Time：2021.10.18 Source：Roller Bearings Suppliers
Electric motors play a vital role in our everyday lives – where we live, work and play. Simply put, they make almost everything that moves, move. Nearly 70 percent of electricity consumed by industry is used by electric motor systems .
Roughly 75 percent of the industrial motors in operation are used to run pumps, fans and compressors, a category of machinery that is highly susceptible to major efficiency improvements2. These applications often operate at constant speed, all the time, even when not needed. This constant operation wastes energy and produces unnecessary CO2 emissions, but by controlling a motor’s speed, we can reduce power consumption, saving energy and reducing environmental impact.One way to control a motor’s speed is through the use of a variable speed drive (VSD), a device that regulates an electric motor’s rotational speed by varying the frequency and voltage supplied to the motor. By controlling a motor’s speed, a drive can reduce power consumption (for example, reducing rotating equipment speed by 20 percent can reduce input power requirements by approximately 50 percent3) and provide a considerable improvement in process control and significant cost of operation savings over the life of the motor. WSBC 23224 CC/W33 bearings online , pls click here :
As useful as VSDs are for saving energy in many applications, they can cause premature motor failure if not properly grounded. While there are many different causes of electric motor failures, the most common issue when using a drive is bearing failure caused by common mode voltage.
Damage caused by common mode voltage
In a three-phase AC system, common mode voltage can be defined as the imbalance present between the three phases created by the pulse width modulated power of the drive, or the voltage difference between the power source ground and the neutral point of the three-phase load. This fluctuating common mode voltage electrostatically induces voltage on the motor’s shaft, and this shaft voltage can discharge through the windings or through the bearings. Modern engineering designs, phase insulation and inverter spike-resistant wire can help protect the windings; however, when the rotor sees a buildup of voltage spikes, the current seeks the path of least resistance to ground. In the case of an electric motor, this path runs directly through the bearings. WSBC 23152 CCK/W33 bearings online , pls click here :
Since motor bearings utilize grease for lubrication, the oil in the grease forms a film that acts as a dielectric, which means it can transmit the electric forces without conduction. Over time though, this dielectric breaks down. Without the insulation properties of the grease, the shaft voltage will discharge through the bearings, then through the motor’s housing, to achieve electrical earth ground. This movement of electrical current causes arcing in the bearings, commonly referred to as electrical discharge machining (EDM). As this continual arcing occurs over time, the surface areas in the bearing race become brittle, and tiny pieces of metal can break off inside the bearing. Eventually, the damaged material works its way between the bearing’s balls and races, causing a grinding effect, which can produce micron-sized pitting, called frosting, or washboard-like ridges in the bearing raceway, called fluting.
Some motors can continue to run as the damage gets progressively worse, without any noticeable issues. The first sign of bearing damage is usually an audible noise, due to the bearing balls traveling over the pitted and frosted areas.
But by the time this noise occurs, the damage has usually become substantial enough that failure is imminent.
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