IMPROVING MOTOR SYSTEM EFFICIENCY WITH HIGH EFFICIENCY BELT DRIVE SYSTEMS

Transcription

1 IMPROVING MOTOR SYSTEM EFFICIENCY WITH HIGH EFFICIENCY BELT DRIVE SYSTEMS Contents Introduction Where to Find Energy Saving Opportunities Power Transmission System Efficiency Enhancing Motor System Performance with Variable Frequency Drives Energy Efficiency Legislation for Motors Improving Motor Efficiency Motor Efficiency and Energy Use Conclusion Additional Resources Introduction Anyone responsible for maintaining a large installed base of motors at an industrial or commercial facility knows that motors consume a lot of energy. According to the Department of Energy (DOE), industrial electric motor systems in the U.S. use about 679 billion kilowatt hours (kwh) of electricity every year. That amounts to about 23% of all electricity used in the country. By another DOE estimate, electric motor-driven machinery uses more than two-thirds of all electricity in industry. Because motor systems are such heavy users of energy, they present industrial plant managers, plant engineers and maintenance personnel a significant opportunity for energy savings. They also offer design engineers the potential for designing more energy-efficient equipment. 1

2 One of the simplest ways to realize savings is to upgrade the power transmission system to a high efficiency synchronous belt drive. Another is to add a variable frequency drive. Finally, consider installing a higher efficiency motor. Combined, these energy-efficient options can produce significant cost savings and a rapid payback, given the proper circumstances. Where to Find Energy Saving Opportunities When looking for energy savings opportunities in motor systems, look first to the highest energy-consuming categories of motor-driven equipment, including: HVAC systems Pumps Compressors Fans Material handling equipment Material processing equipment Facilities with the following conditions present good targets for motor and drive system upgrades designed to save energy: Large numbers of HP general purpose motors of the same size and type, running similar applications Motors that operate continuously or on high duty cycles (4,000+ hours/year) Motors 10 years or older (less efficient pre-epact models) Easily accessible motors Difficult to access drives that don t receive proper maintenance Motors that are undersized or oversized for the task Motors that have been rewound one or more times Motors connected to lower efficiency drive systems (chains, gears, poorly maintained V-belt drives) Also consider these factors to determine whether to invest in motor/belt conversion: When energy costs are high (>$.07/kWh) When plant or building energy use and cost is a concern When motor sizes are 10 HP or higher Power Transmission System Efficiency Putting a motor s energy to work requires a transmission system. Power transmission devices typically include belts, gears or roller chains. From a system design standpoint, the drive system selection can significantly impact overall system efficiency and performance. Saving a unit of energy in the belt drive operation means one less unit of energy for the motor to deliver, reducing motor output. By doing less work, the motor consumes less energy, and the savings goes all the way back to the power plant. 2

3 About one-third of industrial electric motors power belt drives, of which the majority are V-belt drives. When properly installed, V-belts can operate with efficiencies as high as percent. But V-belt efficiency typically drops over time (as much as 10%), especially when belts are not properly maintained. V-belts rely on friction (wedging action) between belt and pulley to transmit power. Friction produces heat (energy) loss. V-belts also slip and creep, especially under high torque loads. Slippage results in speed loss and lower efficiency. And because V-belts have thicker crosssections than synchronous belts, they take more energy to bend around pulleys. V-belt profiles Synchronous belts operate on the tooth-grip principle. Belt teeth mate with corresponding grooves on a sprocket to transmit power. Synchronous belt drives are more efficient than V-belt drives due to a number of factors: Less energy is needed to bend the belt around the sheave, so torque loss is lower Less heat loss is generated, because there s lower friction between belt and sprocket teeth Less speed is lost, because synchronous belts don t creep or slip as do V-belts Synchronous belt profiles Synchronous belt drive efficiency remains at approximately 98% over the life of the drive. On average, synchronous belt drives are about 5% more efficient than V-belt drives. Additionally, a synchronous belt never needs retensioning, as does a V-belt. Nor does it need lubrication, as do chain and gear drives. Less time is required to maintain the drive, which results in lower maintenance costs and can contribute to higher productivity and throughput. Adding a synchronous belt drive to either a NEMA Premium or NEMA standard motor can improve total drive system efficiency, as demonstrated in the following tables. Table 1 compares total motor system efficiency for both NEMA Premium and NEMA standard motors over a range of motor frame sizes. In this simplified scenario, system efficiency is defined by the combination of motor efficiency and belt drive 3

