All About Slip Ring Brushes

by | Jul 15, 2021

Have you ever wondered how electromechanical power and current are transmitted? The slip ring brushes are used in several electromechanical transmissions of signals and power. These include domestic use, industry use, and automotive use.

Here in this article, I will be telling you about slip ring brushes and their uses.

What are slip ring brushes?

Electric_Motor_with_Slip_Rings

The structures of slip rings are designed for the creative performance of brushes. The slip ring brushes are also called the sliding contacts. These brushes are the electrical components used in transmitting electrical signals and power. The signals are conveyed from a static body to a rotating structure. The brushes are made from metals and are identical to wool brushes in shape.

When the electric machine is operated, it causes brushes of slip rings to be lifted. The signal sends an electric current directly from the stator to the rotating part of the slip ring. The slip ring brushes have a wide range in their applications. Some of the standard applications of brushes include cameras, automation, robotics, and medical equipment. The primary determinant of a slip ring brush is the usage, and the amount of power put through the slip ring brushes.

Types of brush material

There are diverse types of brushes depending on the structure of the motor. There are three major brush technologies in the electric slip-ring. These include composite, monofilament, and poly-filament. We can break down these three major brush technologies into;

1. Carbon brushes: Carbon brushes are typically used on AC or DC applications. These applications include commutators and slip rings. The carbon brushes can be made of one (1) or more carbon blocks equipped with one or more shunts or terminals.

2. Copper brush: For a moment, they used Copper brushes in early rotating electric machines. Besides, some slip ring applications still use copper brushes. For transmitting signals, power, and low noise applications.

3. Silver brush: Silver brushes are brushes with reliable conductivity. Silver has a more comprehensive operating range and wind capacity. It is also a wiser preference for longevity and lifetime performance. Silver brushes may be expensive to get for wind operators. But these operators will enjoy fewer repairs or brush changes long term.

4. Graphite brush: Graphite brushes comprise artificial or natural graphite. Graphite is bonded with pitch or resin to form a soft brush material.

Why do we choose carbon brushes over copper brushes?

carbon brush holder for slip ring motor

The reason why people change their brushes is one of the most frequent questions asked. Here are some of the great reasons why we do not use copper brushes but use carbon brushes.

1. Price difference: Copper is more expensive than carbon. That is, cheaper and works almost the same way. Carbon is still good enough to be used in place of copper in some cases where copper would have been a better choice. Copper is so costly that it is not even used in high voltage power lines.

2. The thermal expansion of carbon brushes is minimal and corrosion-free. The low thermal expansion enables proper contact with the slip rings. Thus, carbon brushes are preferable to copper brushes in an accelerator.

3. Overheating: Copper brushes create more heat and cause faster wear than carbon. The carbon brush design makes it wears replaceable without stress. And wear down much faster than the collar on the motor.

4. Resistivity: Carbon brushes have high resistance, which makes exchange easy. In the exchange process, the current passes through brushes and makes arcs. So, there can be a reduction in the arc of current flow using carbon brushes with high resistivity.

5. Self-lubrication: Carbon brushes are self-lubricant. That is, they do not make any spark during commutation. Meanwhile, copper brushes are wounded during exchange due to the hardness of copper.

6. Carbon brushes are softer than copper brushes.

7. Carbon ensures the most critical contact with the commutator segment. And will also get the shape of commutator segments.

Why do we not use carbon brushes and slip rings in a brushless alternator?

As the name implies, a brushless alternator requires no brush!

Hence, we do not need any means to transfer the DC supply to the rotor in a brushless alternator. It makes use of a motor without carbon brushes to generate electricity. A brushless alternator uses a small generator at the edge of the equipment to transmit electrical current. A brushless alternator uses 2-sets of rotors that spin together to transmit and generate an electrical current in place of carbon brushes.

The exciter has static field coils and an armature that rotates. The brushed alternator uses a brush and slip ring to transfer the DC supply to the rotor. And a brushless alternator whole rectifier assembly is positioned in the rotor to transfer current. In a brushless alternator, the excitation is used at the rotor and obtained from the stator.

Without much ado, they made the design of the brushless alternator to use the excitation panel. The excitation current is delivered to the stator of the exciter. Then generates high-frequency AC in rotor winding which is moved to the Diode wheel mounted on a twin shaft through cables.

Summary

In the design of the slip rings, the metal brushes are connected. The connection helps in the conduction of current between the static wires and the rotating shaft.

The slip rings use various brushes such as carbon brush, graphite brush, and copper brush. The copper brush gives the best result, but due to factors like cost of maintenance, resistivity, overheating, etc., carbon brush has been its best substitute.