Archive | Motors & Drives

143

5:49 pm
April 11, 2016
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Motor-Testing Tools Expand Services

Ken Patterson, head of the predictive-maintenance division at Koontz-Wagner, tests a 9,000-hp motor.

Ken Patterson, head of the predictive-maintenance division at Koontz-Wagner, tests a 9,000-hp motor.

New state-of-the-art instruments at Midwest service-provider Koontz-Wagner helped boost the company’s profits by 10%.

Growing companies typically make investments in state-of-the-art tools and equipment so that they may expand their ability to serve their customers. One such company, Koontz-Wagner Services, South Bend, IN, has capitalized on several opportunities since acquiring new energized and de-energized motor-testing equipment.

Koontz-Wagner Services is a leading Midwest provider of repair and maintenance services for rotating-equipment systems, including electric motors, generators, and mechanical power-transmission components, along with electrical-contracting services that range from short-order services to design and complex large-project services.

Kenneth L. Patterson, Koontz-Wagner’s proactive predictive-maintenance (PdM) manager, led the effort to obtain sophisticated motor-testing equipment for the company. In January 2015, after having conducted extensive research on different types of technologies, he turned to products from All-Test Pro, Old Saybrook, CT.

Patterson chose two hand-held, de-energized motor testers: the All-Test Pro 5 and All-Test Pro 31, in addition to the All-Test Pro On-Line II energized motor tester. His 12-person PdM team participated in a standard post-sale training session conducted by All-Test Pro. The team was thoroughly educated on how to perform advanced non-destructive motor testing and analysis for de-energized motor-circuit analysis and energized electrical-signature and power analysis.

Testing motors

As a full-service company, Koontz-Wagner has motor-repair, predictive-maintenance, and construction divisions. Its motor-repair division made immediate use of the de-energized motor-testing equipment by streamlining its inspection processes.

“Using the AT5 and the AT31 has helped us reduce the time it takes to understand the general condition of a motor,” explained Patterson. “The AT5 motor-circuit analyzer shows us if there are bad connections or ground faults, it checks the winding, and it lets us know if there are air gaps, contamination, or broken bars. It gives us a pretty good picture of the motor’s health within just a few minutes; which is important because reducing the time it takes us to inspect a motor has enabled us to lower the cost of that initial inspection,” he continued. “The incoming inspection fees we had been charging customers were a little high, compared with our competitors, so changing our inspection process has allowed us to lower those initial inspection fees and become more competitive.”   

Koontz-Wagner service technician Erik Lehman uses the All-Test Pro 5 to perform initial inspection of a 50-hp motor in his company’s repair shop.

Koontz-Wagner service technician Erik Lehman uses the All-Test Pro 5 to perform initial inspection of a 50-hp motor in his company’s repair shop.

Increasing business opportunities

In September of 2015, Koontz-Wagner’s maintenance-services organization began using its portable de-energized motor-testing equipment outside of the repair shop. A long-time relationship with a large automotive-supply company presented the service providers with an opportunity to offer additional value-added support. The automotive supplier maintains an inventory of approximately 700 spare motors.

Over the course of three months, Koontz-Wagner’s PdM technicians went through this inventory to check the health and condition of all spare units. “Out of that inventory of about 700 motors, we found that about 100 required maintenance,” explained Patterson. “We used the AT5 on-site, which was great because it generates reports quickly, so it was ideal for that particular project. Now, we are scheduling these motors to come into our motor-repair division for service.”

Revenue from energized testing

Energized testing has become another area of sales growth for Koontz-Wagner. “I have generated quite a bit of income using the All-Safe Pro,” Patterson noted. The product is an adaptor installed inside the electrical cabinet that works with the ATPOL II energized testing instrument. This adaptor provides the necessary signals to help preventive-maintenance professionals understand the condition of operating motors with minimal risk and without bulky, protective gear.

Koontz-Wagner’s construction-division team of electricians has installed 15 All-Safe Pro adaptors inside various customer electrical cabinets. Then its predictive-maintenance team members use the All-Test Pro On-Line II energized tester to obtain data on operating motors and further support their customers’ condition-based monitoring and PdM programs.

