Bearings And Lubrication

Very large horsepower (kilowatt) motors are generally supplied with oil-lubricated sleeve bearings with oil supplied from a reservoir. In some cases, pressurized oil lubrication systems are installed by the pump manufacturer along with hydrodynamic thrust bearings. All NEMA frame induction motors are available with ball bearings. These standard ball bearings are normally permanently grease lubricated. The bearings used in a motor must be sealed to keep the lubricant inside the bearings and keep contaminates from getting into the bearings. Double-sealed bearings are common for many pump applications.

Ball bearings are subject to early failure when used in electric motors driven by PWM inverters. This very common problem must be addressed. It is caused by the high carrier frequency used in the inverter to generate the sinusoidal currents for each phase. This results in generation of high common-mode voltages inside the phase windings of the stator. Because there is an excellent electrostatic coupling between the stator/frame and the rotor from the windings, a voltage is induced in the shaft. The ball bearings represent the least-resistant path for a short circuit to the stator. However, the balls seldom actually contact the races because of the film of grease or oil in between. When the voltage builds up in the shaft until it is greater than the insulating capability of the film of lubricant, the voltages arc across the lubrication gap and a flashover current goes through the bearing. In a relatively short amount of time, the bearing races will become grooved, causing the bearings to become noisy. Metal particulate will then egress from the bearing surfaces as the process continues, causing catastrophic bearing failures after a few months.

Therefore, all electric motors that are driven by PWM drives must have a shaft grounding system to provide a low resistance path between the shaft and the motor frame. There are other solutions to this potential problem, which can be discussed with the motor supplier.

Sleeve Bearings Motors that use oil-impregnated porous sleeve bearings are lubricated with an oil-soaked wick. These bearings are available in motors up to approximately 1 hp (0.75 kW). Sleeve-bearing motors larger than 1 hp (0.75 kW) are ring-lubricated. Lubricating oil is drawn up from the bearing sump to the bearing by a ring that rolls over the top of the motor shaft as the shaft rotates. Larger motors, having bearing heat losses that cannot be dissipated directly, may require the use of a pressurized lubrication system wherein oil is pumped into the bearings and allowed to recirculate through a heat exchanger. The oil delivered to each bearing is metered to provide only the required amount. A lubrication system composed of heat exchanger, sump, and pump is normally common to a number of bearings, rather than having a single lubricating pump for each bearing. Other types of bearings must be used in place of, or in addition to, sleeve bearings when thrust loads are present. Smaller sleeve bearings are in the form of a cylindrical shell and are usually made of bronze or steel-backed babbitt metal. Larger sleeve bearings are usually split on a horizontal centerline, allowing easy assembly and disassembly for inspection and replacement. The bearing housing is also split on the horizontal centerline and held together with bolts between the top and bottom halves.

Rolling Element Bearings All new electric motors manufactured to NEMA standards use grease-lubricated ball bearings with high radial and thrust load capacities. They are axially pre-loaded to eliminate any radial or axial play for quiet operation and long life. Most motor end frames include an outer race locking plate on the shaft end bearing to prevent race rotation due to output shaft loads. These bearing mounting features are required for high performance, long life, and high efficiency operation. Most motor manufacturers provide their larger frame sizes with grease fittings for relubrication during the lifetime of the motor. The smaller frames use bearings that are grease-packed and sealed for life at the factory and cannot be relubricated.

NEMA motors subject to very high loads and operating temperature in the larger frames may require oil lubrication to the rolling element bearings. This can be by either oil circulation within the bearing frame or from a pressurized lubrication system similar to that used with sleeve bearings. In addition, for motors required to carry very high thrust loads, quite often in only one direction, taper or spherical roller thrust bearings may be used.

Hydrodynamic Thrust Bearings Certain types of pump applications, such as very large vertical pumps, exhibit very high thrust loads that cannot be accommodated by rolling element bearings. Hydrodynamic bearings, usually with tilting, self-leveling thrust pads, are used for very high thrust loads. These bearings are sometimes referred to as "Kingsbury-type" in recognition of the original manufacturer of this bearing type. This type of bearing is oil-lubricated from a self-contained oil sump or an external pressurized pumping system, depending on size and rating.

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