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Principles and Working of DC and AC machines

Principles and Working of DC and AC machines 11
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JuliyaMadenta,Philippines,Researcher
Published Date:15-07-2017
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Principles and Working of DC and AC machines BITS Pilani Dr Jagadish Nayak Dubai Campus BITS Pilani Dubai Campus Constructional features DC Machines DC Generator BITS Pilani, Dubai Campus DC Machines  A generator consists of a stationary portion called the stator and the rotating portion called the rotor.  A magnetic field is produced when a direct current is applied to windings of the coil in the stator. These coils are called field windings.  The rotor contains the commutator and the conductors across which emf is induced. This part is called armature. BITS Pilani, Dubai Campus DC Machines  The stator has two poles one labeled N and the other S. The field windings are situated on the poles.  The rotor consists of an iron core that has slots which house the armature conductors and a commutator and the brushes.  The figure shows the connections of the coils to the commutator BITS Pilani, Dubai Campus DC Machines  A uniform magnetic field is produced in the small air gaps between the poles and the rotor when current is passed through the field windings. This induces an emf across the coil and looks like as shown below.  When the armature windings are connected to the commutator according to the connections shown earlier , the individual emf gets rectified and all of them add up together. BITS Pilani, Dubai Campus DC Machines  The net effect of the coil connections to the commutator shows a brush voltage which appears as below BITS Pilani, Dubai Campus DC Machines Generated voltage: The generated voltage is given by: N n v p g a 60 Where N Numberof armatureconductors a Numberof parallel pathsbetweenthebrushes pNumber of poles Magnetic flux per pole n speedof therotorinrpm BITS Pilani, Dubai Campus DC Machines BITS Pilani, Dubai Campus DC Machines Generator with a load:  The generated emf v is the voltage across the g armature terminals ( the brushes) when no load is connected to those terminals.  If there is some electrical load connected to the armature, the resulting load voltage will be different from the no load voltage. Now it will depend on the resistance R associated with the armature due to a resitance of the windings and the brush contacts. BITS Pilani, Dubai Campus DC Machines Example :  An 8kW, 200-V dc generator has a full load current of 40nA at 1200rpm. Given the armature resistance is Ra=0.5Ω, armature is modeled as shown below, where RL is a resistive load. Determine the full load voltage for this generator at 900 rpm. BITS Pilani, Dubai Campus DC Machines If the flux per pole φ is kept constant , then the generated voltage is directly proportional to armature speed n.Thus if the field current is kept constant and the flux per pole is kept constant then at 900 rpm the no load voltage is BITS Pilani, Dubai Campus DC Machines Therefore under full load condition of 40 A at 900rpm the full load voltage is, by KVL BITS Pilani, Dubai Campus DC Machines Generator symbol: i is the field current and i denotes the armature f a current. BITS Pilani, Dubai Campus DC Machines  The rotor of a dc generator turning at n rpm induces an emf ( no load armature voltage) of vKn g • Where K = Np/60a is a constant determined by the construction of the generator. BITS Pilani, Dubai Campus DC Machines  The plot of no- load voltage v vs the field current i looks g f like:  These curves are called magnetization curves of the generator. BITS Pilani, Dubai Campus DC Machines Generator field excitation: FIELD EXCITATION When a dc voltage is applied to the field windings of a dc generator, current flows through the windings and sets up a - - steady magnetic field. This is called FIELD EXCITATION. This excitation voltage can be produced by the generator itself or it can be supplied by an outside source, such as a battery. BITS Pilani, Dubai Campus DC Machines If it is supplied by an outside source then it is called separately excited generator as shown below. A generator that supplies its own field excitation is called a SELF-EXCITED GENERATOR. BITS Pilani, Dubai Campus DC Machines CLASSIFICATION OF GENERATORS  Self-excited generators are classed according to the type of field connection they use.  There are three general types of field connections - SERIES-WOUND, SHUNT-WOUND (parallel), and COMPOUND-WOUND.  Compound-wound generators are further classified as cumulative-compound and differential-compound. BITS Pilani, Dubai Campus DC Machines Series-Wound Generator or Series connected generator  In the series-wound generator, the field windings are connected in series with the armature.  Current that flows in the armature flows through the external circuit and through the field windings.  The external circuit connected to the generator is called the load circuit. BITS Pilani, Dubai Campus