Electric Motors (2014)

Topic last reviewed: 1 February 2014

Sectors: Downstream, Midstream, Upstream

Electric motors represent over 80% of electricity use at a refinery since they are used to drive almost all of the minor processes. Of the motors used, 60% are used to drive pumps, 15% air compressors, 9% fans, and 16% other applications [Reference 1]. In the United States industrial applications and process industries (including upstream oil and gas operations such as pump jacks, gas line and compression), electric motors account for 60% to 70% of electricity use [Reference 2]. This may not be true in other parts of the world where direct drives may be used more than electric motors. In addition, it may be appropriate to evaluate other alternatives for power production including direct drive, power from shore (in an offshore development), power from the grid, or self-generated power (such as gas turbines). To best optimize the efficiency of electric motors, a “systems approach” should be used to identify potential savings and performance enhancement. This approach looks at the entire motor system, including the motor itself, coupling to the load, driven load and motor controls, as well as engineering/maintenance practices. By focusing on the motor system as opposed to just the motor itself, the system can be configured to avoid inefficiencies and energy losses. A basic premise of the systems approach is that individual systems do not operate under one condition all the time. Motor and drive systems loads vary according to cyclical demands, environmental conditions, and changes in customer requirements. Taking a component-based approach to the design and operation of a motor and drive system can result in increased costs and maintenance requirements and reduce reliability. Optimizing the design of a motor and drive system can provide motors and drive systems that are well suited to their applications. Key aspects of efficiency improvement for each component of motor systems are detailed below.

Application of Technology

Electric Motor Efficiency Aspects

Higher Efficiency Motors

High efficiency motors have better designs, materials, and higher tolerances than typical motors. They also run cooler, have less vibration, and consequently have higher service factors. Despite these advantages, replacing motors that are still working with high efficiency may not be economically justified in applications where a motor operates for less than 4,000 hours per year. However, when a motor needs replacing or rewinding, it is often an advantage to switch to a high efficiency motor, due to a payback period of typically less than one year [Reference 1]. Savings on motors that run for long hours at high loads are even greater. This relatively quick payback means that it is often beneficial to switch to the most efficient motor available whenever a current motor fails.

National and International Efficiency Classifications – In the US, the Energy Independence and Security Act (EISA) of 2007 focuses on increasing motor efficiency levels. Likewise, the European Union has established the International Electrotechnical Commission (IEC) standards for rotating electrical machinery. Under these directives, the following efficiency classes have been established:

• Standard efficiency = IE1 (Worldwide)
• High efficiency = IE2 (Worldwide), identical to EPAct (US)
• Premium Efficiency = IE3 Worldwide, identical to NEMA Premium (US)

Figure 1 below shows a comparison of the efficiencies for the standard classifications.

Figure 1. Motor Efficiency Classification [Reference 3]

Sizing of Motors

Motors that have been inappropriately sized, either due to incorrect initial calculations or a change in operating parameters, result in excess energy losses. Correcting for motors that are now oversized could save 1.2% of current electric motor consumption, on average, for the U.S. petroleum industry [reference 1]. On the other hand, there may be advantages to designing “modular” systems so that extra capacity can be removed when appropriate.

Coupling Efficiency Aspects

Gears and transmissions are two mechanical elements which offer significant potential for improved efficiency. Accordingly, it can be easier or more cost-effective to change transmissions or gears to achieve overall performance improvement. Gears are used in some systems to convert motor speed to the required speed. Some types of gears (worm gears with very high gear ratios) can be very inefficient. Thus, the larger the gear ratio and the more gear ratios used, the lower the efficiency. High gear losses can be avoided by using a motor with a pole number and respective speed closer to the desired rpm of the driven equipment. If a gear is not used to provide maximum torque at low speed, a Variable Speed Drive (VSD) can provide a better alternative. In many newer applications, gears are avoided by an integrated direct-drive, direct coupling of the motor to the machine (pump, fan, compressor, etc.) thereby eliminating the need for intermediary mechanical equipment.

