Contents
What is brush shifting in DC machines?
Brush Shift A natural solution to the problem appears to shift the brushes along the direction of rotation in generator action and against the direction of rotation in motor action, this would result into a reduction in air gap flux.
What purpose does brush shifting accomplish in motors?
Each conducting segment is wired to a winding of the armature. The function of the brushes is to conduct electricity to the individual segments as they rotate from brush to brush. This allows the motor to turn in one direction at a speed determined by the number of windings in the armature.
What is the use of brushes in DC machines?
A carbon brush is a sliding contact used to transmit electrical current from a static to a rotating part in a motor or generator, and, as regards DC machines, ensuring a spark-free commutation.
Why is the brush shifting method not employed in practical commutation?
Why brushes shifting method is not employed in practical commutation? Explanation: Brushes are located at GNA’s, a small voltage is induced in the commutating coil. It could be partially remedied by shifting the brushes towards MNA but that causes direct demagnetization and is therefore not employed in practice.
Why do brushes spark?
Some sparking at the brushes is normal. Excessive sparking may be due to worn brushes, resulting in reduced spring pressure, or due to roughness of the commutator segments (test with a finger ….. with the power OFF!), or perhaps carbon dust between the commutator segments.
What are the methods used for improving commutation in DC machine?
Methods of Improving Commutation
- Resistance commutation.
- E.M.F. commutation.
- Compensating windings.
What is poor commutation?
A machine is said to be in poor commutation if there is sparking at the brushes and commutator surface. Poor commutation can be caused by mechanical or electrical condition. Mechanical conditions include : 1) Uneven commutator surface. 2) Non uniform brush pressure.
Why are the brushes offset in a DC generator?
Because of this armature reaction, the magnetic neutral axis is slightly offset, and in order to avoid massive sparking at the brushes due to additional current, the brushes are slightly offset to accommodate the shift in the magnetic neutral axis.
Where are the carbon brushes placed in a DC machine?
In a DC machine, the carbon brushes are always placed at the magnetic neutral axis. In no load condition, the magnetic neutral axis coincides with the geometrical neutral axis. Now, when the machine is loaded, the armature flux is directed along the inter polar axis (the axis in between the magnetic poles)and is triangular in wave shape.
What should be the position of brushes in DC?
The coil is short-circuited every time its segment is passed through the brush, so the brushes should be placed along the point where the EMF induced in the armature is minimum, and that point is along Magnetic Neutral Axis or MNA or q-axis. where ‘θ’ is the angle between the motion of conductors and magnetic flux and ‘θ’ is 0° along q-axis.
How to reduce the armature reaction in a DC machine?
Usually, no special efforts are taken for small machines (up to few kilowatts) to reduce the armature reaction. But for large DC machines, compensating winding and interpoles are used to get rid of the ill effects of armature reaction. Compensating winding: Now we know that the armature reaction is due to the presence of armature flux.