As an example, Patterson pointed to having performed vibration testing on a compressor motor for a customer and the fact that the ATPOL II confirmed the results. “Maintaining the health of a 200-hp compressor motor,” he said, “is critical because the unit provides air to this automotive customer’s facility.”

According to Patterson, All-Test Pro’s technologies are helping his company in many ways, so much so that he’s hoping to expand Koontz-Wagner’s capabilities even more in 2016 using these instruments. No wonder: The company credits the new motor-testing equipment with helping increase its profits by 10%, proving that there are real benefits to investing in modern tools and technology. MT

For more information on these motor-testing tools and other All-Test Pro technologies, visit alltestpro.com.

145

5:17 pm
March 18, 2016
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Prevent Motor Bearing Failures

Understanding the top five causes of failed motor bearings will help stop these problems in their tracks.

According to the bearing experts at SKF (Gothenburg, Sweden, and Lansdale, PA) these five damage mechanisms are the most common causes of motor-bearing failures. Understanding them as you examine a failed bearing can help you prevent their recurrence.

Electrical erosion
Electric erosion (arcing) can occur when a current passes from one ring to the other through the rolling elements of a bearing. While the extent of the damage depends on the amount of energy and its duration, the result is usually the same: pitting damage to the rolling elements and raceways, rapid degradation of the lubricant, and premature bearing failure. To prevent damage from electric-current passage, an electrically insulated bearing at the non-drive end is usually installed.

Inadequate lubrication and contamination
If the lubricant film between a bearing’s rolling elements and raceways is too thin, due to inadequate viscosity or contamination, metal-to-metal contact occurs. Check first whether the appropriate lubricant is being used and that re-greasing intervals and quantity are sufficient for the application. If the lubricant contains contaminants, check the seals to determine whether they should be replaced or upgraded. In some cases, depending on the application, a lubricant with a higher viscosity may be needed to increase the oil-film thickness.

Damage from vibration
Motors transported without the rotor shaft held securely in place can be subjected to vibrations within the bearing clearance that could damage these components. Similarly, if a motor is at a standstill and subjected to external vibrations over a period of time, its bearings can also be damaged. To prevent these problems, secure the bearings during transport in the following manner: Lock the shaft axially using a flat steel bent in a U-shape, while carefully preloading the ball bearing at the non-drive end. Then radially lock the bearing at the drive end with a strap. In case of prolonged periods of standstill, turn the shaft from time to time.

Damage caused by improper installation and set-up
Common mistakes in installation include using a hammer or similar tool to mount a coupling half or belt pulley onto a shaft; misalignment; imbalance; excessive belt tension; and incorrect mounting resulting in overloading. To prevent these problems, use precision instruments such as shaft-alignment tools and vibration analyzers and other appropriate tools and methods when mounting bearings.

Insufficient bearing load
Bearings always need to have a minimum load to function well. If they don’t, damage will appear as smearing on the rolling elements and raceways. To prevent these problems, be sure to apply a sufficiently large external load to the bearings. This is crucial with cylindrical roller bearings, since they are typically used to accommodate heavier loads. (This, however, does not apply to preloaded bearings.) MT

SKF is s a global supplier of bearings, seals, mechatronics, lubrication systems, and services that include technical support, maintenance and reliability services, and engineering consulting and training. For more information on motor bearings and other technologies and topics, visit skf.com.

125

4:50 pm
February 24, 2016
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Prevent Motor-Bearing Failures

Learn the latest on the top five causes of failed motor bearings to help stop these problems in their tracks.

By examining a failed motor bearing and understanding the clues that various types of damage often produce, you can keep these problems from plaguing your motor fleet in the future.

By examining a failed motor bearing and understanding the clues that various types of damage often produce, you can keep these problems from plaguing your motor fleet in the future.


According to the bearing experts at SKF (Gothenburg, Sweden, and Lansdale, PA) these five damage mechanisms are the most common causes of motor-bearing failures. Understanding them as you examine a failed bearing can help you prevent their recurrence.

Electrical erosion
Electric erosion (arcing) can occur when a current passes from one ring to the other through the rolling elements of a bearing. While the extent of the damage depends on the amount of energy and its duration, the result is usually the same: pitting damage to the rolling elements and raceways, rapid degradation of the lubricant, and premature bearing failure. To prevent damage from electric-current passage, an electrically insulated bearing at the non-drive end is usually installed.

