buildre proved accurate grasp of the slip ring, brush the key points, and take effective technical measures set brush life and reliability of the electrical device is fully improved and improved. To this end, we made a lot of experiments, the results show that as long resolve slip ring, brush friction, line speed, and lubrication and other key issues, even the life of an ordinary brush can reach 27,000 hours (about three years ) or more, while the handle slip ring finish, hardness, conductivity, its service life can be up to 15 years. Obviously, such a performance can meet the requirements of equipment operation. However, the reality of a brush collector device and a far cry from the experimental results, the average life of the brush only about 2000 hours or so, slip ring life is very short, and the failure rate is higher. We have access to large amounts of information, trying to find the crux of the problem, but with little success, so we focused a lot of slip rings, brushes damage phenomenon independent research experiments and found two new rules, made a breakthrough.

1.1 Jitter and sparks

       Sparks is a slip ring, brush the number one enemy of reliability, once produced, high-temperature, high-energy will slip ring surface erosion, destruction becomes rough finish, then brush wear intensified. At the same time, so that the slip ring and brush contact ablation area decreases, the current density increases, the spark intensity is further increased, whereby the vicious cycle, until the slip ring is damaged.
       Sparks brush and slip ring due to poor contact between the current density is too large to produce a discharge phenomenon sparks due to the current density is too large (ie, the electric field strength is too high), then what is causing the slip ring and brush The current density is too big? According to the current density, since the rotor current I is determined by the load, even in the start state, it is impossible to increase the level of the discharge, therefore, the current density is clearly huge increase sharply due to the conductive area S. Further, when the slip ring stationary state, the brush contact is usually good, poor contact occurred in the rotation state, the moment generated by the motor is running radial jitter of the slip rings, brushes produced from (although the brush spring action , but not transient response), causing the brush and slip ring contact area dropped even zero, so the current density generated great cause sparks. A lot of practice proved this conclusion.
       Then slip ring jitter caused what is the reason? There are three:

1) additional torque

       Additional torque induced jitter and sparks occurred in the start-up phase, during which the motor starting current is large, low speed (rotor frequency higher). According to motor learning, the additional torque of the motor rotor and stator fundamental harmonic imbalance torque generated by the role, it will produce a strong motor noise and vibration, seriously affecting the normal operation of the motor. Additional torque caused by jitter usually occurs only in the motor starting process, a specific speed range, when the starter is completed, additional torque becomes very small, negligible, however, the jitter caused by the instantaneous spark hazard has been formed. It is noted that the design and manufacturing from the motor to overcome the source of additional torque is extremely difficult, realistic approach is to constrain the slip ring, to get rid of the effect of additional torque, or to minimize the range of motion, but this requires the traditional winding machine difficult to achieve.

2) slip ring runout

       If the slip ring and rotor coaxial, requiring slip ring and shaft must ensure strict concentricity, runout should not exceed 0.01_ 0.05mm, otherwise, when the motor at high speed, slip ring and shaft eccentricity error will have a great unbalanced centrifugal force, balancing the destruction of brush and slip ring pressure is not balanced, serious will produce sparks, ablation slip ring, even minor, will also lead to uneven wear slip ring, forming an ellipse, while the slip ring surface destruction increased smoothness, significantly reduced life.
       The problem is the traditional winding induction machine by limiting the production process, can only be completed before the processing of the slip ring mounted on the rotor shaft and the rotor winding cable to pass, the slip ring end of the hollow shaft to be difficult to the sliding ring is installed after the whole process, so the slip ring runout large result can not be avoided.
       Bildt through a lot of practice shows that as long as no spark erosion, purely mechanical friction on the slip ring surface smoothness damage is very small, the average life expectancy slip rings up to 10 years, even though there is local damage due to impurities in the brush, it will not significant impact on the conductivity, but does not produce sparks.

3) slip ring cantilever structure and defects

        External power collecting device is typical of the end of the slip ring and the axis of cantilever structure, in this structure, jitter constraint slip ring bearing only the end of the motor, slip ring the other end is open and is not binding. Therefore, when the additional torque produced by the motor, or the misalignment of the slip ring and the shaft with eccentric force, a corresponding slip ring bound jitter, amplitude jitter with the slip rings and the end bearing is proportional to the distance increases. Therefore, the cantilever structure external power collector inhibiting the ability of poor jitter, resulting slip ring ignition is widespread, and once problems occur, there is no simple and effective approach.
      For example, a heating company 250KW wind turbine, motor capacity is not to limit the starting current is only about 2 times, but the motor starting to close the rated speed, sudden and sharp jitter, amplitude of up to 2mm above the brush and slip ring spark generated between intense, severe erosion slip ring surface, the motor can not operate normally. To this end the manufacturer of the motor Depot, for bearings, slip rings repeated handling, but the result remains the same.
        Additional torque due to the asynchronous machine is not completely eliminated, and it is difficult to accurately quantify, therefore, to solve the resulting jitter problems can only be attributed to how to suppress the jitter amplitude, and can not imagine eliminating the root causes of jitter.
        Built-in slip ring collector means mounted between the motor bearings at both ends, in theory effectively suppress jitter, but produced a new problem. First, the increased length of the rotor shaft, deflection lowered if bold shaft diameter, not only increases the size and cost of the motor, but also affect the electromagnetic properties of the rotor; second complex structure, the slip ring brushes and the windings be isolated to prevent damage to the winding insulation graphite dust, but also to consider replacing the brushes easy to design caused many difficulties.

 Line speed and wear:

        If you solve the problem sparks, brushes, slip rings lifetime depends on the friction coefficient and the line speed. Since the coefficient of friction factors determine - smoothness (including lubrication), the spring pressure is easy to be satisfied in the processing, so that the relative movement of life issues focused line speed.
According to the literature description, relative motion of objects and the wear rate is proportional to the square of speed, therefore, to reduce the brushes, slip rings increase the life of the wear rate, the key is to reduce the speed of relative movement between the two.
       Slip ring brush relative speed
       Wherein the motor speed depends on the number of poles n and production requirements, not changed, so to reduce the line speed is only reduced the diameter of the slip ring D.
       Conventional slip ring winding machine set on the rotor shaft, slip rings can not imagine a diameter smaller than the diameter of the rotor shaft, the rotor load which is based on the strength requirements of the design, the larger the high-voltage power motor, the rotor shaft diameter of at least of 100mm or more, plus the slip ring and shaft insulation thickness, slip ring diameter is impossible to reduce to the desired level.
Diameter slip ring brushes and slip rings seriously affect the working life of the parameters on the grounds that the slip ring diameter and the line speed is proportional. Brushes and slip rings to extend the wear life of the most intuitive and simple way is to reduce the diameter of the slip ring, but it is for conventional induction machine structure is almost impossible.
       Both built-in or external type collector device, the slide ring is rigidly mounted through an insulating layer on the rotor shaft, slip rings can not be smaller than the diameter of the diameter of the rotor shaft, while the latter is designed according to the motor load strength, can not take into account the slip ring speed.
       Looking at the domestic and international pressure wound type induction machine, slip ring shaft diameter is approximately 2.5 - 3.5 times more than reasonable slip ring diameter of at least twice as large or more, and the resulting line speed is doubled, electricity brush increases the wear life significantly.