SECTION 3 - BRUSH THEORY AND CHARACTERISTICS
Question 3:5 What Is Current Density?
Current density is the value of the current passing through a particular brush in relation to its contact area and is expressed as Amps per cm2 or Amps per square inch.
The actual current a brush can carry is widely influenced by operating conditions such as type of ventilation, continuous or intermittent duty, speed and other factors. The published data sheet ratings for electrographite brushes are generally conservative, some allowance having been made for short term overloads above those listed in the published data.
The current carrying capacity of a brush depends ultimately on the operating temperature. On well-ventilated machines having small brushes with larger surface area in proportion to their volume and where brushes cover only a small percentage of the commutator or ring surface, conventional current densities for electrographite grades can often be doubled without seriously jeopardising their performance.
On
the
other hand, increasing the current density without making provisions for
maintaining a suitable low brush temperature may reduce the brush life
dramatically.
In
practice low current density in a machine caused by running a machine below full
rated load is potentially more damaging than a moderate overload. For good
operating temperature and performance as a general rule, the actual operating
current density should be not lower than 60% of the published rated current
density.
Question 3:6 How Is Current Density Calculated?
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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