SECTION 3 - BRUSH THEORY AND CHARACTERISTICS
Question 3:3 What Is Contact Drop Or Voltage Drop?
The general intent of voltage drop figures is to group carbon types for the total voltage drop over two brushes, (1 positive and 1 negative) contacting on the slipring of defined dimensions and specified speed and operating conditions.
The term "contact drop" is not strictly correct because the voltage drop across the brush and collector surface is only one part of the total drop from terminal to terminal.
This is well illustrated by the following example:
OHM VOLT DROP
AT 40 A
------------------------------------
Resistance of brush body = 0.003 0.12
Resistance between brush and
flexible shunt (Tamped connection) = 0.0015 0.06
Resistance between shunt and spade terminal = 0.0002 0.008
Total resistance excluding contact resistance = 0.0047 0.188
------------------------------------
Whereas the contact resistance is 0.325 ohms
The published total contact drop for two brush is 2.6V
Therefore for one brush is 1.3V
True contact Drop at the brush face 1.112V
The IEC recommendation may eventually influence manufactures to adjust the published figures to true voltage or contact drop. Care should be taken when comparing figures from different manufacturers to ensure the best conditions were identical, otherwise they cannot be directly compared.
Contact drop increases from standstill to operating speed by as much as 30%.
What influence does voltage or contact drop have on the brush selection and performance?
Because the brush has to handle part of the current reversal in the armature coils, the brush contact drop should be greater than or at least equal to the armature reactance otherwise too much current with flow in the brush face.
In operating conditions the contact drop varies with the condition such as the commutator condition and the spring force ect.
In this section we are dealing only with the inherent characteristics of particular brush materials in relation to each other tested under similar conditions as defined by the manufacture.
This could mean for example that a material may have the same reistance as another similar grade but have a higher contact drop and therefore could make it more suitable for a machine with difficult commutation.
On the other hand too high contact drop may cause too much heat at the brush face under certain circumstances.
The brush specialists are best able to make these kind of selections.
Measuring and control applications using sliprings often run into problems because the contact drop, which can typically be in the order of 1-2 Volts, is too high in relation to the voltage being measured.
For application involving thermocouples, strain gauges or analogue signals of low voltage the contact drop in the brushes can interfere with the accuracy of the readings. Also the variable contact drop can indicate varying signals which are in reality stable.
The use of silver plated rings and silver graphite brushes can help but special mercury wetted enclosed self contained sliprings of the "Mercotac" type or similar are excellent for this duty as the contact resistance can be less than one milliohm.
Question 3:4 What Relevance Is Brush Hardness?
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.
以上資訊為台灣慕根耐公司應用工程專家提出, 未經書面許可請勿下載複製及引用