This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Armature Reaction -1”. 1. In a DC machine, the form of. Armature reaction is effect of armature flux on main field flux. Basically there are two windings in a dc motor - Armature winding (on rotor) and field winding (on stator). When we excite the field winding, it produces a flux which links with the armature. This causes an emf and hence a current in the is the use of compensating for the armature reaction in a DC. Since armature current depends on the electrical or mechanical load on the machine, armature-reaction effects are therefore load-dependent. In dc machines.
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The second figure in the above image shows armature flux lines due to dc machine armature reaction armature current. Field poles are de-energised.
Now, when a DC machine is running, both the fluxes flux due to the armature conductors and flux due to the field winding will be present at a time. In dc machines, the armature reaction causes a distorted flux-density distribution in the dc machine armature reaction gap.
Figure 1 shows the magnetic field produced in the air gap of a two-pole dc motor or generator by a the field mmf, b the armature mmf, and c both armature and field mmf acting together.
Armature Reaction in DC Machine
Figure 2 shows the distorted flux distribution in the air gap, curves a, b, and c corresponding to the above three cases. So, for a given rating of machine, a wave winding is more suitable for large currents and low voltages.
The exact distribution of dc machine armature reaction windings and selection of the number of slots per pole of the field greatly influences the design of the machine and its performance, affecting such factors as commutation in a DC machine or the waveform of an AC machine.
A schematic winding diagram for dc machine armature reaction DC machine with a commutator, showing a wave winding - shown as if the surface of the armature was flattened out. Winding materials[ edit ] Armature wiring is made from copper or aluminum.
Copper armature wiring enhances electrical efficiencies due to its higher electrical conductivity. Aluminum armature wiring is lighter and less expensive than copper.
Armature reaction - AccessScience from McGraw-Hill Education
Now, if we place another winding in close proximity of the armature winding and if it carries the dc machine armature reaction current but in the opposite direction as that of the armature current, then this will nullify the armature field.
Such an additional winding is called as compensating winding and it is placed on the pole faces. Compensating winding is connected in series with the armature winding in such a way that it carries the dc machine armature reaction in opposite direction.
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Interpoles are the small auxiliary poles placed between the main field poles. Winding on the interpoles is connected in series with the armature.
Each interpole is wound in such a way that its magnetic polarity is same as that of the main pole ahead of it. Interpoles nullify the quadrature axis armature flux.
In view of this, brush shift is limited only to very small machines.
Here also, the brushes are fixed at a position corresponding to its normal load and the mode of operation. Due to these limitations, this method is generally not preferred.
Armature Reaction in DC machines
Inter Pole The limitation of brush shift has led to the use of inter poles in almost all the medium and large sized DC machines. Inter poles are long but narrow poles placed in the inter polar axis. They have the polarity of succeeding pole coming next in sequence of rotation in generator action and proceeding which has dc machine armature reaction behind in rotation sequence pole in motor action.
The inter pole is designed to neutralize the armature reaction mmf in the inter polar axis.