Transmissions are used in some applications to adjust the motor speed to the application and to allow some soft connection between the two to allow for vibration etc. The traditional V-belt has maximum friction but also has high losses. The efficiency is around 95% to 98% when new but drops to 93% after significant use [Reference 4]. Flat belts provide far lower friction losses and can reach 98% to 99% efficiency. Roller chains made from steel can reach 98% efficiency. As with gears, many newer systems are integrating direct drive, direct coupling to avoid intermediary equipment.

Efficiency Aspects of Motor Driven Loads

Integrating the components of an electric motor drive system can provide opportunities for improved energy efficiency. Pumps used for transport of fluids such as water or oil are available integrated or separately, with motors and pump wheels which are assembled at the application site. A major impact on the efficiency of pumps is the operating point versus the optimal point. Constant flow systems can be sized to operate close to the maximum efficiency point, however most applications have a variable load so the pump must work at changing flow and pressure moving away from optimum efficiency. Optimal pump systems offer significant potential energy savings in various applications. For example, a system that uses two pumps at the same time with an integrated sequencing control system has been developed which is claimed to save considerably more energy under par-load conditions than a variable speed system that operates at a constant speed [Reference 4]. Fans are available as integrated systems as fan sets (up to 2 kW) and separately as motors and fan wheels. The efficiency of fans varies with size (flow, diameter, power) and type of gas. As with pumps, fans with constant flow systems can be sized to the maximum efficiency point, but changing flow and pressure causes the fan to move away from the optimal efficiency. Only large fans with adjustable blades can avoid this reduction in efficiency. The energy demand of fans is dependent on the design of the entire installation and the components as well as the running conditions. The use of optimal design and technology along with better time management can reduce the energy consumption by a factor of greater than 60%. Compressors used to compress air, liquid natural gas, and for gas transport use reciprocating, rotary screw and centrifugal systems. Most compressors come in packaged systems in which the motor and compressor are in a full or semi-hermetic enclosure. Many compressed air and pneumatic control systems can be replaced by more efficient systems such as electric servo or linear motors. Some manufacturers that offer standard compressor packages are moving towards more energy efficient compressor systems. Such systems may employ Permanent Magnet (PM) motors and Variable Frequency Drives (VFDs) as a way of making both cooling and compressed air systems more efficient.

Efficiency Aspects of Motor Controls

Variable Speed Drives (VSDs)

Adjustable speed drives (ADSs) and variable speed drives (VSDs) are used to match the speed of the motor to load requirements, particularly in scenarios that might have variable speed requirements. Prior to the widespread adoption of VSDs, load matching was achieved using transmissions, dampers, throttles, control valves, and bypasses. These types of load control are frequently replaced with VSDs.

VSDs offer the best efficiency advantages in variable torque applications (centrifugal loads like pumps and fans) because the energy required to drive the load is proportional to the cube of the load speed. Therefore, any decrease in the magnitude of driven load results in a significant decrease in the power required.

Decreasing the flow and pressure in a pump by a small amount can have significant reductions in cost [1]. In constant torque applications (non-centrifugal loads like air compressors and horizontal conveyors), efficiency benefits are not as dramatic, however, VSD allow for more stable operation compared to on/off control. The increase in efficiency comes from eliminating mechanical resistance due to artificial brakes, which are a major source of partial-load losses. Additional energy savings are achievable by eliminating components such as gears, clutches, and transmissions. [Reference 4].

The control technology used for adjusting the frequency and voltage delivered to the motor to match exactly the required speed and torque is called a variable frequency drive (VFD). This electronic controller sits between the grid and the motor and based on pulse width modulation can adjust the electrical input to the motors as demand changes. New motors are moving towards this technology and away from the fixed-speed design of a set number of poles, due to the increase in flexibility and efficiency adjustable-speed provides. However, VFDs have additional costs, which can be greater than or equal to high-efficiency motors. VFDs also have increased losses, which are minimal at nominal torque and speed, but can reach 30% at 25% torque and speed [Reference 4].

