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Category Archives: Motors and Drives

Motors and Drives

Overload Protection for Electric Motors

Infrared imager (Courtesy of Fluke Electronics)

Thermal images are an easy way to identify apparent temperature differences in industrial three-phase electrical circuits, compared to their normal operating conditions. By inspecting the thermal differences of all three phases side-by-side, technicians can quickly spot performance anomalies on individual legs due to unbalance or overloading. Read more

Motors and Drives

Bearing Protection of Inverter-duty Motors


Although it is now common knowledge that inverters—also known as variable frequency drives—often induce unwanted motor shaft voltages, many customers who purchase three-phase alternating current (AC) induction motors do not realize that models labeled “inverter-duty” or “inverter-ready” might not prevent inverter-caused bearing damage. While many such motors have inverter-rated insulation to protect their windings, their bearings are too often ignored. If they are to be truly ready for use with inverters, these motors also need bearing protection. Read more

Motors and Drives

Variable Frequency Drive Cable Best Practices


Correcting common mistakes that affect performance and safety

BY PETER COX, Belden, Inc.

Most industrial operations today are looking for ways to reduce costs in these tough economic times. Variable-frequency drives (VFDs) are popular in many applications because they provide energy savings and a “more precise” control over the electric motor. Moreover, VFDs use sensitive communications data to control mechanical devices, allowing operators to increase precision, reduce waste, and increase motor life by reducing wear and tear on motors.

Good installation practices and the right cables for integrated systems are important to the long-term success of any installation. However, poor installation or use of the wrong power and communications cables on these systems can result in reduced or even unsafe performance. This increases the possibility of system failure and expensive downtime and it shortens the life for both motors and cables.

Motors and Drives

Monitoring Stator Winding Deterioration


The importance of partial discharge testing in electric motors


Comparing partial discharge (PD) results obtained from a single machine is a valuable tool that enables companies to observe the gradual deterioration of a machine stator winding. Proper monitoring of stator winding deterioration allows companies to plan appropriate maintenance for the machine in accordance to International Electrotechnical Commission’s IEC 60034 standard: “Rotating Electrical Machines – Part 27-2: Online Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines”, also known as IEC/TS 60034-27-2.

However, a few recent case studies have shown that modern air-cooled stator windings are suffering from premature deterioration of the insulation.

Motors and Drives

Understanding Short Circuit Motor Contribution

Photo credit: GE Industrial Solutions

A back to basics approach

BY DARRELL BROUSSARD, GE Industrial Solutions

An electric motor’s contribution is the current generated by a motor or motors during a short circuit condition. It represents a small but important value that is needed to determine the maximum short circuit current available, thereby establishing the short circuit rating of electrical equipment.  Regardless of the size or voltage rating of a motor, it can be demonstrated that motor contribution is present during a fault.

During normal operation, a motor converts electrical energy into mechanical energy. Current flowing in the stator produces a rotating magnetic field with the poles facing toward the rotor. This rotating magnetic field induces a current into the rotor. A magnetic field with the poles facing out is produced in the rotor due to the stator-induced current. This causes the rotor (motor shaft) to rotate. As long as the stator
is supplied to a stable voltage supply, the motor shaft will continue to rotate.

During a short circuit condition, the system voltage will decay and a stable voltage supply will no longer exist. The rotating magnetic field in the rotor will attempt to support the reduced voltage condition by becoming a power source. The motor is now providing additional current into the faulted electrical system. This phenomena
is called motor contribution. Read our expanded digital magazine for more information on short circuit motor contribution.

Motors and Drives

Overload Relays for Motor Protection


Improve system reliability and efficiency

BY KEVIN TRIMMER, Eaton Corporation

Nearly a third of global energy demand is attributable to manufacturing. Overall, the industry’s use of energy has grown by 61 percent between 1971 and 2004. According to the International Energy Agency (IEA) analysis, there are substantial opportunities to improve worldwide energy efficiencies.

More specifically, motor systems use more than 75 percent of a plant’s electricity. Driving energy efficiencies is more than being green. It makes good business sense, and delivers improved system productivity, increased reliability, and reduced energy and maintenance costs. As energy becomes more difficult and costly to find, refine and use, it is critical to understand available methods of motor control and protection.

Nearly every motor in an industrial facility uses some form of protection. Selecting the appropriate motor control can help to optimize maintenance operations and reduce energy costs.

Standard overload protection has evolved beyond strictly monitoring current for an overload condition. Today’s electronic overload relays for motor protection are capable of monitoring power, power factor, current and voltage, while continuously providing this data through standard industry protocols.

There is a wide array of motor control and protection solutions available. The challenge is determining what level of motor protection is required for a specific application. To find an appropriate solution, it is important to first understand the various failure modes within an industrial facility and the impact that each can have on uptime and throughput.

