DC traction systems have inherent differences to their industrial counterparts having to cope with increased levels of vibration and thermal shock as a result of rapidly changing load currents. The advent of fast switching thyristor control and regenerative braking have added to the harsh conditions already experienced by the carbon brush which is a vital part of the current carrying chain, so it is imperative that the correct grade of brush is chosen to optimize the performance and reliability of the equipment.
Traction applications may be divided into three categories:
1. On pure electric systems: ventilator motors, lighting alternators or generators, motor alternator/generator sets.
Note: In general, these machines can be considered little different from their Industrial counterparts. They are subject to only slightly higher vibration levels and no special features need normally be incorporated in them.
Assam Carbon, India’s number one traction brush supplier offers a wide range of materials and design expertise to minimize the effects of harsh running conditions.It is an established, experienced high performance brush manufacturer with over 50 years’ experience in serving the traction industry. Optimum brush grades are developed in the centers of excellence, using our own testing facilities for in house development and customer support.Sharing knowledge and experience, Assam Carbon will give local technical support backed up by an expert team of application engineers.
Industrial brush applications cover an extremely wide field: from fractional horse power delivering or consuming less than 750 watts to the very large motors found in heavy industry and the turbo alternators used in power generation plant.
Industrial FHP machines
The smallest industrial machines are mainly motors or tachometer generators. They usually differ from “consumer” motors of similar power in two major ways, consumer motors are:
If fed from a relatively high voltage supply of say 120 volts or greater, then industrial FHP motors consume only moderate currents. Grades from IM or PM class may be used both here and also if thyristor control is used, as rapid current changes can induce high inter-segmental emf, which worsens commutation.
Motors running on pure DC supply usually present no great problems and grades such as Link A or B from the carbon graphite class are used. The lower resistivity electrographites such as Grade EGO, EG12 and EG260 are also used.
Most FHP machines have recessed insulation but where flush insulation is used, then the range of brush grades is restricted to A2Y or grade C4, or where current density is low, to IM or PM grades.
Permanent magnet motors form a greater proportion of the total motors manufactured. There are normally no extra problems likely to occur because of this.
Tachometer generators are machines specifically used for showing rotational speed of a shaft. They are designed to ensure that their output is directly proportional to the speed and thus many are used to feed a meter, built in or in a control panel. They may also be incorporated into a servomechanism that maintains constant speed. Their output current is generally low and internal losses are low. Low loss but consistent contact drop brushes as found in silver graphite grade SM3 is popular, although some motors, where the output is higher, use an electrographitic grade such as EGO.