Motors and Drives

Quick Fact:  The cost of electricity for a heavily used electric motor can be up to 10 times the purchase cost of the motor – EACH YEAR!

General Description:[1]
Electric motors fall into two classes, based on the power supply: alternating current (ac) or direct current (dc). Alternating current (ac) motors are the most widely used in industry. Industry’s preference for ac motors springs from their wide application, simplicity, low cost, and efficiency. There are two primary types of ac motors: induction (also referred to as asynchronous) and synchronous. Induction motors are used widely because of their simple design, rugged construction, relatively low cost, and characteristically long operating life. Synchronous motors, on the other hand, have some useful advantages and are used in more specialized applications.

 Alternating current (ac) motors can be single-phase or polyphase. In terms of quantity, single-phase motors are the most common type, mainly because many small motors are used for residential and commercial applications in which single-phase power is readily available. However, several operating constraints on these motors limit their widespread use in industrial applications. Integral single-phase induction motors tend to pull large starting currents relative to the motor’s size. In general, they operate less efficiently than three-phase motors of comparable size, and are not available in larger sizes.

In contrast, polyphase motors are used widely in industrial applications. They consume more than half of all the electricity used in industry. Polyphase motors can be found in almost every industrial process, and they often operate continuously to support production processes. These motors can achieve high efficiencies with favorable torque and current characteristics. The effectiveness and low cost of three-phase motors are major reasons why three-phase power is used so widely in industry. In terms of energy consumption and efficiency improvement opportunities, three-phase motor systems predominate.

The advantages of dc motors include excellent speed control and the ability to provide high torque at low speeds. Since electric power is supplied as alternating current, additional equipment that converts ac to dc power, such as motor generator sets or rectifier systems, is needed to run dc machines. Because batteries supply dc current, dc motors have an advantage in applications in which the motor is supplied by a dc bus as part of an uninterruptible power system. 

To select the proper motor for a particular application, the engineer needs to consider the basic requirements of the service. These include the load profile, environmental conditions, the importance of operating flexibility, and reliability requirements. Motors should be sized to operate from 75% to 100% of rated load. The principal consequence of operating a motor above its rated load is a higher winding temperature, which shortens the operating life of the motor. If the motor has a service factor of 1.0, the motor lifetime may be only a few months if it is operated above rated load or if it is operated at rated load when there is a power quality problem.

Synchronous ac motors operate at the speed of the rotating magnetic field.  Many applications require speeds different from these, however.  So motors are usually combined with various types of speed adjustment devices. These devices include gears, belts, eddy-current couplings, hydraulic couplings, variable frequency drives (VFDs), etc.  Many applications that are currently served by constant speed motors are well suited for variable speed motors. For example, in many pumping system and fan system applications, flow is controlled by using restrictive devices, such as throttle valves or dampers, or bypass methods. Although these flow control methods have advantages, speed control is often a more efficient and cost-effective option for many systems.

Potential Energy and Cost Savings Opportunities:
The below energy conservation opportunities or energy efficiency actions (EE ACTIONS) are provided as a partial list of potential savings opportunities in your plant.  They are grouped as no-to-low cost, moderate cost, and long-term cost investments.  Consider each for your plant and feel free to contact us for clarification or any assistance you may need in assessing specific projects.

No-to-low cost investment

EE Action: Replace v-belts with cogged or synchronous belt drives [2]
About one-third of the electric motors in the industrial and commercial sectors use belt drives. Belt drives provide flexibility in the positioning of the motor relative to the load. Pulleys (sheaves) of varying diameters allow the speed of the driven equipment to be increased or decreased. A properly designed belt transmission system provides high efficiency, decreases noise, requires no lubrication, and presents low maintenance requirements. However, certain types of belts are more efficient than others, offering potential energy cost savings.

EE Action: Eliminate voltage unbalance [2]
Voltage unbalance degrades the performance and shortens the life of a three-phase motor. Voltage unbalance at the motor stator terminals cause phase current unbalance far out of proportion to the voltage unbalance. Unbalanced currents lead to torque pulsations, increased vibrations and mechanical stresses, increased losses, and motor overheating, which results in a shorter winding insulation life.

EE Action: Improve motor operation at off-design voltages [2]
Motors are designed to operate within +/- 10% of their nameplate rated voltages.  When motors operate at conditions of over- or under-voltage, motor efficiency and other performance parameters are degraded.  The best motor performance occurs when power supplied to the motor terminals is close to the nominal utilization voltage.

EE Action: Turn motors off when not in use [2]
Motors use no energy when turned off. Reducing motor operating time by just 10% usually saves more energy than replacing a standard efficiency motor with a NEMA premium efficiency motor. In fact, given that 97% of the life cycle cost of purchasing and operating a motor is energy-related, turning a motor off 10% of the time could reduce energy costs enough to purchase three new motors.

Moderate investment

EE ACTION: When to purchase NEMA premium efficiency motors [2]
The new EISA 2007 standards went into effect on December 19, 2010, taking most of the guesswork out of specifying almost all motors up to 500 horsepower. The new standards mean that the standard motors that you buy today have the same efficiency as the premium efficiency motors that you bought in 2010. Some motor manufacturers will be releasing new lines of ultra-high efficiency motors over the coming years, which will have even higher efficiency than the NEMA premium efficiency standards require.

Based on the EISA standards, the efficiency of standard motors will mean that these motors will make the most financial sense in most applications. If annual running hours are very high, 8000 or more hours, then condsider using the new ultra-high efficiency motors. The following reference tools will help you decide if a special motor will save you money, or if it is time to replace an existing, lower efficiency motor.

Reference tools: 
https://www.motorboss.com/ecat/search/html/calculator.html
http://www.reliance.com/prodserv/motgen/energy_calc.htm

EE Action: Eliminate excessive in-plant distribution system voltage drops [2]
Studies indicate that in-plant electrical distribution system losses—due to voltage unbalance, over- and under-voltage, low power factor, undersized conductors, leakage to ground, and poor connections—can account for less than 1% to over 4% of total plant electrical energy consumption. In a study at three industrial facilities, average electrical distribution system losses accounted for 2% of plant annual energy use. Losses due to poor connections represented one-third of these losses and accounted for 40% of the savings after corrective actions were taken.

Poor connections or inadequate conductor sizes result in excessive energy losses. The increased resistance converts electrical energy into heat and imposes additional loads on the distribution system. Maintenance of connections is generally referred to as termination maintenance. Termination maintenance is generally a cost-effective electrical distribution system energy savings measure.

EE Action: Use adjustable speed drives where economical [2]
An adjustable speed drive (ASD) is a device that controls the rotational speed of motor-driven equipment. Variable frequency drives (VFDs), the most common type of ASDs, efficiently meet varying process requirements by adjusting the frequency and voltage of the power supplied to an AC motor to enable it to operate over a wide speed range. External sensors monitor such things as flow, liquid levels, or pressure and then transmit a signal to a controller that adjusts the frequency and speed to match process requirements.

EE Action: Is it cost-effective to replace old eddy-current drives? [2]
New pulse-width-modulated (PWM) adjustable speed drives (ASDs) may be cost effective replacements for aging or maintenance-intensive eddy-current drives.

References

[1] exerpts from Improving Motor and Drive System Performance: A Sourcebook for Industry, Industrial Technologies Program (U.S.), 2008, United States Department of Energy, Washington, D. C., pp. 78 (cited 9/13/10)

[2] Energy Tips – Motors, Industrial Technologies Program (U.S.), United States Department of Energy, Washington, D. C., (cited 9/13/10). To find more tip sheets go to EERE Publication and Product Library.