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Brushes and Brushgear
Introduction
Brushes are an essential part of any rotating electrical machine that has either a commutator or slip rings. ('Commutator' is used hereafter to mean both commutator or slip rings unless slip rings need to be referred to specifically.) The brushes conduct the electricity to and from the rotating element as they slide over the commutator. Brush material is a careful compromise between conductivity and minimum wear of both brushes and commutator.
Reducing uneven wear on the commutator
To prevent brushes from wearing grooves in the commutator, they should be staggered so that their tracks overlap. If there are more than two brush spindles, then alternate pairs of spindles and not alternate spindles should be staggered. Each positive brush then runs on the same track as a negative brush and uneven wear due to polarity effects is eliminated.
Classes of Brushes
The following classifications are those from The Morganite Crucible Company.
| Morganite | 
with a number inside the symbol | Morganite brushes differ in one essential respect from all other graphitic brushes in that the specially selected plumbago from which they are made is not taken up to a high temperature during the course of manufacture and so the lubricating properties remain unimpaired. They are also distinguished by their very high electrical and thermal conductivity. Because of their superior lubricating properties, they are used largely on high-speed machines and are well known for their silent and sparkless operation. |
| Hard Morganite | 
with a number following the symbol | Hard Morganite brushes also possess high electrical and thermal conductivity. The term Hard Morganite is used to denote a class of brushes composed of pure graphite specially selected for its capacity to withstand a high temperature during manufacture without losing its lubricating qualities. The brushes are not hard in the ordinary sense, but on the contrary, are soft and resilient compared with ordinary carbon brushes. HM brushes are made in various degrees of density because in some applications it is useful to have low density brushes so that they have less inertia and therefore maintain a more intimate contact with the commutator. |
| Electro-graphitic | 
with a number following the symbol | Brushes that are made from natural graphite and having the characteristics of Morganite and Hard Morganite are somewhat fragile. For traction and similar work, it is often essential to use a brush possessing properties similar to Morganite and Hard Morganite and yet capable of withstanding very severe mechanical conditions. Such a brush must be capable of carrying very heavy overloads, of running cool at high speeds and of successfully resisting mechanical shocks. This demand brought into being the EG series. EG brushes have no natural graphite in their composition, but are still made from various forms of carbon. The final process in manufacture is the conversion of the carbon and its binding material into artificial graphite by baking at a high temperature in an electric furnace. This treatment produces striking changes in the characteristics of the material, increasing its electrical and thermal conductivity, rendering it highly refractory, softer than before graphitization yet at the same time very tough. This material is also very suitable for switch contacts which are subjected to sparking and mechanical shocks. |
| Copper Morganite | 
with a number following the symbol; the higher the number, the higher the graphite content | Prior to the introduction of carbon and graphite brushes, copper gauze was almost universally employed for making dynamo brushes. Such brushes were, however, apt to devour the commutator or rings on which they ran with remarkable rapidity, but at least they possessed the virtue of very low contact resistance. This is a feature of great importance when heavy currents have to be dealt with and commuting difficulties are trivial or non-existent. CM brushes contain admixtures of copper and other materials in a very fine state of division. The brushes at the highly metallic end of this series approximate to copper gauze brushes but do not have the same destructive tendencies. |
| Battersea Carbon |
with a letter inside and a number following the symbol | Battersea Carbon brushes are made from various forms of carbon, with or without a certain amount of graphite. This class satisfies a large range of applications where operating conditions are not particularly exacting. Group A contains a moderate amount of graphite, group B a larger proportion and group C, no graphite at all. These brushes all have a higher contact drop that the more graphitic classes. |