Q & A INDEX

SECTION 3 - BRUSH THEORY AND CHARACTERISTICS

Question 3:17 What Is The "S Cooling Factor"?

　

As discussed in Q 3:9, Q 3:10 & Q 3:12 the heat generated from friction, current in the brush and ambient temperature has a big influence on how the brush performs.

　

In well ventilated machines the brushes can operate at higher current densities. See Q 3:5.

　

In respect of slipring brushes the selection of electrographitic or copper graphite brushes is not only influenced by current density but by the surface area of the collector and the type of enclosure is also involved. 

　

For this reason a so-called "S Factor" has been formulated to give a guide to brush selection. 

　

The main cause of brush or slipring wear is brushes which contain too high a percentage of copper in proportion to the cooling surface and current density.

　

If high wear problems arise with sliprings the actual secondary current should be checked. See Q 3:6. To correctly determine a suitable brush grade one must also calculate a factor for the cooling surface.

　

This factor is called ''S'' and is calculated by the Formula 

　

S=  π x D x W

             I

　

Where D = Diameter of Rings in cm 

            W = Width of Rings in cm 

             I   = Actual Current In The Ring 

　

Suitable Values of "S'' for bronze Sliprings  

　

　 Approximate 85% Copper 20A/cm2 Approximate 75% Copper 15A/cm2 Approximate 50% Copper 12A/cm2 Electrographite No Copper   10 A/cm2
Enclosed 0.7 0.7 - 1.0  1.0 - 1.3 >1.3
Ventilated 0.5 0.5 - 0.7  0.7 - 1.0 >1.0
For Steel Sliprings  >3.0

 For Commutators 1 - 250KW                           

                                 >250KW 

 6.0 - 10.0 

 3.5 - 5.0

　

EXAMPLE  

　

Slipring 200 mm diameter 25mm wide 

2 Brushes per ring each 20 mm X 32 mm cross section

Total operating current for that ring   120A 

Currently experiencing high ring wear and excessive dusting.

Existing Grade 75% copper graphite 

　

Collector contact area - π x D x width  = 3.142 x 20 cm x 2.5 cm = 157.1 cm2 

　

 S Factor = Contact Area Current   = 157.1 = 1.31 

                            Current                          120 

　

Check Brush Current Density 

　

2 cm x 3.2 cm x 2 =   brush contact area 12.8 cm2 

　

Current density     Amps          120   =  9.38 A/cm2 

                           Contact Area 12.8 

　

From these calculations we can see that the brushes can be changed from 75% copper to electrographite which is within the current density rating (less than 10A/cm2) and the S Factor cooling guidelines (greater than 1.3 factor). 

　

lf however the ring was only 180 mm diameter the S Factor would become 1.18 and therefore unsuitable for an electrographitic brush. A 50% copper graphite brush would need to be used in this case.  

　

Q & A INDEX

The information has been drawn from experience of Morganite Taiwan Limited Application Specialists combined with a wide range of published information from major carbon supplier and motor manufacture. However, the material can not be downloaded, copied or used without written permission.

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