Selection of brush holders
A carbon brush holder assembly fulfils a significant function for a complete and satisfactory slip ring system.
It is therefore very important that you allocate enough time to identifying the spring system and spring pressure as well as placement and configuration of the carbon brush holder. The objective is to achieve the best possible combination of contact and voltage drop. This is because it affects system stability, mainly in the form of thermal loss.
The most common spring system assemblies are:
- Pressure finger – Most common alternative where the carbon brush length does not exceed 65 mm.
- Constant force spring – Most common where carbon brushes have longer dimensions e.g. carbon brush holder assemblies for generators in wind power turbines.
- Tension spring – Usually used for smaller carbon brushes and are a constituent part of the carbon brush design.
Selection of spring system
Choice of a spring system is usually related to the dimensions of the carbon brush. It is important to be aware of the fact that variations in spring pressure, depending on the characteristics and movement of the spring, can be up to ±10%.
It is also very important that the brush’s regular weight is taken into account. When many brushes are arranged in the same phase it is essential that they are at the same angle of inclination to the slip ring’s contact surface. This is to avoid pressure differentials and any possible imbalances in distribution of currents between the brushes.
There are many different theories and schools of thought regarding the best spring pressure. It is important to ensure that the pressure is sufficient for the function and application, where vibrations, contact and voltage drop are all taken into account. Spring pressure usually varies between around 175-250 cN. For train locomotives and applications where vibrations prevail with high amplitude, significantly higher pressure can be used.