SECTION 2 -BRUSH DESIGN, MANUFACTURE AND APPLICATION
Question 2:11 How Is The Flexible Connection Lead Size And Number Determined?
Generally this has all been worked out in the original design of the brush by the manufacturer. However some conditions may require revision of the original design.
The physical measurements of the lead are:
:1 Flexible Lead Length
The length of the lead is measured from the top of the Brush to the centre of the terminal stud position.
The lead must be long enough to allow for full travel of the brush to its shortest position in the brush holder.
If the lead must be long enough to allow for full travel of the brush to its shortest position in the brush holder.
If the leads are excessively long there is the a possibility that they could foul in rotating parts, particularly in the case of Schrage type motors where the leads could catch in the moving gears and teeth on the racks.
Another problem with long leads can be that in motors with high velocity cooling air the turbulence may cause some leads to move around excessively. This can eventually harden and fracture the lead or chaft it against the constant force spring, if fitted, and eventually cut through.
Placing a band on adjacent pairs of leads approximately midway along their length provides additional support. In some special applications the copper leads can incorporate some steel strands to strengthen it.
If the flexible leads are relatively small in diameter placing a single insulation over two leads also gives additional support and keeps the leads tidy.
:2 Flexible Lead Diameter
The original lead diameter will generally be selected in relation to the maximum current that the particular brush can handle considering its dimension, the type of material used and the ambient temperature.
For high current carrying capacity copper graphite brushes the leads would be much larger than for an electrographite brush of the same dimensions because of the higher current carrying capacity of the material.
Another consideration of the lead size is potential short term overload.
The brush material can stand overload to varying degrees, however the lead may be the limiting factor, particularly with high starting currents that can occur with, for example, electric forklift motors.
In these cases the lead is best dimensioned to the largest lead practical to fit in the brush, taking into consideration how the flexibility of the lead my affect the free movement of the brush up and down in the brush holder.
When increasing the size of a single lead it may be necessary to change to two smaller leads for the benefits of better flexibility or because the material will be too weak around the hole and may crack the brush during tamping of the lead. In these cases the configuration of the brush and number of leads can usually be altered to allow for the higher current.
This not only has the benefit of increasing the current capability because of the larger cross section of lead but the additional tamped connection in the brush gives a lower resistance in the joints.
:3 Flexible Lead Insulation
Though lead insulation prevents the live lead shorting to earth or other live components nearby, it is better not to fit it as a standard option, if it is not necessary. This is because it adds to the cost and by enclosing the lead there is a de-rating of the current carrying capacity which may require the size to be increased.
Question 2:12 Are There Any Warning Devices To Indicate Short Brushes?
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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