DC Motors: High Efficiency Designs

The cost of electrical energy in the U.S. is increasing¹ and since electric motors account for over two thirds of industrial energy (60-70% in a manufacturing plant²) and about 23% of all electricity sales in the U.S., according to the U.S. Department of Energy³, motor energy management and efficiency has generated immense concern. Motor manufacturers have responded by introducing energy efficient motors that reduce energy consumption. In addition, the replacement of bulky wound field motors with compact, brushless D.C. and permanent magnet motors that have high efficiency/power density is a purchasing trend that will continue for many years to come. But the issue of motor energy management and efficiency is more complex than merely selecting a new motor replacement. This complexity merits a thorough examination because successful energy management is really a systems approach.

An historical aside may be useful at this point to illustrate where the industry was a few decades ago and where it is now. Energy management was not always a concern. From post-WWII to the early 1970s, motor design was inefficient and emphasized low initial costs without much concern about lifetime or operating costs. But, in 1973, the first oil crisis⁴ occurred and completely changed the industry as electricity costs skyrocketed. A new imperative was born in motor design: limiting operating costs (Table 1). In response, most motor manufacturers added higher efficiency motors, featuring “optimized designs, more generous electrical and magnetic circuits and higher quality of materials.” ⁵

In 1997, the U.S. Energy Policy Act, which created mandatory efficiency standards, was passed. At about the same time, the Consortium for Energy Efficiency (CEE) developed a voluntary premium efficiency standard, which evolved into the NEMA Premium designation. Premium efficiency motors offer an efficiency improvement in the range of 4% for 1 hp motor to 2% for a 150 hp motors. ⁶

Table 1: Lifecycle Costs of An Electric Motor ⁷

Motor Losses

Electric motors experience a variety of losses⁸ that reduce their efficiency. These losses⁹ include:

• I²R (power) losses
• Core losses
• Mechanical losses

Power losses (I²R losses) can account for 20-30% of a motor’s total losses. ¹⁰ Power losses depend on the winding resistance and motor current and result in wasted power in the form of heat. Since winding resistance increases with temperature, controlling motor temperature can improve efficiency. Typically, winding resistance reductions are achieved by using more copper. High efficiency motors can have about 20% more copper in the windings than standard motors.

Magnetic core losses are the results of hysteresis and eddy currents.¹¹ Hysteresis losses can be reduced by using high quality steel in the core, such as silicon steel instead of carbon steel. Eddy currents are caused by circulating currents in the motor core. They can be reduced by using thinner and longer laminations and ensuring adequate insulation between the laminations. ¹²

Stray load losses are the “difference between total motor losses and the sum of the other four losses, including I²R, iron/magnetic, friction and windage.” ¹³ Some causes of stray load losses include, imperfections in the slotting of the stator and rotor and saturation effects. They are eliminated by design and careful manufacturing processes. High efficiency motors have much less stray load losses than standard motors. ¹⁴

Mechanical losses are the result of windage and friction. Fan blades, brush-commutator contact and bearings are the typical sources of mechanical losses. High quality bearings, improved ventilation fan designs and brush-commuator improvements are ways to alleviate these losses. ¹⁵

Other Factors that Affect Motor Efficiency

Beyond motor losses, there are other factors that can impact electric motor efficiency. They include¹⁶:

• Proper sizing
• Electrical power quality
• Type of control
• Distribution Losses
• Type of transmission
• Maintenance
• Operating Temperature¹⁷
• Application (Mechanical efficiency of driven equipment)

The oversizing of motors (lightly loaded) is a common cause of electric motor inefficiency. An oversized motor, defined as less than 50% loaded, not only lowers the efficiency also but also the power factor. “This decrease in performance is especially noticeable in small motors and standard efficiency motors.”¹⁸ In addition, at light loads, iron core losses are high. To increase efficiency, some motor controls will “automatically lower the motor voltage when the load drops.” This will lower energy costs.¹⁹