Inadequate lubrication and contamination
If the lubricant film between a bearing’s rolling elements and raceways is too thin due to inadequate viscosity or contamination, metal-to-metal contact occurs. Check first whether the appropriate lubricant is being used and that re-greasing intervals and quantity are sufficient for the application. If the lubricant contains contaminants, check the seals to determine whether they should be replaced or upgraded. In some cases, depending on the application, a lubricant with a higher viscosity may be needed to increase the oil-film thickness.

Damage from vibration
Motors transported without the rotor shaft held securely in place can be subjected to vibrations within the bearing clearance that could damage these components. Similarly, if a motor is at a standstill and subjected to external vibrations over a period of time, its bearings can also be damaged. To prevent these problems, secure the bearings during transport in the following manner: Lock the shaft axially using a flat steel bent in a U-shape, while carefully preloading the ball bearing at the non-drive end. Then radially lock the bearing at the drive end with a strap. In case of prolonged periods of standstill, turn the shaft from time to time.

Damage caused by improper installation and set-up
Common mistakes in installation include using a hammer or similar tool to mount a coupling half or belt pulley onto a shaft; misalignment; imbalance; excessive belt tension; and incorrect mounting resulting in overloading. To prevent these problems, use precision instruments such as shaft-alignment tools and vibration analyzers and other appropriate tools and methods when mounting bearings.

Insufficient bearing load Bearings always need to have a minimum load to function well. If they don’t, damage will appear as smearing on the rolling elements and raceways. To prevent these problems, be sure to apply a sufficiently large external load to the bearings. This is crucial with cylindrical roller bearings, since they are typically used to accommodate heavier loads. (This, however, does not apply to preloaded bearings.)

Source
SKF is s a global supplier of bearings, seals, mechatronics, lubrication systems, and services that include technical support, maintenance and reliability services, and engineering consulting and training. For more information on motor bearings and other technologies and topics, visit skf.com.

569

10:05 pm
February 8, 2016
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Select the Best VFD for Your Application

Choosing the right variable frequency drive for an application involves several important considerations. For example, based on acceleration requirements, sensorless vector control may be more suitable than volts-per-hertz (V/f) control. While V/f control is effective in dragging logs up a slope, it’s not appropriate for dockside hoists that position 12-ton shipping containers to within inches.

Choosing the right variable frequency drive for an application involves several important considerations. For example, based on acceleration requirements, sensorless vector control may be more suitable than volts-per-hertz (V/f) control. While V/f control is effective in dragging logs up a slope, it’s not appropriate for dockside hoists that position 12-ton shipping containers to within inches.

According to the technical experts at Mitsubishi Electric Automation, Vernon Hills, IL, there are several factors to consider when selecting variable-frequency drives (VFDs). Among them:

What is your load type: constant or variable torque?

For a constant-torque load, the torque is independent of speed (ignoring momentary shock loads). Examples include conveyors and hoists. For a variable-torque load, torque varies as a function of speed. Examples include fans and pumps. This primary distinction underlies every decision you’ll make about the type of drive.

What are your acceleration requirements?

Does it matter how fast your load accelerates up to speed? For a fan, probably not. For a centrifuge, almost certainly. In the latter case, you may want to select sensorless vector control, rather than volts-per-hertz (V/f) control. While the V/f approach is effective for many applications, it doesn’t allow a motor to develop near-full torque at near-zero speeds (unlike sensorless vector control). V/f control can be appropriate for dragging logs up a slope, but not for a dockside hoist that needs to position a 12-ton shipping container to within inches.

Controlled deceleration presents its own challenges.

During decelerations, the motor acts as a generator. The resulting energy needs to go somewhere, and is typically dissipated as heat in a braking resistor. Controlled-deceleration capability is a good solution for constant-torque loads, changing loads, or even unbalanced loads.

What is your speed range?

Although a conveyor belt may operate consistently at 60 Hz, for an unspooling module on a printing line, the motor needs to deliver torque as effectively at 0.5 Hz as 60 Hz. This is another application where garden-variety V/f control won’t do the job. Sensorless vector control will (and most VFDs these days include it). Keep in mind, however, that not all offerings are created equal. Be sure to double check specifications against your requirements.

