Armature Reaction - DC Equipment 206 PDF

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Summary

Armature reaction and how they occur...

Description

Armature Reaction  Armature reaction drop is the effect of a magnetic field on the distribution of the flux under main poles of a generator  All current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a dc generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called ARMATURE REACTION. Armature windings are wound on magnetic structure  When current is passed through armature windings then it produces a magnetic field  Magnetic field produced in armature is 90 electrical degrees from that produced by the field winding  These two fields become vectorially combined in a distorted result  The magnetic field on one side of the field pole is in effect swept aside and reduced  The voltages generated in the windings are generated in direct proportion to actual field that is present. Armature reaction results in the neutral voltage point being appreciably moved in relation to the brush position  As a result, the commutator and brush switching function is no longer spark free, and resulting brush and commutator life drastically reduced

Compensating Methods 1. Rotate the brush hanger mechanism to find the correct but distorted neutral point. This is very effective at a fixed current load. But the brushes must be moved each time the load is changed. 2. Shape or modify the ends of the field pole shoes so that

high flux cannot exist on the ends because of high reluctance. It reduces but does not eliminate the problem Add interpoles or commutating poles to the field structure. Interpoles are small main field poles and are installed midway b/w the main poles. They modify the resultant of the main field and armature caused distorted magnetic field. The result of combined field caused by the main field, the armature reaction field and the commutating field is that locally around the brushes there is no effective field. Therefore, there is so unwanted local voltage generation to spoil the required commutator and brush switching process. The effect of armature reaction is related to armature current, its counteraction by commutating fields is also needed in relation to the armature current. the commutating field windings in series with the armature. Commutating fields are used in medium and large dc machines. 4. Compensating windings are laid directly into the pole shoe face of the main fields. They are parallel to the armature shaft and carry the current in the opposite direction to the armature windings immediately adjacent to them. The result is that the main field flux symmetry is no longer distorted, since the armature reaction magnetic flux is equally and oppositely opposed by the compensating winding flux. The function of the commutating field is still required, but

to a reduced degree. Compensating windings are expensive and cumbersome.

Self Excited Generator  A DC generator whose field magnet winding is supplied current from the output of the generator itself is called a self-excited generator.  There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature  Series self excited Generator  Shunt self excited Generator  Compound self excited Generator Generator Magnetization curve  Generated voltage depends on the field magnetic flux, for a constant speed  So Output voltage is related to field excitation current in a curve known as Magnetization curve.  This are also known as internal characteristics of generator.  Polarity of output voltage depends on the direction of rotation and direction of current in the field. Magnetization curve  Magnetic conditions within the generator can be investigated by testing a separately excited generator using a voltmeter connected to the output, where VT is indicated, and an ammeter in series with the field circuit to measure field current. The test procedure is to adjust the

speed of the prime mover to the rated value as indicated on the generator name plate and then to adjust the variable resistor for increasing values of field current (including zero) as read on the ammeter. For each setting of field current, the voltmeter indicates the corresponding induced voltage. Data is usually taken for voltages up to 25 to 50 percent above the rated value. The data is then plotted on a graph called “magnetization curve”....