Low Voltage Effects on AC Motors A drop in voltage will result in a proportionate increase in current. If the current exceeds the nameplate rating and is not corrected, this can result in damage to the motor from overheating Nameplate Rating An electric motor will have a nameplate rating for both voltage and amperage. When a load is on a motor, the motor must draw a fixed amount of power. The required power is about equal to volts times amps. Should the voltage fall below the nameplate rating, the amperage will increase. This can result in increased heat that will shorten the motor’s life, according to Motorsanddrives. om. Torque The motor is subject to several kinds of torque. The starting or start-up torque is the amount present when the motor is stationary and power is applied. The pull-up torque is the minimum amount necessary for the motor during the starting sequence. Effects of Low Voltage on Torque A reduced amount of voltage will reduce the amount of torque. This can result in difficulty for start loads. A reduction of voltage to 80 percent would result in a torque value of only 64 percent. In light loads, a reduction in voltage may actually be preferable because it will result in increased efficiency.
Even a small voltage unbalance will result in large current unbalance during the running of motor by a factor of 6 times. • Negative phase sequence components will lead to heating of motor • Negative phase sequence currents leads to reduction in motor output torque. • Motor is forced to run at higher slip leading to increased rotor loss and reduced efficiency. • Electricity boards should look in to this phenomenon seriously where irrigation pump sets suffer the voltage imbalance, even 1% loss of efficiency for the country like India would mean a great loss. HP irrigation pump sets with a quantity of 6 Lakh approximate, the loss of power would be 22MW and annual wastage of Rs. 47. 5 millions (At 3 Rs/unit, 4 Hrs of Pump working and 6 month’s season Keys to maximize the service life of industrial motors Why do motors fail? Certain components of motors degrade with time and operating stress. Electrical insulation weakens over time with exposure to voltage unbalance, over and under-voltage, voltage disturbances, and temperature. Contact between moving surfaces causes wear.
Wear is affected by dirt, moisture, and corrosive fumes and is greatly accelerated when lubricant is misapplied, becomes overheated or contaminated, or is not replaced at regular intervals. When any components are degraded beyond the point of economical repair, the motor’s economic life is ended. For the smallest and least expensive motors, the motor is put out of service when a component such as a bearing fails. Depending upon type and replacement cost, larger motors—up to 20 or 50 horsepower (hp)—may be refurbished and get new bearings, but are usually scrapped after a winding burnout.
Still larger and more expensive motors may be refurbished and rewound to extend life indefinitely. An economic analysis should always be completed prior to a motor’s failure to ensure that the appropriate repair/replace decision is made. Extend Motor Life with Improved Bearing Care Bearing failures are the root cause for the great majority of electric motor downtime, repair and replacement costs. Bearing and motor manufacturers are aware of the situation. Motor repair shops can attribute much of their business to bearing failures.
And motor users see bearing failure as the fundamental cause of virtually every electric motor repair expense. Studies conducted by the Electrical Apparatus Service Association also demonstrate that bearing failures are by far the most common cause of motor failures. Knowing that shaft bearings are the Achilles’ heel of industrial electric motors is not a new idea in maintenance departments, but what is new is recognizing that something can be done to prevent most motor bearing failures. Factors Affecting Bearing Life Electric motors actually present a relatively easy duty for shaft bearings.
The motor rotor is lightweight, yet because of its large shaft diameter, the bearings are large. For example, the bearings supporting the 140 lb. Rotor for a typical 40 hp. 1800 rpm industrial motor are so large that they have an L-10 minimum design fatigue life of 3000 years, or 10 percent of the bearings are statistically expected to fail from fatigue after 3000 years of operation. Plant operating experience, however, strongly contradicts such optimistic estimates of motor bearing life. In actual industrial environments, bearing failure is rarely caused by fatigue; it is caused by less-than-ideal lubrication.
Because of contaminated lubrication, bearings fail well before they serve their theoretical fatigue life. There are many reasons for less than-ideal bearing lubrication. Lubricants can leak out; chemical attacks or thermal conditions can decompose or break down lubricants; lubricants can become contaminated with non-lubricants such as water, dust, or rust from the bearings themselves. These lubrication problems can be eliminated. Motor bearings can last virtually forever by simply providing an ideal contamination-free, well-lubricated bearing environment.
