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Tuesday, 10 December 2019

Brush Holder in DC Machine

Brush and Brush Holders in DC Machine :

The basic function of a brush is to collect current with the least possible voltage drop from the rotating coil to a stationary position. The type of brush used depends upon the speed of rotation of the. commutator and the voltage involved in it. While doing its function it should

a. not have abrasive action on the commutator segments
b. not offer high resistive path resulting in voltage drops between the segment and the brush
c. not wear easily giving rise to carbon coating on commutator segments.
d. not offer high frictional resistance between the surface.
e. not be brittle resulting in breakage of brushes
f. not be costly, and should be easily available.

There are mainly four types of brushes in DC Machine. They are

a. Carbon Brush : These are made out of pulverised graphite carbon with binder material mould to shape and baked at 1200°C.

b. Graphite Brush : These are made out of natural plumbage and are suitable for operation at higher current density.

c. Metal Carbon Brush : In these some amount of copper is mixed with graphite carbon to increase conductivity. Their contact voltage varies between 0.1 to 2.2 V and is suitable for higher current density operations.

d. Electric Graphite Brush: These are more robust mechanically and are generally used in tractions.

The length, width and thickness of the brush vary from machine to machine. For smaller machines, brushes of 20 mm width and thickness 6 to 9 mm are used but for larger machines brush width vary from 30 mm to 40 mm width and thickness from 6 to 18 mm. A common shape of the brush is shown in Figure. A flexible wire is attached to one end of the brush for external connection and is called pig tail.

Brush holders in DC Machine not only keep the brush in position but also in continuous contact with the commutator segments. The application of brush is either radial or tangential. The brush holder with clamping screws keeping the brush in tension with the help of the spring. All brush holders for carbon brushes are radial type. There are mainly four different types of brush holders generally used. They are

a. Plunger type brush holder
b. Box type brush holder
c. Lever type brush holder
d. Composite brush holder

For smaller dc machines where the current density is less plunger type brush holders are used. A sectional view of the plunger type brush holder is shown in Figure. A tube is moulded in brass box either round or square section containing the brush with compression spring as in the case of Tullu pump or mixer grinder. The compression spring which is made of brass is retained by means of an insulating cap with a collar in the inner side of which serves as an attachment for the wire.

For larger dc machines box or lever type is used. The body of the brush top clamps into a round brush pin, a metal finger with a friction roller at the end presses on the end of the brush. The tension is regulated by worm and worm wheel adjustments, the whole forming a neat fitting. An isometric view of the different brush holders are shown in above figure. Excellent continuity is obtained in the lever type between the terminal and the brush but the inertia and weight of the moving parts is much greater than the brush leading to sparking. It is pivoted on one end and the brush is kept in tension with a lever and spring, and provision is made for minute adjustments of the tensions.

For larger dc machines when the current density is more and the area of surface of contact has to be more, a composite brush holder is used. A simple figure of composite brush holder is shown in Figure where more than one brush in parallel will be in operation. Multiple brush box which is bolted to the bracket holding the brushes has a plunger type arrangements with compression springs to keep the brushes in tension. The brushes are separated from one another by a thin division of partition. Separate pressing device is employed for each brush. Terminal box is used for terminal connection.

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