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.
2. On diesel electric systems: main and auxiliary generators. (Often replaced by brushless alternators with diode rectifiers).
3. Traction motors: traction motors are generally quite different from industrial motors. In order to obtain the required torque/speed characteristic, they are almost always series wound motors. This gives a fairly constant powder output over the speed range, with torque declining as speed rises. Thus, on starting, the high armature current gives a high field current, which provides the torque needed to accelerate the locomotive from rest. As the speed increases so does the back emf; thus the current falls, the field weakens and the speed rises further.

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:

• 1) Series wound in order that they can be used on both AC and DC current;
• 2) Designed to run at very high rotational speeds. FHP machines used in industry are more likely to be DC fed, often from a thyristor -rectifier control system, or else to be an ac induction motor and consequently without brushes.