The explosion protection of electrical and mechanical machines serves the protection of persons and production- and storage sites (e.g. power plants, offshore sites, chemical industry) in hazardous areas.
The best type of explosion protection is the physical segregation of equipment capable to cause an explosion and explosive atmosphere. This separation is not always possible and therefore it is fundamental to protect electrical equipment insofar, that it cannot cause an explosion.
The EC directive 94/9/EC ATEX is defining minimum requirements for electrical equipment
The directive differentiates between areas where a gas- or dust explosion may occur.
direct current motors for areas above ground with potentially explosive gas atmospheres in protection degree "d" flameproof enclosure:
The service life of the carbon brushes will be influenced by many factors, such as load alternation (also idle speed), vibration, dust, humidity and chemical influences.
As carbon brushes have to be considered as wearing parts, a first control check after an adequate time of about 500 operating hours is necessary. The level of observed abrasion gives you the basis for the maintenance intervals to be defined.
When the indicated level of remaining carbon length is reached, the carbon brushes have to be exchanged with new ones of the same material.
Dimensions of the brushes
Remaining carbon length
8 x 10 x 20
8 x 16 x 23
10 x 16 x 25
12,5 x 16 x 25
10 x 25 x 32
12,5 x 25 x 32
16 x 25 x 32
20 x 32 x 40
It is absolutely necessary to grind in new carbon brushes to the diameter of the collector before first start up.
Within one set of brushes only carbon brushes of the same material may be used.
The position of the brush rocker (marking of the neutral zone) needs attention and may not be changed. If the motor is running with an incorrect adjusted brush rocker, brush arcing and increased carbon brush wear-out will occur.
At the surface area of the collector patina is generated. The patina serves as sliding coating between the collector and the carbon brushes. In order not to destroy this sliding coating, the collector- and the carbon brush contact surface should not be touched with fingers, neither be contacted with fats nor mechanically damaged. If the patina gets damaged the consequence will be an increased abrasion of the carbon brushes.
Carbon brushes exist since 1870 when the first collector machines were developed.
Facing the advancing development of the DC drive loaded with brushes towards the concepts without brushes you should think that carbon brushes are a dying product. But there is no tonic like reading your own obituary. Even AC drives without brushes often do need a carbon brush for the dissipation of the capacitive shaft currents.
Improved formulas, matched impregnation and intelligent solutions in connection with the brush holder, life time of carbon brushes have increased in the past years and due to this the reputation of the DC motor has improved accordingly. A brush service life of 20.000 hours is no problem anymore in many applications.
More than 500 different standard materials out of 10 materials categories are offered by named carbon producers to solve the problems of their customers. Year by year it becomes even more. Combined with the various brush versions, the user is faced with an enormous choice of product variety, hardly to overlook, but thus offering the chances of a customer orientated or application orientated solution of the problem. In close teamwork with the motor producers the optimal solution is filtered out.
Carbon brushes are ceramic carbon materials, which are processed out of carbonic raw materials like coke, graphite, carbon black, tar pitch or resin. Metals are creating the adequate voltage drop in brushes e.g. for low voltage motors.
The carbon materials suiting the respective applications are produced in various processing steps using heat treatments up to 3000 °C. After-treatments with metals, oil or waxes can optimize the running performance. The shape of the brushes e.g. block- or twin brush can have besides others a major effect on the commutating properties of the brushes. The whole production process can take up to one year.
In the course of the last decades the carbon brushes were faced with new challenges.
Besides the obvious control of the commutating properties on DC motors, difficult environmental conditions like chemical influences, high or low air humidity or load problems are the focus of attention nowadays.
In times of reduction of maintenance staff, the interpretation of commutator- or ring surfaces and the carbon brushes themselves become increasingly important. When applied correctly carbon brushes are good indicators for the condition of the machine respectively are leading to remedial measures.
Too low capacity utilization of the motors or too low collector temperature creates so called scorings and collector attack. The optimal temperature is between 60 and 90 °C.
Corrective measure can be achieved by adaption of the number of brushes or modification of the cooling.
Difficult environmental conditions, such as e.g. vapors of alkaline solutions or acids, oil, silicon, dust, etc. are demanding more from the carbon brush materials. High wear and tear and collector attack are often the consequences of inappropriate materials chosen.
Changed cooling air supply respectively coating adjusted materials can often be of help.
These trapezoidal burnings at the edge of the commutator segments are typical for faulty commutation. Reasons can be damages at the windings, incorrectly justified commutating poles or a wrong position of the brush bridge.
Shiny side faces of the carbon brushes and damaged power lines are an indication for brush vibrations. These are triggered by too large distance of the holder, non-circularity of the collector, too smooth surface of the collector or external vibrations.