4 efficiency. Premium and standard motors are paired with V-belt and synchronous belt drives to demonstrate the resulting potential changes in system efficiency. NEMA Premium Eff rpm Enclosed NEMA Standard Eff rpm Enclosed Nominal HP Nominal Efficiency (%) System Efficiency (%) 93% V-Belt Drive 98% Sync Belt Drive Nominal Efficiency (%) System Efficiency (%) 93% V-Belt Drive 98% Sync Belt Drive Table 1 Motor efficiency ratings are based on the assumption that motors are operating at or near their full load capacity. (A less than fully loaded motor operates at lower efficiency, so new replacement motors should first be right sized for the job in order to gain the greatest system efficiency.) Table 2 shows the efficiency gains accomplished by pairing a NEMA Premium motor and synchronous belt drive, compared to a NEMA standard motor and V-belt drive: Table 2 NEMA Standard Efficiency 1800 rpm Enclosed NEMA Premium Efficiency 1800 rpm Enclosed 93% V-Belt Drive 98% Synchronous Belt Drive Nominal HP System Efficiency (%) The motor system efficiency gains described in Tables 1 and 2 can be translated into significant energy savings potentials as seen in the following table. To realize the full savings potential, count the number of motors with similar horsepower and efficiency ratings, and multiply accordingly. 4

5 NEMA Standard Efficiency Motor + V-Belt Drive To NEMA Premium Efficiency Motor + Synch Belt Drive Nominal HP Annual $ Savings Potential 10 $ $1, $2, $3, $4, $5, (Calculations based on individual motors operating 4,160 hours per year with an energy cost of $0.08/kWh) Enhancing Motor System Performance with Variable Frequency Drives Motor systems can operate more efficiently and save energy with Variable Frequency Drive (VFD) speed control. A VFD regulates motor speed (and energy consumption) electronically by changing the frequency of the alternating current supplied to the motor. VFDs are known by other names as well, including: AC drives AC adjustable speed drives Inverter drives On some types of flow-generating equipment, such as fans, pumps and compressors, the flow is controlled with valves, dampers or other mechanical means. The motor runs continuously at full speed. It s like accelerating and braking at the same time. By adding a VFD to a conventional AC induction motor, speed and torque output can be matched to the needs of the application. There s no need to run at the largest possible anticipated capacity all the time, which wastes energy. Nor is there a need for a motor that s too large for the job. Especially when motors are connected to belt drives, VFDs offer another advantage. AC motors in industrial plants are often subject to harsh starts, or high levels of starting torque, which can strain components connected to the motor (such as belts and bearings). A VFD starts a motor with reduced voltage and reduced frequency, resulting in a soft start. This places less mechanical stress on motor system components and reduces energy consumption. There is, however, a price to pay for this ability to adjust motor speed. First, VFDs are costly to acquire. Second, motors with VFD controls will operate at a lower efficiency than non-controlled motors when both are 5