Do you need to optimize energy usage?

Instead of wasting all of the energy harvested when your motor is overhauling, you can apply it to your next move, courtesy of a regenerative VFD. These drives have internal capacitors that temporarily store energy for reuse.

Do you need an encoder?

Not all drives are the same at low speeds. A drive with a 200-to-1 speed range, for example, can provide 100% speed from your motor down to about 1/3 Hz. This might be acceptable for some applications, but not others—in which case you’ll need an encoder, Since not all drives work with encoders, you’ll want to determine your need for this capability in advance. MT

VFD Sizing Matters

Back in the day, an undersized drive—a common situation in plants—would simply trip when it exceeded operating specifications. Today, VFDs are very good at limiting themselves so they don’t trip.

The bad news? While a properly sized drive today might allow the system to complete acceleration in one second, as commanded, one that’s too small for a task will take two or three seconds. In short, an undersized drive will compensate, but the system won’t perform as desired and the compensation could mask the true problem.

Mitsubishi Electric’s technical experts also note that it’s crucial to size a drive based on peak current command—not on horsepower. Because of horsepower’s connection to motor size, it’s easy to focus on it. In reality, though, you want to size a drive so that the maximum current in the worst-case scenario is always within the mode of the unit’s continuous-current rating.

For more information on selecting VFDs, including various design, installation, and operational factors, visit us.mitsubishielectric.com/fa/en.

813

8:23 pm
December 17, 2015
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Three-Phase Motor Tips: How To Evaluate Winding Temperatures

Before pulling what might be a hot-running unit, take the time to confirm your suspicion.

By Mike Howell, Electrical Apparatus Service Association (EASA)

Suspect a three-phase motor is running hot? If you’re right, the unit is either producing more heat than it’s designed for or dissipating less. With excess heat, the main concerns are typically the health of the bearing-lubrication and the winding-insulation system.

Before incurring the expense of pulling the motor, evaluate its winding temperature. Here’s how.

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Once the winding temperature of a suspect motor has been determined, compare it to the unit’s nameplate (allowable) temperature. Often, only the insulation class is listed. For electric motors, the class is usually B, F, or H. Motors that get close to or exceed their class temperature limits could be having problems.

Measure the stator current of all three phases.

Compare these readings to the nameplate ratings and, if possible, to the current readings of any sister motors in the same application. The heat produced by the stator winding is proportional to the winding resistance and square of the current. Extra current equals extra heat.

Don’t conclude the winding is too hot by simply touching the motor’s frame.

Winding temperatures can’t be properly evaluated based on outside frame temperatures.

If the motor is equipped with temperature detectors that are capable of providing readings, use them.

A controller isn’t always necessary. Thermistors may require a controller, but thermocouples and resistance temperature detectors (RTDs) can be easily read with common, handheld meters and reference tables.

A thermocouple or RTD can be affixed to the stator core back iron, using proper electrical-safety precautions.

In such cases, the winding temperature is usually around 5 to 10 C above the back-iron temperature. If temperature detectors are not installed or operable, winding temperature can be extrapolated using a time series of winding-resistance measurements taken after shutdown. (Ask your service center for assistance with this if needed.) While this approach requires shutting down and opening the motor terminations, depending on the unit, it may be worth it.

Once you know the winding temperature, compare it to the motor’s nameplate temperature, i.e., allowable temperature.

Frequently, only the insulation class will be listed. For electric motors, the class is usually B, F, or H (see table). If the motor is getting close to or exceeding its class temperature limit, you may indeed have a problem.

Be proactive

If your evaluation of the winding temperature points to a problem, take steps to solve it. Don’t wait for a failure to occur. While overheating in the stator winding can lead to stator winding failures, it can also damage the stator core and/or mechanical components due to heat transfer from the stator winding to the bearing lubrication. MT

Mike Howell is an electrical support specialist with the Electrical Apparatus Service Association (EASA). Based in St. Louis, EASA is an international trade association of more than 1,900 electromechanical sales and service firms in 62 countries. For more information, visit www.easa.com.

