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How to Find a Solution for Bearing Currents |
Author:Oreste Scioscia, Jr., Eaton 2014-6-27 16:40:05 |
Source:http://www.pump-zone.com/ |
Bearing currents occur in specific motor and drive installations and can lead to motor fatigue and failure. While no system is inherently immune to bearing currents, few installations experience bearing currents during normal operation. While many installations have preventive measures installed, knowing that protection is in place is sometimes outside the owner or operator’s scope. Therefore, end users must understand the symptoms and causes of bearing currents to properly identify and address this problem. What Are Bearing Currents? Bearing currents occur when an induced voltage on the motor shaft is high enough to overcome the breakdown voltage of the bearing lubricant. This is typically greater than 50 volts. These shaft voltages will do one of the following:
Figure 2. Circulating current flow in a motor
Figure 3. Magnetic fields from driving current Either current type results in fluting in the bearing race. Therefore, the bearing’s rotation is no longer smooth (see Image 1).
Either current type results in fluting in the bearing race. Therefore, the bearing’s rotation is no longer smooth (see Image 1). - See more at: http://www.pump-zone.com/bearings-motors/june-2014-how-find-solution-bearing-currents#sthash.WLjYO7ys.dpuf
Image 1. Fluting in bearing race caused by bearing currents
Bearing Current Causes Voltage in the shaft is induced by the magnetic fields generated by current flowing through the motor windings. For motors connected across the utility line (not connected to a drive), these magnetic fields are ideally balanced and yield a net voltage of zero with respect to earth on the rotor and shaft. This is because the stator windings are symmetrical. When motors have asymmetry within the windings, it can cause a net voltage different than zero, which can lead to bearing currents. Drives can cause imbalances in the supply voltage through pulse width modulation (PWM). Motors supplied by PWM are unbalanced because the motor is supplied by pulses. This can result in a neutral point that is not equal to zero (such as a direct-current [DC] offset). The new neutral voltage is proportional to the common-mode voltage, or DC-link voltage, of the drive. Figure 4 is an example of common-mode voltage. Image 1. Fluting in bearing race caused by bearing currents - See more at: http://www.pump-zone.com/bearings-motors/june-2014-how-find-solution-bearing-currents#sthash.WLjYO7ys.dpuf
Figure 4. Three-phase voltage source inverter driving voltage with a common mode other than zero volts This new neutral voltage has a frequency equal to the switching frequency of the inverter and can yield high-frequency currents in the bearing with limited amplitude and duration. Motors and drives with proper low-impedance grounding rarely exhibit bearing currents. Often, bearing currents will occur because of specific installation conditions. How to Prevent Bearing Currents Many methods for preventing bearing currents exist. Depending on the cause, some are better options than others. Different options include:
These options are highlighted in Figure 5. The red arrow displays shaft to ground currents. The green arrow displays circulating currents, and the yellow arrow displays currents from the shaft through the load and to the ground.
Current flow from shaft to ground
Figure 5. Three-phase voltage source inverter driving voltage with a common mode other than zero volts Solution Example A large grain elevator in North Dakota had a 400 horsepower motor and variable frequency drive (VFD) installed on top of it. While both the motor and VFD were grounded, the length of cable to the ground was more than 1,000 feet. Loud squealing and noises began coming from the motor bearings. This indicated fluting in the bearing race caused by bearing currents.
A motor grounding brush was installed upon startup as a preventive measure. However, the long cable length between the motor/drive and ground had allowed for large, common-mode voltages to occur during normal operation. As a result, a large flow of current moved through the grounding brush, which did not remove the difference in voltage potential between the motor shaft and ground. As a result, the motor shaft voltage increased with time and was then arcing through the bearing. Two measures were taken to resolve the situation. First, insulated motor bearings were installed. This prevented current from flowing through the bearings. The second was to replace the installed grounding cable with stranded grounding cables. This allowed for more current to flow through the grounding wires by eliminating the problem of skin effect. Skin effect occurs when high frequency currents pass through a conductor of a certain size. Because of the high frequency, current will only flow through the outer portion of a conductor and not through the inside (similar to glazing only being able to stick to the outside of a doughnut and not to the center). By using stranded cable with high frequency currents, the current is forced to flow through each strand, which reduces resistance of the grounding cable. This removed the large difference in voltage between the motor shaft and ground. Expert Help to Find the Cause These solutions are all viable options, depending on the source of the problem. No solution is a cure all for bearing currents because they can have several causes. If an operator suspects that a system is experiencing bearing currents, consulting an engineering service with the proper measurement tools is the best way to identify the source of the problem. References
Author Bio: Oreste “Rusty” Scioscia, Jr., completed his undergraduate degree in mechanical engineering at the University of Pittsburgh with magna cum laude honors. He graduated cum laude from the University of Pittsburgh in December 2013 with his master’s in electrical engineering with a concentration in electric machines/power systems. He performed electric machine and transformer research using advanced high frequency magnetic materials under a grant from the U.S. Advanced Research Project Agency – Energy. He is a contributing author for the Institute of Electrical and Electronics Engineers, including the Advanced Power Electronics Conference (APEC). Scioscia is senior application engineer at Eaton.
Bearing currents occur in specific motor and drive installations and can lead to motor fatigue and failure. While no system is inherently immune to bearing currents, few installations experience bearing currents during normal operation. While many installations have preventive measures installed, knowing that protection is in place is sometimes outside the owner or operator’s scope. Therefore, end users must understand the symptoms and causes of bearing currents to properly identify and address this problem. What Are Bearing Currents?Bearing currents occur when an induced voltage on the motor shaft is high enough to overcome the breakdown voltage of the bearing lubricant. This is typically greater than 50 volts. These shaft voltages will do one of the following:
Bearing currents occur in specific motor and drive installations and can lead to motor fatigue and failure. While no system is inherently immune to bearing currents, few installations experience bearing currents during normal operation. While many installations have preventive measures installed, knowing that protection is in place is sometimes outside the owner or operator’s scope. Therefore, end users must understand the symptoms and causes of bearing currents to properly identify and address this problem. What Are Bearing Currents?Bearing currents occur when an induced voltage on the motor shaft is high enough to overcome the breakdown voltage of the bearing lubricant. This is typically greater than 50 volts. These shaft voltages will do one of the following:
Bearing currents occur in specific motor and drive installations and can lead to motor fatigue and failure. While no system is inherently immune to bearing currents, few installations experience bearing currents during normal operation. While many installations have preventive measures installed, knowing that protection is in place is sometimes outside the owner or operator’s scope. Therefore, end users must understand the symptoms and causes of bearing currents to properly identify and address this problem. What Are Bearing Currents?Bearing currents occur when an induced voltage on the motor shaft is high enough to overcome the breakdown voltage of the bearing lubricant. This is typically greater than 50 volts. These shaft voltages will do one of the following:
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