May 2009
You are currently browsing the articles from circuitmaniac.com written in the month of May 2009.
Critical Field Resistance for a Shunt Generator We have seen above that voltage build up in a shunt generator depends upon field circuit resistance. If the field circuit resistance is R1 (line OA), then generator will build up a voltage OM as shown in Fig. (3.5). If the field circuit resistance is increased to R2 [...]
Written by John on May 17th, 2009 with no comments.
Read more articles on D.C. Generator Characteristics and Direct Current Machines and Electrical Machines.
Let us see how voltage builds up in a self-excited generator. (i) Shunt generator Consider a shunt generator. If the generator is run at a constant speed, some e.m.f. will be generated due to residual magnetism in the main poles. This small e.m.f. circulates a field current which in turn produces additional flux to reinforce [...]
Written by John on May 16th, 2009 with 3 comments.
Read more articles on D.C. Generator Characteristics and Direct Current Machines and Electrical Machines.
The obvious disadvantage of a separately excited d.c. generator is that we require an external d.c. source for excitation. But since the output voltage may be controlled more easily and over a wide range (from zero to a maximum), this type of excitation finds many applications. (i) Open circuit characteristic. The O.C.C. of a separately [...]
Written by John on May 16th, 2009 with 8 comments.
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The O.C.C. for a d.c. generator is determined as follows. The field winding of the d.c. generator (series or shunt) is disconnected from the machine and is separately excited from an external d.c. source as shown in Fig. (3.1) (ii). The generator is run at fixed speed (i.e., normal speed). The field current ( If) [...]
Written by John on May 16th, 2009 with 1 comment.
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The speed of a d.c. machine operated as a generator is fixed by the prime mover. For general-purpose operation, the prime mover is equipped with a speed governor so that the speed of the generator is practically constant. Under such condition, the generator performance deals primarily with the relation between excitation, terminal voltage and load. [...]
Written by John on May 14th, 2009 with 3 comments.
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We know that the armature circuit in lap winding of a multipolar machine has as many parallel paths as the number of poles. Because of wear in the bearings, and for other reasons, the air gaps in a generator become unequal and, therefore, the flux in some poles becomes greater than in others. This causes [...]
Written by John on May 12th, 2009 with no comments.
Read more articles on Armature Reaction and Commutation and Direct Current Machines and Electrical Machines.
In this method, an arrangement is made to neutralize the reactance voltage by producing a reversing voltage in the coil undergoing commutation. The reversing voltage acts in opposition to the reactance voltage and neutralizes it to some extent. If the reversing voltage is equal to the reactance voltage, the effect of the latter is completely [...]
Written by John on May 12th, 2009 with 3 comments.
Read more articles on Armature Reaction and Commutation and Direct Current Machines and Electrical Machines.
Improving commutation means to make current reversal in the short-circuited coil as sparkless as possible. The following are the two principal methods of improving commutation: (i) Resistance commutation (ii) E.M.F. commutation We shall discuss each method in turn. Resistance Commutation The reversal of current in a coil (i.e., commutation) takes place while the coil is [...]
Written by John on May 12th, 2009 with 1 comment.
Read more articles on Armature Reaction and Commutation and Direct Current Machines and Electrical Machines.
Fig. (2.6) shows the schematic diagram of 2-pole lap-wound generator. There are two parallel paths between the brushes. Therefore, each coil of the winding carries one half (Ia/2 in this case) of the total current (Ia) entering or leaving the armature. Note that the currents in the coils connected to a brush are either all [...]
Written by John on May 10th, 2009 with 3 comments.
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With the brushes in the G.N.A. position, there is only cross-magnetizing effect of armature reaction. However, when the brushes are shifted from the G.N.A. position, the armature reaction will have both demagnetizing and crossmagnetizing effects. Consider a 2-pole generator with brushes shifted (lead) Īøm mechanical degrees from G.N.A. We shall identify the armature conductors that [...]
Written by John on May 10th, 2009 with 2 comments.
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With no current in armature conductors, the M.N.A. coincides with G.N.A. However, when current flows in armature conductors, the combined action of main flux and armature flux shifts the M.N.A. from G.N.A. In case of a generator, the M.N.A. is shifted in the direction of rotation of the machine. In order to achieve sparkless commutation, [...]
Written by John on May 9th, 2009 with 5 comments.
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In a d.c. generator, the purpose of field winding is to produce magnetic field (called main flux) whereas the purpose of armature winding is to carry armature current. Although the armature winding is not provided for the purpose of producing a magnetic field, nevertheless the current in the armature winding will also produce magnetic flux [...]
Written by John on May 8th, 2009 with 10 comments.
Read more articles on Armature Reaction and Commutation and Direct Current Machines and Electrical Machines.
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