346

3:54 pm
November 25, 2015
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Schneider Electric Furthers IIoT Evolution

“The Industrial Internet of Things [IIoT] is an evolution, not a revolution,” was the lead statement at the Schneider Electric SPS Nuremberg show in Nuremberg, Germany, Nov. 24, 2015. To support that claim, Clemens Blum, Schneider’s executive vice president of industry business, referred to the description of a Schneider 1999 Computerworld Smithsonian Award that talked about connected products and systems that operate as part of a larger system of systems and smart plants and machines, with embedded intelligence, that are integrated to enable the smart enterprise, improve efficiency and profitability, increase cyber security, and improve safety.

Marketing director of machine solutions, Rainer Beudert, followed with a discussion of smart machines and how they fit in the evolving IIoT. He described the Schneider definition of a smart machine as one that intuitively interacts with operators; assists with predictive maintenance; minimizes its environmental footprint; and provides modularity, connectivity, plug and work setup, self awareness, reusable design, digital mobility, and data management. It also makes available information about status, configuration, conditions, quality, and features.

To learn more about what Schneider is doing to further the IIoT evolution, view the press-conference video at: https://www.youtube.com/watch?v=WI7JnKh3eV0

1439

6:34 pm
August 6, 2015
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Cool Advice On Hot Motors

0815hotmotors1

Increased operating temperatures damage motors. Protection starts at the source.

By Jim Bryan, Electrical Apparatus Service Association (EASA)

The effects of excessive temperature on motor performance are notorious. After moisture, they are the greatest contributor to bearing and winding failures. Understanding the source of increased temperature is key to correcting the problem and improving the reliability of your facility’s motor fleet.

Figure 1 illustrates the theoretical impact of increased operating temperature on motor-insulation systems. The chart addresses thermal aging and not other conditions that affect motor life. In effect, this graphic indicates that every 10 degree C increase in operating temperature cuts insulation life expectancy by half.

This illustration of the theoretical impact of increased operating temperature on motor insulation systems addresses only thermal aging. As shown, every 10 degree C (50 degree F) increase in operating temperature cuts insulation life expectancy by half. Conversely, a decrease of 10 degrees C (50 degrees F) could double insulation life expectancy.

This illustration of the theoretical impact of increased operating temperature on motor insulation systems addresses only thermal aging. As shown, every 10 degree C increase in operating temperature cuts insulation life expectancy by half. Conversely, a decrease of 10 degrees C could double insulation life expectancy.

Conversely, the chart in Fig. 1 also shows that decreasing the temperature by 10 degrees C could double insulation life expectancy. While this is true anywhere on the curve, at some point the rule of diminishing returns dictates that the cost of building and operating a cooler-running motor will outweigh the benefits. This article focuses on several factors that contribute to increased temperature and what to do about them.

Overload and service factor

Overload is a common culprit in temperature problems. Due to load variations in the driven equipment, the condition is sometimes intermittent. At other times, the designer has chosen to let the motor operate above the rated load—which is permissible if the service factor is greater than 1.0.

The NEMA Std. MG 1-2011 definition of service factor says a motor is thermally capable of overload to that point within the insulation class at normal service conditions (rated voltage and frequency). Of course, any overload will increase the operating temperature. Most motors are designed to be most efficient—run cooler and consume less power for the same job—at about 75% of rated load.

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Fig. 2. Click to enlarge.

The insulation class determines the maximum allowable operating temperatures that yield “normal” service life (see Fig. 2). If a motor consistently fails prematurely in an application, and little can be done to mitigate the temperature, one solution may be to rewind it with a higher-temperature-class insulation system. Don’t forget the bearings in this attempt. The lubricant is the limiting factor in temperature-related bearing problems, so be sure it will work in your operating environment.

Pulse-width modulated (PWM) adjustable-speed drives (ASD) produce relatively low negative-sequence currents that essentially add load to the motor by trying to make it run in the opposite direction. These negative-sequence currents also greatly increase rotor temperature. A properly designed inverter-duty motor will compensate for this.

Ventilation

Motor designs include a system for dissipating the heat produced by the winding and bearings. Often called the “cooling circuit,” this system can be affected by such factors as fan diameter, shaft speed, the presence and location of air ducts and deflectors, and altitude.