Motors and Drives

Enhance Performance with VFD Technology

Electric motor

Energy savings for pumping applications


In the early days of variable frequency drive (VFD) technology, the typical application was in process control for manufacturing synthetic fiber, steel bars and aluminum foil. Because VFD improved process performance and reduced maintenance costs, they replaced motor generator sets and direct current (DC) drives. When the energy crisis occurred in the early 1970s, saving energy became a critical goal, and the use of
VFDs quickly spread into large pump applications and eventually into HVAC (heating, ventilation and air conditioning) fan systems.

In many flow applications, a mechanical throttling device is used to limit flow. Although this is an effective means of control, it wastes mechanical and electrical energy. Figure 1 represents a pumping system using a mechanical throttling valve and the same system using a VFD.

If a throttling device is employed to control flow, energy usage is shown as the upper curve in Figure 2 while the lower curve demonstrates energy usage when using a VFD. Because a VFD alters the frequency of an alternating current (AC) motor, speed, flow, and energy consumption are reduced in the system. The energy saved is represented
by the green shaded area.

Motors and Drives

Reviewing Electric Motor Reliability Research

electric motor

What did the studies really say—then and now?


One of the most frequently quoted studies related to electric motor reliability is a 1983 Electric Power Research Institute (EPRI) project, entitled “Assessment of the Reliability of Motors in Utility Applications: IEEE Transactions on Energy Conversion”. It has been used to support a variety of programs, equipment, and other electric motor strategies.

In fact, this author has cited other papers that referenced the study over many years and had been searching for a copy of the original in order to provide additional detail. Recently, the paper that covers the details of the study has been made available through the Institute of Electrical and Electronics Engineers, Inc. (IEEE) and a review has brought some statements attributed to the study into question.

The good news is that this was not the only study on electrical and electric machine reliability. Studies had been performed by several groups, including an IEEE Power Engineering Society group from 1962 all the way through 1995 and then supported by other industry groups as recently as 2010.

What is particularly interesting about these studies is that they focus on different industries such as utilities, petrochemical, general industry and commercial buildings, yet have very similar results. While each study looked deeper into the issues, and the results were different than represented by many papers and books, the actual findings were much more interesting and far more supportive of the programs and strategies presented in those cases.

In the 2013 Motor Review in Electrical Source Magazine, this feature will cover what these studies really represent in relation to larger machines, which was the primary purpose of many of the papers. This includes the identification of the reliability issues that were identified and the recommended strategies with supporting information. While the full breadth of the related studies is far more than we can cover in this feature, the information that will be discussed will have a significant impact on how you look at your motor system.

Motors and Drives

Maximizing Electric Motor Efficiency

Photo credit (electric motor): Paul Wright

How to get the most from your rotating machines


An area of great interest is improving the efficiency of electrical devices. More electric motor efficiency means less electrical energy consumed and thus less impact to the environment. By far, the largest consumers of electric energy are AC (alternating current) induction motors. In the United States, 50 percent of all electrical energy is consumed by electric induction motors.

Worldwide, AC induction motors consume over 70 percent of all electrical energy produced. Improving the efficiency of electric induction motors can have a dramatic impact on worldwide power consumption.

Improving electric induction motor efficiency has become the top priority of many organizations charged with developing innovative ways to reduce electric power carbon dioxide (CO2) emissions.

In a perfect world, AC induction motors would operate at 100 percent efficiency—in other words, every kilowatt of power delivered to the motor terminals would be converted to useful work at the motor shaft. In reality, the motor only delivers a percentage of the AC power as rotating mechanical energy to the shaft of the motor.

Motors and Drives

Improving Motor Maintenance Practices


Useful approaches to building an effective operation and maintenance program


The pressures to be more competitive and cost-effective in today’s deregulated market have driven plants to reduce resources by making more informed, condition-based decisions regarding equipment maintenance activities. These decisions go beyond the traditional technical specifications and acceptance criteria that were used when plants where designed and manufactured.

Motor maintenance programs must balance safety and economic risk when allotting resources. It is difficult to ensure that each electric motor receives its appropriate share of resources for testing and maintenance to ensure reliability of plant operation. Many factors must be considered when designing a motor maintenance and testing management program. Age and operating hours are factors, however, newer isn’t always better as some of the problems present in past designs are now reemerging.

A motor maintenance program must consider the prevailing plant or utility maintenance philosophy established by management. All utility plant managers strive for high reliability and availability with minimal operation and maintenance (O&M) costs. Because business owners need guidance when it comes to managing their motors, the following guidelines are provided to help economically maximize the reliable life of motors and improve motor maintenance practices.

The primary consideration when determining and scheduling motor maintenance is the safety classification of a motor. In general, safety-related motors are a specially identified class of motors with regards to a maintenance program and might receive more scrutiny than other motors. In many cases, safety-related motors do not operate continuously and have minimum run-time hours. This type of low-duty cycle can limit the amount of maintenance necessary; however, due to the safety nature, reliability is critical.

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