Conventional wisdom teaches that such an ideal motor bearing environment can be provided by using a dry-running lip seal or using sealed (lubricated-for-life) bearings. Indeed, for many light-duty applications, such bearing protection techniques are often sufficient to allow bearings to last as long as the equipment itself. However, these bearing protection methods have not significantly reduced the rate of bearing failure in severe-duty industrial motors. Bearings in industrial applications continue to fail because of inadequate lubrication caused by lubricant loss, contamination, and decomposition and break-down.
Lip seals invariably wear out well before the bearing fails, and sealed bearings inherently foreshorten the life of a bearing to the service life of the contained grease (usually only about 3,000 to 5,000 hours for most industrial services). Maintenance professionals may find the following suggestions on how to forestall motor hearing failure obvious, but some new techniques and technologies are available. Lubricate Bearing at Correct Intervals Despite years of warnings from bearing manufacturers, over lubrication continues to plague many motor bearings. Too much grease can cause overheating of the bearings.
The lubrication instructions supplied by the motor manufacturer will specify the quantity and frequency of lubrication. Generally, two-pole motors should be greased twice a year, four-pole and slower motors only once a year. Use the Best Available Grease The most commonly used bearing grease is polyurea-based, a low-cost, low-performance, highly compatible lubricant. However, it does not handle water well, a serious drawback for many industrial applications. It reacts readily with water and loses its ability to lubricate bearings. Industrial motor bearings should be lubricated with a synthetic-based aluminum complex grease.
A high-quality grease pays for its additional cost in reduced motor downtime and repair costs. Keep Out Moisture Unless the motor is being hosed down or it operates in a humid environment, reasonably shielded motor bearings may not become seriously contaminated with moisture while the motor is running. However, when the rotor is shut down, moisture and condensation can collect on the surface of the bearing components. Eventually, this water breaks through the oil and grease barrier, contacts the metal parts of the bearing, and produces tiny particles of iron oxide.
These rust particles make an excellent grinding compound when mixed with the grease. resulting in premature failure of the bearing because of surface degradation. Preventing water contamination is a major challenge to bearing housing design. Close shaft-to-endbell clearances cannot stop the movement of humid air. Contact seals will quit contacting, resulting in large gaps that allow movement of air and water vapor across the bearing. Vapor-blocking bearing isolators, such as the one illustrated, are among the more successful devices presently available to prevent water vapor from entering a stationary bearing.
When the motor shaft is rotating, the isolator opens, eliminating the possibility of friction and wear. However, when the shaft is stationary, the isolator closes, preventing movement of air or water across its face. With no wear from rotating friction, the seal may last indefinitely, and surely as long as the fatigue-failure life of the bearing. Keep Out Dirt Lip seals, contact seals, and frequent grease replacement help minimize the amount of dirt and other air-borne abrasives that can contaminate bearing lubricant. These solutions, however, have some drawbacks.
Lip seals have a short service life, and frequent grease displacement is expensive and messy. One successful approach to keeping air-borne dirt and liquids out of an operating bearing is to install a labyrinth-type non-contact seal over the bearing housing. These bearing isolators, readily available from suppliers, combine a tortuous labyrinth path with impingement and centrifugal forces to trap and remove air-borne dirt and liquid; virtually no contamination can reach the bearing. Because the bearing isolator is a non-contact device, it will generally be the longest-lasting component of the motor.
Although not intended as such, a bearing isolator could serve as an emergency sleeve bearing if the primary bearing fails, possibly preventing damage to the motor’s stator and rotor. In emergency situations, the bearing isolator can allow continued operation for a short time and still prevent the need to rewind the motor when the bearing is replaced. Bearing isolators constructed of bronze or other non-sparking materials also can prevent hazardous sparks that could otherwise occur when the bearing’s rolling elements fail. Other Suggestions
Improved bearing protection and lubrication will reduce downtime and the maintenance costs of electric motors, but other important motor design features contribute to long service life, including over-sized high quality bearings, high-tech winding insulation, superior fan design, high-performance paint (such as epoxy) and a strong, rigid cast iron frame. These features, usually standard or readily available, are found in most industrial-grade severe-duty electric motors. High-performance bearing protection systems. however, are not universally accepted as essential for long motor life.
Specifying permanent bearing protection for new motors, or retrofitting isolators onto existing equipment, usually requires initiative on the part of the user’s maintenance or engineering staff. Permanent, absolute bearing protection has a greater effect on motor life than any other decisions made in specifying, equipping, and caring for electric motors. Keeping bearings lubricated with the right amount of clean, uncontaminated, high-quality lubricant allows bearings in most industrial motors to outlast all other motor components.
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