Fan. The amount of air that a fan provides varies as the cube of its diameter and is directly proportional to the speed. In a totally enclosed fan-cooled (TEFC) motor, the fan is often the main source of objectionable noise. The designer must make sure it provides sufficient cooling air without creating too much noise.

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In larger open motors, air ducts distribute the cooling air through the rotor and stator cores to improve cooling efficiency. Deflectors may be used in open or enclosed motors to direct the air to locations that need it and to reduce turbulence.

Air ducts and deflectors. In larger open motors, air ducts distribute the cooling air through the rotor and stator cores to improve cooling efficiency (Fig. 3). Deflectors may be used in open or enclosed motors to direct the air to locations that need it and reduce turbulence. Turbulent air doesn’t cool efficiently, so the location of deflectors can be critical for optimizing the cooling circuit (Fig. 4). Clogged ducts or missing or incorrect air deflectors could make the motor run hotter.

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Turbulent air doesn’t cool efficiently, so the location of deflectors can be critical for optimizing a motor’s cooling circuit. Clogged ducts or missing or incorrect air deflectors could make a unit run hotter.

Operating temperature. A motor doing a given amount of work will produce a level of temperature increase called temperature rise. This, plus the ambient temperature, equals the motor’s operating temperature:

Temperature rise + Ambient temperature = Operating temperature

Because ambient temperature directly affects operating temperature, NEMA motors list the maximum allowable ambient temperature on the nameplate. By contrast, IEC 60034-1, 5.3 limits the maximum ambient for IEC motors to 40 C. For these motors, the design-temperature rise at rated load plus this maximum ambient should not exceed the temperature-class rating of the motor’s insulation system.

If a motor is located outdoors, not only is the ambient temperature subject to change, factors such as sunshine come into play. For example, repainting gray pump motors white at an open-pit mine in the Sonoran Desert reduced operating temperatures 10 to 15 C. Building a structure to shade the motors produced a similar result.

Altitude. As altitude increases, air gets thinner, reducing its ability to carry heat away from the motor. If a motor is to be operated at an altitude above 1,000 m (3,300 ft.), its design should be adjusted to accommodate the less-efficient cooling that results.

Voltage variation

Motors are designed to perform optimally when the applied voltage equals the nameplate rated voltage. NEMA Std. MG 1-2011 requires motors to be capable of starting and operating at the rated voltage ±10%; IEC requires ±5%. Although both standards include a frequency tolerance that affects voltage tolerance, for purposes of this discussion, consider the frequency variation to be zero.

The NEMA standard also says motor performance may be affected by voltage variation. Playing on the NEMA requirement that motors be able to operate successfully at ±10% of rated voltage
(230 V – 10% = 207 V), some manufacturers will indicate that their 230/460-V designs are “Suitable for Use on 208 V.” If the 208-V voltage supply varies, however, there’s no margin for the motor, and its performance may suffer. Unless the nameplate or some other communication from the manufacturer indicates “Suitable for Use” on an alternate supply voltage, it’s not a good idea to use your motors in such a manner. That’s because the equipment would produce less torque and higher full-load amps, while running hotter.

Under-voltage. Under-voltage results in higher amperage being required to produce the needed power or work. Ohm’s Law states that P = IE (where P = power, I = current, and E = voltage). If E decreases and P is constant, then I must increase. Since the heat produced varies as the square of the current, this additional current increases resistive losses in the motor winding and, therefore, raises the operating temperature.

Over-voltage. The converse is also true. If E goes up, I will decrease when P is constant. This is one reason motors are designed with a rated voltage of 460 V when the nominal voltage applied is 480 V. The higher voltage helps the motor to run cooler.

The slip of induction motors, however, is inversely proportional to the applied voltage. The higher the voltage, the lower the slip and the faster the motor (and fan) will turn. As noted earlier, the fan will move more air at higher speeds, so more power is required to turn it. This could have an impact on motor current as much as or more than over voltage, offsetting a portion of the decrease in motor current.

Apply this principle carefully, though, because the magnetic flux produced in the core iron also increases with a higher voltage until it reaches the saturation point (maximum flux/cross-sectional area) for that grade of electrical steel. If the voltage is increased beyond the saturation point, additional flux is possible only with a disproportionately large increase in current. This generates more heat, as is discussed in the following electrical steel section.

