Basic Motor Theory (12)


The maximum voltage from an armature winding can be obtained when the brushes are in contact with those conductors, which are midway between the poles. This will result in the greatest possible number of conductors cutting the magnetic lines in one direction between a positive and negative brush. This brush position is known as the no load neutral position of the brushes. The current in a given armature coil reverses in direction as the coil sides move from one pole to another of opposite polarity, whereas the function of the commutator is to keep the current unidirectional. This reversal of current is known as commutation. The commutator acts as a switch to keep the current flowing in one direction.

However, the fast rate of change in direction of the current in any given coil induces an appreciable voltage in that coil which tends to keep the current flowing in the original direction. Therefore, the current reversal is delayed causing an accelerated rate of change near the end of the commutation period. This results in an arc if the reversal is not completed before the brush breaks contact with the coil involved. Any arcing is detrimental to the operation of the machine and must be counteracted.

Armature Reaction
Since the armature conductors carry current they set up a magnetic field which distorts or opposes the main field. This is called armature reaction and is a function of the amount of load present. Figure 21 shows the MMF and flux wave shapes due to the armature reaction only; and Figure 22 shows the combined effect of both. It can be seen that armature reaction causes the flux to shift, thus tending to saturate one pole tip. If this effect is appreciable, it can be detrimental to the satisfactory performance of the machine. If severe enough, it may result in a flashover, which is the progressive arcing over successive bars until the arc extends from positive to negative brush, thus short circuiting the machine terminals.

Figure 20. MMF and Flux Wave Shape due to Main Field only

Figure 21. MMF and Flux Wave Shape due to Armature Reaction only

Figure 22. Flux Wave Shape, combined effect

Brush Shifting
One method of reducing the arcing due to non-linear commutation is to shift the brushes away from the geometrical neutral position. Then commutation will occur when the applicable coil is under the influence of a weak magnetic field that will generate a voltage in the coil, which opposes the induced voltage due to current change. Therefore, this new voltage will assist rather than hinder the current reversal. In a generator, it is necessary to shift the brushes forward in the direction of rotation for good commutation. This is true because the current flow through the conductors is in the same direction as the voltage and, it commutation is delayed until the coil sides are under the next pole, it will be assisted by the current reversing voltage. In a motor, it is necessary to shift the brushes against the direction of rotation because current flow is in opposition to the induced voltage. The amount of shift necessary depends on the load so a given shift will not be satisfactory for all loads. One effect of shifting brushes is that a demagnetization component of armature reaction is introduced. In other words, when the brushes are shifted, the armature reaction will not only distort the main field flux but it will also directly oppose the main field. This will result in a reduction of the field flux. Another effect is that if the brushes are shifted far enough, it is possible to reduce the number of effective turns because there will be voltages in opposition to each other between two brushes.
In generators the demagnetization component of armature reaction would be detrimental because there will be a decrease in generated voltage with increase in load. However, in a motor, the effect would be beneficial because the speed would tend to remain constant.

Another method to combat the induced voltage caused by current reversal is the use of interpoles. The interpoles are located at the geometric neutral points midway between the main poles and provide reversing magnetic field of proper strength and polarity. They eliminate the need for brush shifting and, because of this, the demagnetization effect of armature reaction is eliminated. The interpole must have sufficient strength to overcome the armature reaction and provide a reversing field, therefore, it is connected in series with the armature winding. When the armature current is increased in the same proportion. In a generator, the interpole must have the same polarity as the next pole in the direction of rotation while in a motor the interpole must have the same polarity as the last pole.


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