6 Energy Efficiency Legislation for Motors Energy efficiency has become a national priority. Beginning with the Energy Policy Act of 1992 (EPAct), the federal government set minimum efficiency levels for certain categories of electric motors. As of 1997, newly manufactured motors in the HP range in these categories had to meet efficiency levels ranging from percent. Congress broadened coverage on motor efficiency with the Energy Independence and Security Act of 2007 (EISA). These new motor standards will take effect December 19, Federal legislation has heightened awareness of motor system efficiency and energy consumption. Users and designers of motor systems can use the impetus behind this legislation to save energy and reduce costs. delivering full rated power. Nonetheless, adding VFD controls to a motor system may offset this inherent inefficiency by allowing adjustment and fine tuning of the speed of the driven equipment (so that air movement or fluid flow/pressure is not greater than necessary, for example). And they provide the added benefit of being easier on belt drive systems, since motor speed can be ramped up slowly, avoiding the high starting torque of a non-controlled motor. Improving Motor Efficiency The Motor System A motor is one part of a system of components, all of which contribute to the system s efficiency. At the heart of the system is the motor itself, a device that converts electrical energy into mechanical power to turn a shaft. Upstream from the motor is the electrical power supply, wiring and controls. Downstream from the motor is the drive transmission and the driven device. Motors are classified into two broad categories: alternating current (AC) and direct current (DC) types. Alternating current induction motors are most commonly found in industrial and commercial applications, and are thus the focus of motor energy efficiency legislation. All AC motors have three basic components the stator, rotor, and enclosure and operate on the same principle. The stator is the stationary motor frame, containing wire windings (the core) that form one or more pairs of magnetic poles. The rotor and shaft are an integral component. The rotor rotates within the stator. The stator s magnetic field induces an opposing magnetic field in the rotor, and the pushing interaction between the two fields produces the rotational force. The enclosure holds everything together, supports the shaft bearings, dissipates heat and provides protection for the internal motor components. Motor enclosures are broadly classified into two types: open and totally enclosed. There are many types of AC motors. The type most commonly found in commercial/industrial applications is the three-phase squirrel cage induction motor. NEMA recognizes five different designs of three-phase motors, each with different speed/torque characteristics. They are designated as NEMA Design Types A, B, C, D and E. Design B motors are considered the standard for industrial applications. NEMA s newest classification, Design E, designates the newer ultra-high-efficiency motors. NEMA also classifies motor frames using an alpha numeric code. Each frame code relates to a series of physical motor frame dimensions. 6

7 Purchase, Installation, Maintenance 5% Frame codes are helpful when designing belt drive systems in order to verify the motor shaft size. Motor Efficiency and Energy Use Converting electrical energy into mechanical power consumes energy. The measure of total energy used by the motor in relation to the rated power delivered to the shaft is the motor efficiency rating. In other words, how well an electric motor converts input electrical power (measured in kilowatts) to output power at the shaft (measured in horsepower) defines efficiency. Electricty Costs 95% The purchase price of a motor is a small fraction of the cost of energy consumed over its lifetime. Source: Motor Decisions Matter SM Power losses within the motor can reduce its inherent efficiency. Typical internal power losses result from factors including the following: 1. Iron core losses 2. Stator resistance 3. Rotor resistance 4. Windage and friction 5. Stray load losses These factors explain why motor efficiencies vary so widely, from about 80% for small motors to about 95% for large motors. Motor efficiency is a function of design. Improved materials and better designs result in higher efficiency motors, such as NEMA Premium efficient motors. Replacing pre-epact standard electric motors with high efficiency motors can reduce energy consumption by 3 to 9 percent. Does it pay to invest in high efficiency motors? Consider that the cost of purchasing, installing and maintaining a motor is a mere fraction of the cost of energy it consumes over its working life. Therefore, it s important to consider the total cost of ownership, not just initial cost, when purchasing or replacing a motor. 7

8 Conclusion The energy and long-term cost savings gained by replacing older standard efficiency motors with NEMA Premium efficient motors can be multiplied by including a high-performance synchronous belt drive for even greater overall system efficiency. The key is to analyze the complete power transmission system (motor and belt drive), not just the motor. Selecting a NEMA Premium efficient motor and a high-efficiency synchronous belt drive offers the best solution for keeping energy costs to a minimum. And a synchronous belt drive offers additional advantages in lower maintenance and downtime costs. Additional Resources Engineering design assistance with belt drive systems is available from Gates Corporation. Contact a Gates Product Application Engineer, (303) , ptpasupport@gates.com, or visit 8