Unbalanced voltage. Unbalanced voltage in a three-phase motor supply will also result in high temperatures, particularly in the phase that has the highest voltage applied. NEMA Std. MG 1-2011 calculates voltage unbalance as:

% Unbalance = Maximum deviation from average/Average

This equation is used to calculate the average voltage and current. NEMA Std. MG 1-2011 says the percentage of current unbalance may be 6 to10 times that of the voltage unbalance, so accurate voltage measurements (within 0.5%) for all three phases are important. Further, with any voltage unbalance greater than 1%, the rated load should be reduced due to the additional heating.

Electrical steel

Several factors determine the ability of electrical steel to transmit flux, including its thickness, quantity, and type or grade. These properties are defined for the various grades. Some modern grades are capable of handling higher flux levels, which is one reason why higher horsepower ratings can be developed in smaller frame sizes. Of course, these grades are more expensive, which always figures into the design equation.

AC-motor cores are constructed by laminating specially insulated electrical steel tightly together. A core’s length and diameter determine its quantity or volume.

The thickness of each lamination is important in controlling eddy currents, (circulating loop currents induced within the steel by the changing magnetic field), in that piece and in the entire core. The thinner the lamination, the smaller the circulating loops and the lower the current. Eddy currents don’t contribute to work done by the motor, they just produce heat (losses).

The interlaminar insulation also helps control eddy currents. If the insulation is damaged, eddy currents can cross to adjacent laminations and increase in size, causing the magnetizing (no-load) current to increase. Core-loss testing can reveal this increase and indicate whether there is a need to repair or replace the bad steel.

Current density

Another derivative of Ohm’s Law says P = I2R (where I = current and R = resistance). In the case of a motor winding, P = wasted power (losses).

As wire size decreases, the resistance per foot increases so, for a given current, a smaller wire produces higher P (losses). Since these losses are manifested as heat, in an AC motor it is always best to use the largest total cross section of wire per turn that comfortably fills the slot.

The resin used in the winding process also conducts heat better than does air, but it won’t bind the wires together effectively if the stator slot is less than about 45% full. The resulting voids will cause higher operating temperatures.

When designing a motor, one trade-off involves the number of turns per coil versus the pitch of the winding. The pitch of the motor winding is the number of winding slots in the stator core that are spanned by the wire coil.

Generally, as the pitch is increased (up to and including full pitch minus one slot), the number of turns may be reduced. With fewer turns in each coil, a larger cross section of wire per turn is possible.

The trade-off here is the length of the end turns, especially in two-pole motors. As the coil spans more slots, the width of the coil increases, making the coil and the end turns longer. This increases the coil resistance and thereby increases the losses. Also, the wider the pitch, the more difficult the winding process becomes. To optimize the design, it’s important to use the longest pitch practical, while keeping in mind windability and total length of turn.

Circulating currents

Circulating currents are produced in the winding under certain conditions. These don’t contribute to the work being done by the motor; they are losses that produce additional heat.

Beware of coil groups in parallel that don’t contain the same number of turns. Circulating currents will produce high temperatures in circuits with fewer turns or coils. In the case of odd grouping (where the number of slots per phase isn’t equally divisible by the number of poles) the uneven number of coils must be distributed equally through all phases. Rewinders should count the total number of coils in each phase to confirm they are the same.

Beware of coil groups in parallel that don’t contain the same number of turns. Circulating currents will produce high temperatures in circuits with fewer turns or coils. In the case of odd grouping (where the number of slots per phase isn’t equally divisible by the number of poles) the uneven number of coils must be distributed equally through all phases. Rewinders should count the total number of coils in each phase to confirm they are the same.

Uneven coils. Beware of coil groups in parallel that don’t contain the same number of turns (Fig. 5). Circulating currents will produce high temperatures in circuits with fewer turns or coils. In the case of odd grouping (where the number of slots per phase isn’t equally divisible by the number of poles) the uneven number of coils must be distributed equally through all phases. When rewinding, the service center should count the total number of coils in each phase to confirm they are the same.

