.
12.what are the conditions to be fulfilled for a shunt generator to build up Voltage?
a) There must be some residual magnetism in the field poles.
b) The shunt field resistance should be less than critical resistance.
c) The field coils should be connected with the armature in such a way that current flowing through them should increase the EMF induced by the residual magnetism.