SLIP RING SYSTEMS

ventilation
brush holders
carbon brushes

Slip ring range

Carbex offers a complete range of power and signal slip rings.

Everything from simple standard solutions to complex devices. In our test lab we can test and simulate complete devices according to customer specific driving cycles. In our controlled climate chamber we can test and verify the performance of products regarding, for example, temperature, humidity, air flow and vibrations to simulate the actual operating conditions of the application.

Check out our inquiry portal, PDF data sheets with product information or contact us.

A slip ring assembly is a complex system consisting of four main parts:

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For a start, you have to take account of things that you cannot control, or can only partially influence, such as cooling temperature, humidity, radiant heat and polluted air. The most important things to consider when designing your system are power and speed.
To begin with, power current is the most important. The total size of brushes (t x a) must be able to withstand the maximum current and the slip ring’s speed.
Because the carbon brush is not a fixed component, the voltage drop will fluctuate between the carbon brushes. Another parameter that will affect the voltage drop is the brush pressure. If you place a carbon brush on the upper side of the slip ring and another on the underside, the carbon brush’s weight will exert a significant difference on the brush pressure.

The friction between the carbon brush and slip ring generates heat. The maximum temperature is around 80°C. If it gets any hotter the excess heat must be diverted away or the system must be cooled in some other way.

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Slip rings

A slip ring can be made in different ways and with different materials.

Make the slip ring small
The important thing with rings is to make them as small as possible. The reason for having a small slip ring diameter is to keep both the voltage drop and friction down (minor losses to cool off). It is always better to make the rings wider than to go up in diameter to create an effective area.

Ensure cooling of the slip ring
Helical grooves are used for many applications. The cooling capacity is enhanced with helical grooves in the slip ring but, at the same time, the contact surface for the carbon brushes is reduced resulting in higher losses and higher temperatures. Another advantage of helical grooves is that they can remove carbon dust from the contact area between the brush and slip ring. To avoid hot spots it is advisable to have two connections for each phase that comes from the cable connection.

Select the best material
Stainless steel rings are more resistant to mechanical wear, but at the same time not as good a conductor. It is therefore important to make a choice between bronze or steel rings from the outset.

1. Prefabricated slip ring units

Advantages

  • Parts are integrated
  • Balanced
  • Easy to assemble
  • Usually welded contacts between bolt and ring.

Disadvantages

  • Only cooling on the visible part of the ring material

2. Assembled slip rings units

Advantages

  • Cooling.

Disadvantages

  • Difficult to balance
  • Thermal problems can arise in connection plugs

3. Bronze (brass) rings.

Advantages

  • Good conductors
  • Easier to cool
  • Smaller dimensions

Disadvantages

  • Sensitive to high and low temperatures

4. Stainless steel rings.

Advantages

  • Mechanically stable.

Disadvanatges

  • Inferior conducting properties
  • More difficult to dimension than brass rings

5. Helical grooves

Advantages

  • Removes carbon brush dust from the slip ring surface and brush holder.
  • Better cooling capacity.

Disadvantages

  • If helical grooves are used you have to ensure how to calculate current density (A/cm²).

Ventilation

The most common ventilation systems are usually one of the following types:

1. Axial fan that sucks air from or blows air into the slip ring unit.

Advantages

  • Price.
  • Easy to assemble.

Disadvantages

  • No optimal air flow as it is difficult to get air to the parts which need to be cooled
  • The air flow depends on speed and not temperature

2. Auxiliary fan that blows air into the slip ring unit

Advantages

  • Important parts of the slip ring are cooled with the correct design

Disadvantages

  • Price
  • Usually runs at the same speed and same cooling capacity

3. Heat exchange systems

 

Advantages

  • Ability to maintain good working temperature of the rings even with low loads

Disadvantages

  • Use of dry heated air that can affect the humidity and result in bad brush behaviour .

Cooling air from outside or inside?

There are mainly two ways to take in air. One way is to use outdoor air. The disadvantage with this solution is complicated as the external temperature and humidity change season to season.
The second way is to take the air from indoors. This gives a better temperature, but the air can be contaminated with oil meaning filtration is required.

Brush holder

A good brush holder system depends on several things.

For shorter brushes (up to 64 mm length) pressure fingers are a solution. For longer brushes, constant force springs are usual.
At least two pocket brush holders are used for each ring. This is to ensure you always have good contact between the brush and slip ring. When there is more than one brush on the ring it is important to mount the brush holders at the same angle to the ring.

The size and placement of the carbon brush affect the carbon brush pressure
Large copper brushes affect the weight and angle pressure. Many generators, for example in wind turbines, use carbon brushes in the dimensions 40 x 20 x 100 mm. The weight per carbon brush is about 300 g depending on the material content. With a brush pressure of 250 cN/cm² the total pressure will be around 2000 cN. If one of the carbon brushes is placed at the top of the slip ring and one at the bottom, there will be a difference on the brush pressure of up to 30%. This will cause uneven current distribution between the brushes and can even cause thermal problems. Another thing to consider is that the brush pressure changes a little when the brushes wear down.

It is always better to use two brush holders with three pockets as opposed to three brush holders with two pockets. In other words, three brushes instead of two for one slip ring slot. This also improves the air circulation around the brush for cooling.

Carbon brush holders
Carbon brush holders
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Carbon brushes

There are many different carbon brush materials. The most common are electrical, silver and copper graphite. Contact us for guidance on the choice of carbon brush material.