Variable Voltage Controls (VVCs)

Variable voltage controls (VVCs) are similar to ASDs and VSDs, the difference being that they can provide constant speed with variable loads. The economic principles for VVCs are the same as for ASDs/VSDs. Basic voltage control can be achieved using a resistor in series with a diode (or a series of diodes) because changes in the current drawn only change the voltage across a diode slightly. If a more efficient or precise VVC is needed, a feedback voltage regulator can be used. This measures the actual output voltage and calculates the voltage error, which is used to control the regulation element in a negative feedback control loop. However, this method causes a reduction in the stability of the voltage supply due to the nature of the control to adjust it constantly.

Engineering/Maintenance Practices

Rewind vs. Replace

Frequently large motors are rewound (rebuilt) when they fail, rather than replaced due to the low cost of rewinding a motor compared to purchasing a new motor. Rewinding motors can result in a 1 to 2% drop in efficiency. For this reason, a cost benefit analysis should be completed prior to rewinding any motor to determine if purchasing a new, higher efficiency motor is economically advantageous when considering operating costs. For non-specialty motors with long run times (4,000 hours per year), the best decision is typically to replace the failed motor with a new premium-efficiency motor, or to use extraordinary care while rewinding the old motor [Reference 5].

Reduce/Eliminate Idle Time

Motors are often left to idle, due to the common belief that it is more economical than stopping and restarting the motor. While startup energy demand is higher than operating demand, the startup period is brief and is usually less than one minute of additional running time. Since motors are designed to be started, the stresses caused by more frequent starts should not be an issue, provided the start rate given in NEMA MG 10-2001, Revision 2007, is not exceeded. When the NEMA start rates are exceeded, soft starters can be used to reduce the risk of premature motor failure. Idling motors use energy unnecessarily and should always be shut down if they will not be needed for intervals longer than those identified in the NEMA document. Automatic shutdown timers are effective at reducing the idling time of motors at a facility without the need for additional oversight [Reference 6].

Voltage Unbalance

Voltage unbalance, where the voltage of power supplied to the motor does not match the voltage at which the motor is rated, can cause vibrations and mechanical stress, increased losses, and motor overheating. These issues increase the maintenance costs of running the motor and shorten its life. If the input voltage is lower than the rated voltage of a motor, the current drawn increases proportionally in order to provide the proper wattage. The high current through the motor causes overheating and reduces the life as well as the efficiency of the motor. If the input voltage is too high, the current drawn is not decreased, although it may seem reasonable. High voltages on a motor cause the magnetic portion to become saturated, meaning that the motor attempts to magnetize the iron beyond the point to which it can be easily magnetized. While attempting this, the motor draws excessive current, also causing overheating. This shows why it is important to reduce voltage unbalances: any difference between the input voltage and the rated voltage will cause higher current to flow through the motor, decreasing its lifespan due to overheating [Reference 7].

Typically, the goal is to keep the unbalance from exceeding 1%, although even this disparity reduces motor efficiency at a part load operation, and higher unbalances can reduce even full load efficiency. Since voltage unbalances can be caused by open circuits, unbalanced transformer banks, or faulty operation of voltage correction equipment, they are relatively easy to fix. By instituting a plan to regularly monitor the motors and their input voltage, or increasing the frequency of current inspections, any voltage unbalances can be identified and rebalanced.

Technology maturity

Key metrics

Decision drivers

Alternative technologies

Gas engines - For certain applications, gas engines can be used in place of electric motors where a fuel source is readily available. These engines can be reciprocating, rotary or turbine engines and are commonly used in the oil and gas industry in remote locations with limited or no access to electrical power. GTs can be used to produce power and EM can still be used efficiently, especially in upstream where the load profiles vary tremendously over the life of field and turn down after plateau is required. EM and VFD in this situation can improve overall efficiency. However, the project needs to take into consideration the life cycle costs of using direct drive vs EM.