Two-speed, two-winding motors. Two-speed, two-winding motors can also produce circulating currents. If one or both windings are connected delta or multiple parallel wye circuits, a closed circuit will be present when that winding is not energized. A special connection with four leads can open this circuit on motors connected with a one-delta circuit. For this approach to be effective, the motor starter must have four contacts rather than three. Energizing the other winding will induce a voltage in the un-energized winding, and the closed circuit may allow current flow. This unintended current flow produces additional heat in the motor. For this reason, it’s always advisable to use a one-wye connection, since it does not have this type of closed circuit. Where this isn’t possible, a reputable service center can help identify the connections with the highest probability of success.

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The odd harmonics of the fundamental AC waveform (except multiples of three) will produce negative torques when the rotor speed is above the synchronous speed for that harmonic. By opposing a motor’s fundamental torque, negative torques add load and increase heat. As shown here with effects of the 5th and 7th harmonics, total harmonic distortion (THD), expressed as a percentage, can be measured with a power quality analyzer.

Harmonics

The odd harmonics of the fundamental AC waveform (except multiples of three) will produce negative torques when the rotor speed is above the synchronous speed for that harmonic. Negative torques oppose a motor’s fundamental torque, adding load and, thus, increasing heat. Figure 6 shows the effects of the 5th and 7th harmonics on the fundamental waveform. Such effects can be measured using a power-quality analyzer to find the total harmonic distortion (THD), expressed as a percentage. IEEE 519 states that THD should not exceed 5% at the point of common coupling (the facility service entrance).

These harmonics are produced by non-resistive loads supplied by the same power feeder as the motor. Motors themselves are a source of harmonics since they are mostly inductive loads. Ballasts, rectifiers, and power-factor-correction capacitors are a few examples of other sources.

The higher the motor operating temperature, the shorter its expected life. Anything that can be done to lower the temperature, whether it be improving the ventilation or optimizing the design, will provide better service life and reliability in your operation’s motor fleet. MT

Jim Bryan is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis. EASA is an international trade association of more than 1,900 firms in 62 countries that sell and service electrical, electronic, and mechanical apparatus.

0815hotmotors8To learn more, visit
easa.com
motorsmatter.org
ptda.org

 

Note: This article was updated August 21, 2015.

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6:04 pm
July 10, 2015
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Use These Keys to Achieve Effective Balancing

Proper installation procedures can eliminate a number of machinery defects and associated problems early. Aligning equipment to the correct tolerances and true targets and balancing it correctly at the time of installation—before the equipment is put into service—can eliminate the root cause of many bearing, rotor, seal, and coupling failures.

Proper installation procedures can eliminate a number of machinery defects and associated problems early. Aligning equipment to the correct tolerances and true targets and balancing it correctly at the time of installation—before the equipment is put into service—can eliminate the root cause of many bearing, rotor, seal, and coupling failures.

Looking for keys to successful dynamic balancing of your plant’s equipment? Use the following eight points from Ludeca Inc.’s (Doral, FL) Gary James. More important, include them as must-do elements in your balancing procedures.
—Jane Alexander, Managing Editor

— Inspect the equipment structure/mounts to make sure no cracks or loose bolts are present.

— Confirm that the belt on belt-driven equipment is in good condition and properly tensioned. (Remember that the second harmonic of a belt frequency can be very close to the rotational speed of the drive.)

— Inspect the rotating element for build-up and clean as necessary. (Remember that even a slight dust build-up can cause an unbalance.)

— If the equipment’s rotating element is a blower, count the number of blades. (Since correction weights must frequently be attached to blades, it may be best to use a fixed-location balancing method.)

— If equipment is down when you arrive, replace the reflective tape or attach new tape as required. (This ensures accurate phase data.)

— If possible, when acquiring your initial phase data, turn off the averaging function and monitor the data for a brief time to ensure stability. Doing this could identify potential problems.

— Document, document, document. That means keeping written notes on your findings with regard to:

  • phase and amplitude data
  • number of blades
  • correction locations
  • when weights were attached or removed
  • how much weight was attached or removed
  • sensor placement
  • tachometer placement.

— If the equipment is variable speed, with a variable-frequency drive (VFD) or DC drive, make sure the speed is repeatable to within 5% or less, run to run. MT

For more information on balancing and alignment issues, visit ludeca.com.

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