CALCULATION TO DETERMINE TYPE OF CARBON BRUSHES

CALCULATION TO DETERMINE TYPE OF CARBON BRUSHES

Passing of current between brush and slip ring is called electrical greasing.

Types of Brushes

Group	% Cu	Current Density(A/cm2)
1(metal graphite )	85	20
2(metal graphite )	75	17
3(metal graphite )	65	12
4(electro graphite )	0	8

DIMENSIONS OF DIFFERENT BRUSHES

TR ₅ – 38X32X22

NGEF-1> 315R-40X40X20

           2>280-40X32X16

TR₄ – 38X32X16

Let us take example of NGEF 315R GROUP 3.

Area touching the slip ring- 40x20=8cm²           

Capacity of a brush= 12x8=96A

Let us take a motor whose secondary current is 260A

 2 brushes needed, so capacity of one brush will be 130A

So 3 Group brush will not work, we take Group 2 brush.

For group 2, capacity of each brush will be 17x8=136A

We can use this brush.

Therefore, to order a brush we will tell the vendor specifications as follows:

NGEF 315R brush, Group 2.

Calculation of Spring Tension of Brush

Range available- 180-200 gm/cm²

                               200-220 gm/cm²

For NGEF 315R Group3, area=8cm²

Let’s take 200 range.

So, tension will be 200x8= 1.6KG

Coefficient of Friction

 

                                                           12 A, 120°C

At the point 12 A coefficient of friction is 0.1 which is lowest, here heat developed will be minimum, so an increase or decrease in temperature will result in an increase in coefficient of friction, which  is undesirable.