Copper Rotor Motors - For applications up to 20 horsepower, new copper rotor motors have been developed by Siemens and the Copper Development Association Inc. They are ultra-efficient due to the replacement of the aluminum squirrel cage with a copper one. The losses on this new motor are up to 15% below those of comparable NEMA premium efficiency motors, and have a long motor life [Reference 8].

Operational issues/risks

Utilizing a systems approach to optimizing motor efficiency has been common practice for over 20 years and presents no risk to adopters in the oil and gas industry. Poor system selection and analysis that can lead to high operating costs or excess wear on the system might include:

• Decreased production, due to lack of power being supplied to a key unit;
• High operating costs, due to providing excess power to units as well as maintenance issues due to wear and tear on the system; and
• Expensive fixes or replacements and lost production from excessive downtime.

Opportunities/business case

Improving the efficiency of electric motors is typically a long-term process for operational facilities. The immediately beneficial strategy is to begin monitoring for voltage unbalances for all motors, as it is easy to adjust on a case-by-case basis. Evaluations should be done on how to reduce motor idling time, which is analysis that requires the entire process be taken into consideration. While these evaluations are more involved than fixing unbalances, they can increase the motor’s power factor when performed in conjunction with adding capacitors and/or adjusting the voltage to key motors. The long-term, more significant improvements occur each time a motor fails or needs to be rewound. Replacing the old equipment with highly efficient motors and variable speed drives, instead of just choosing a standard motor, has a quick payback period. Particularly for equipment handling large loads and long hours, this decision will save money in the long run. However, since it is rarely economical to make this replacement for a motor that is still functional, these improvements must wait until the old motor is no longer working.

Industry case studies

Motor Resizing / Variable Speed Drives (VSDs)

This case study is an example of extremely oversized pumps that were fixed by resizing existing motors and adding variable frequency drives (a type of VSD) to others. At a San Francisco refinery, the vacuum gas oil plant was converted to a Diesel Hydro Treater. This caused all the pumps to be oversized, with some operating at efficiencies as low as 40%. The refinery kept using the pumps, but suffered significant additional vibration, decreased hydraulic efficiency, and had failures in seals and bearings almost once a year. For a total investment of $1.2 million, the refinery installed two variable frequency drives: one on the product transfer pump and one on the primary feed pump. Together, these drives saved $340,000 per year. In addition to the VSDs, two other pumps were resized to operate more effectively in their new roles. An additional savings of $410,000 per year was gained by the resizing, giving the entire project a payback of about 1.6 years. Since the upgrade, there has been much less vibration and many fewer seal and bearing failures, and the process control at the facility has improved [Reference 9].

References:

1. Worrell, Ernst, and Christina Galitsky. "Energy Efficiency Improvement and Cost Saving Opportunities For Petroleum Refineries: An ENERGY STAR® Guide for Energy and Plant Managers." Energy Star. Ernest Orlando Lawrence Berkeley National Lab, Feb 2005
2. “Improving Motor and Drive System Performance: A Sourcebook for Industry", U.S. Department of Energy, Energy Efficiency and Renewable Energy, September 2008
3. -removed-
4. Waide, P. and Brunner, C., Energy –Efficiency Policy Opportunities for Electric Motor-Driven Systems, International Energy Agency, 2011
5. "Impact of Rewinding on Motor Efficiency." GE Industrial Solutions. General Electric, Feb 2002. Web. 6 Sep 2013
6. "Energy Tips: Motor Systems; Turn Motors Off When Not in Use." Energy Efficiency and Renewable Energy: Advanced Manufacturing Office. U.S. Department of Energy, November 2012. Web. 6 Sep 2013
7. Cowern, Edward. "Baldor Motors and Drives: Cowern Papers." . BALDOR, April 1999. Web. 6 Sep 2013
8. "New Motor Technologies Boost System Efficiency." RELIABLEPLANT. U.S. Department of Energy, 2008. Web. 6 Sep 2013
9. "Variable Speed Pumping: A Guide to Successful Applications, Executive Summary." Energy Efficiency and Renewable Energy. U.S. Department of Energy, May 2004. Web. 5 Sep 2013