| Bearing protection for inverter-driven motor -> Monitor Keywords |
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  Bearing protection for inverter-driven motorRelated Patent Categories: Pumps, Motor Driven, Electric Or Magnetic Motor, Rotary Motor And Rotary Nonexpansible Chamber Pump, Having BearingThe Patent Description & Claims data below is from USPTO Patent Application 20070201995. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The subject invention generally pertains to a hermetically sealed compressor with an inverter-driven motor and more specifically to a means for protecting the motor's bearings against certain induced voltages and currents. [0003] 2. Description of Related Art [0004] For years, it has been known that shaft induced current, driven by shaft induced voltage, can damage motor bearings. In some cases, the energy generated by shaft induced current causes deterioration of the lubricant, which can ultimately damage the bearings. If the induced voltage is sufficiently high, electrical arching across the bearing can erode the bearing's surfaces directly. [0005] Shaft induced voltage can come from different sources. It can be electrostatically generated within the motor, or the voltage can arise from imbalanced ampere-turns in the stator, or from stator or rotor asymmetries. In cases where the motor is driven by an inverter or variable speed drive, the induced voltage is known as common mode voltage, which can be caused by the switching frequency of the inverter's SCRs (silicone controlled rectifiers), BJTs (bipolar junction transistors), GTOs (gate turn off tyristors), or, more recently, IGBTs (insulate gate bipolar transistors). Additional background on shaft induced voltage and related information can be found in U.S. Pat. Nos. 5,313,129; 5,914,547; 6,030,128; 5,735,615; 5,139,425; 5,059,041; 4,109,978; 4,378,138; 4,220,879 and 6,555,943. [0006] Although electrically grounding the shaft or electrically isolating the bearing can reduce the effects of shaft induced voltage, such measures are usually not necessary due to improvements in the design and manufacture of modem day motors and their variable speed drives. More recently, however, the SCRs, BJTs and GTOs of inverters have been replaced by much faster IGBTs. While SCR's, BJTs and GTOs operate at relatively low frequencies, IGBTs operate at switching frequencies of 2-4 kHz and higher. At these higher switching frequencies, IGBT's appear to generate common mode current in the range of 2 to 10 MHz, which can be very difficult to limit to a conductive path that bypasses the bearing. [0007] The increasing popularity of IGBTs for variable speed drives has not only resurrected the problem of induced common mode voltage, it has raised the problem to a new level where conventional methods of correction no longer work. Consequently, a need exists for a way to protect the bearings of a motor driven by an inverter with IGBTs. A better method is particularly needed for hermetically sealed motors whose bearings are relatively inaccessible for repair. SUMMARY OF THE INVENTION [0008] It is an object of the invention to help prevent certain induced currents from damaging a rolling element bearing of a refrigerant compressor system driven by an inverter. [0009] Another object of some embodiments is support a motor/compressor shaft with two different style bearings, a rolling element bearing and a journal bearing, where only the rolling element bearing needs a shaft-grounding device, which is generally accessible for servicing. [0010] Another object of some embodiments is to use both shaft-grounding and electrical insulation to protect a bearing against induced common mode voltage originating from an inverter's IGBTs. [0011] Another object of some embodiments is to provide a way of servicing a shaft-grounding device without adversely affecting the refrigerant charge of a hermetically sealed compressor system. [0012] Another object of some embodiments is to contact the end of a shaft with a shaft-grounding device that applies an ideal magnitude of contact force. [0013] Another object of some embodiments is to electrically insulate a bearing from an adjacent supporting member by coating the member with a ceramic layer that is harder than the outer periphery of the bearing and harder than the material of the supporting member, wherein the hardness of the coating is by virtue of the ceramic layer having certain proportions of titanium dioxide and aluminum oxide. [0014] Another object of some embodiments is to provide an outboard with an adjacent labyrinth seal that inhibits excessive gas flow when the shaft-grounding device is momentarily removed. [0015] Another object of some embodiments is to screw a shaft-grounding device into a threaded hole that is sufficiently small to minimize any gas exchange between the compressor and the atmosphere when the shaft-grounding device is temporarily removed. [0016] Another object of some embodiments is to ground the end of a shaft using a stranded copper wire brush rather than using a carbon block, as the wire brush is more effective at conducting induced common mode current. [0017] Another object of some embodiments is to align a shaft-grounding device with a rotational axis of a shaft to minimize wear between the shaft and the shaft-grounding device. [0018] Another object of some embodiments is to provide the brush of a shaft-grounding device with some axial movement to ensure contact between the brush and the end of the shaft even after the brush experiences some wear. [0019] Another object of some embodiments is to restrict relative rotation between a brush and an outer housing of a shaft-grounding device to prevent the shaft from rotating the brush and creating wear within the shaft-grounding device. [0020] Another object of some embodiments is to use a stranded grounding wire to effectively convey high frequency common mode current from a shaft. [0021] One or more of these and/or other objects of the invention are provided by a compressor system that employs both a serviceable shaft-grounding device and a ceramic coating to protect a rolling element bearing that could otherwise be damaged by high frequency induced common mode voltage and current originating from an inverter that includes a plurality of IGBTs. [0022] The present invention provides a compressor system powered by an AC voltage supply for compressing a refrigerant. The system includes a compressor housing defining a suction inlet and a discharge outlet; a motor housing extending from the compressor housing; a bearing bracket extending from the motor housing; a bearing having an outer periphery supported by the bearing bracket; and a shaft supported by the bearing and being rotatable about a rotational axis. The shaft includes an outboard end and an inboard end. The bearing is closer to the outboard end than to the inboard end. The system also includes a compressor element driven by the shaft and rotatable relative to the compressor housing to force the refrigerant from the suction inlet to the discharge outlet. The compressor element is closer to the inboard end than to the outboard end. The system includes a minimally conductive coating disposed on the bearing bracket. The coating is between the bearing bracket and the bearing to provide electrical resistance therebetween, and the coating is harder than the bearing bracket and the outer periphery of the bearing. Continue reading... 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