Electrical Spec and Structure Design of Slip Rings
For various slip rings with hole there are common electrical specifications: insulation resistance and rings which should consider factors of matching, shielding, noise voltage. The Insulation resistance is the leakage resistance from the ring to ring Ci and from the ring to the ground. It mainly depends on the performance of insulation material and the spacing of adjacent circuit. For the average slip ring, it is fine when the minimum insulation resistance is 100megohm under the humidity of 60% and the voltage of 500V. While for the high pressure slip ring, it needs higher insulation resistance.
The material of the gasket and the insulating strip should have a good dimensional stability and low hygroscopicity under the condition of the working environment. Because when the material becomes damp, the insulation resistance would be reduced; and when the temperature has changed, the unstable size and the damp part of the slip rings would have some bad impacts on the contact resistance and the work reliability of the slip ring, so that shortening the life span.
1) Contact Resistance
The tiny electric resistance between the ring and the brush contact surface is called the contact resistance. It is the direct current d1between the terminal resistance and the corresponding conductive ring. The terminal resistance is the contact resistance of the brush connection, and it is very tiny (generally less than 0.005) It is relatively hard to measure, and when in the process of measurement, the probe should be as close as possible to the contact point between the ring and the brush.
When the slip ring rotates, the contact resistance would change. And when we magnify the contact point between the brush and the conductive ring with the microscope, we can find that it is made up of many tiny peaks and valleys. In the process of rotation, the contact area change with no rules, so the dynamic contact resistance is usually considered sound absorption. The measurement methods of the dynamic contact resistance is generally through a certain value of the direct current and observe the peak voltage between the conductive ring and brush on the oscilloscope, and to determine the contact resistance according to the voltage.
2) Dielectric Strength
The dielectric strength is measured through adding certain voltage in the adjacent circuit of the conductive ring. The voltage could be the direct current or 50Hz alternating current. When the arc discharge occurs among circuits (usually is destructive) or the leakage current reaches extreme value, then we consider it as failure. In the high voltage test, the relay of the current should be limited. When the leakage current exceeds the limit value, then cut off the circuit.
High voltage test and insulation test are both part of the dielectric properties, and they are different in the aspect that the insulation resistance is the total resistance measuring the two insulation circuits, while the high voltage breakdown is to measure the dielectric strength of the weakest point between the two circuits. Breakdown generally occurs in the place where there has the most narrow air gap, such as along the ring of an insulator or between the slip ring terminals. Because the strength of most insulating material is three to six times of that of the air, so increasing the purity of insulation piece can effectively increase the distance of the air gap. Under the standard room temperature and wind speed, the dielectric strength of the air is approximately 3 KV/mm. and in the actual design, we usually take half of this value. In the design of the slip ring, the shape of the ring, brush, and the gasket should be smooth and continuous, and avoid sharp corners so that reducing the possibilities of corona. Most of the circuit should be able to withstand the dielectric strength test with twice of the rated voltage plus 1000 v.