Poor power quality can also decrease the efficiency of an electric motor.²⁰ Power quality problems can include phase voltage imbalances (i.e., undervoltage, overvoltage), harmonics, interference, frequency imbalances, etc. Phase voltage imbalances can cause high input currents (I²R losses). Voltage flicker (brownouts or sags) are short-term voltage variations. They add to motor losses and cause motor current to increase. Powerline harmonic distortions increase motor losses and cause motor reactive heating, and decrease efficiency.²¹

Motor temperature affects efficiency. Cooler running motors will have a lower winding resistance and result in lower motor losses. For example, a “10^oC reduction in motor temperature will reduce the D.C. resistance losses of the conductors by 3-4%.” ²²

Design Features That Improve Motor Efficiency

Motor efficiency is improved by reducing motor losses: power, core and mechanical. This is accomplished in any of the following ways²³:

• Build with closer tolerances
• Reduce vibrations
• Increase the amount of copper in the stator windings
• Use higher-grade electrical steel
• Improve the power factor to reduce reactive current heating
• Use high efficiency mechanical loads (pumps, fans, etc.)
• Use electronic controllers instead of across-the-line start/stop controls
• Use energy efficient belts and/or gear reducers
• Use power conditioning equipment.

Closer tolerances, vibration control, low-friction bearings, and energy efficient belts are some of the ways to reduce mechanical losses. Improved mechanical load designs help improve motor efficiency by “reducing internal friction through smoother and more carefully contoured internal surfaces, tighter tolerances, higher quality bearings.” ²⁴ “Additional savings are possible by replacing V-belts with energy-efficient cogged belts. This can raise system efficiency by up to 2 percent. Switching a right-angle worm speed reducer to an inline helical or right-angle bevel gear reducer can raise efficiency by 20 to 50 percent. This means that a lower-horsepower motor that consumes less electricity [ ] can be used to drive the load.” ²⁵ Core losses are reduced with high-grade steel and better lamination designs. Power conditioning equipment can improve the electrical mains’ power factor. This reduces motor running temperature (I²R losses) by eliminating the reactive component of motor current. In manufacturing, winding resistance can be reduced by maximizing slot fill²⁶ and minimizing the end-turn radius²⁷.

D.C. Motors: High Efficiency Designs

Improvements in the efficiency of D.C. motors break down into three broad categories:

• High efficiency designs
• Advances in the brush-commutator area
• New D.C. motor types: Permanent Magnet and Brushless types.

Much of the effort in producing high efficiency D.C. motors revolves around ways to understand thermal pathways through the motor as well as limiting I²R losses (heat). In addition, “software modeling tools [are] used get a better understanding of both the magnetic flux and thermal flows in the motor laminations … Slight changes in lamination geometries, metallurgy, and insulating materials allow for increased power density and smaller motors.” ²⁸

Since the brush-commutator area of a D.C. motor is a source of mechanical losses (as well as high maintenance), there has been research conducted on how to improve these motor parts. One of the ways is to pursue designs that increase the power density of the motor by decreasing the size of the commutator. “As the circumference of the commutator shrinks, there’s less brush wear with every turn of the rotor. Reduced brush wear results in extended intervals between brush changes. Engineers also have redesigned brush blocks, pressure fingers and springs to allow for longer brushes. With longer brushes, the interval between brush changes extends further, providing for longer periods of operation without a maintenance shutdown. DC motors can be purchased with brush wear sensors, which warn that a brush is worn down to its lowest level and requires changing. Brush wear sensors often prevent commutator damage from a worn brush being left in too long and resulting in costly repairs.” ²⁹

Improvements in D.C. motor power density have led to the introduction of brushless D.C. (BLDC) motors and permanent magnet D.C. (PMDC) motors. Both of these motors types are inherently efficient. PMDC³⁰ motors use permanent magnets in lieu of power-consuming electromagnets to generate the motor’s magnetic field. These high performance, permanent magnets are made from neodymium-iron-boron alloys which have a large energy density and “offer the possibility of achieving high efficiency and compact motors.” ³¹ The permanent magnets are mounted in the stationary field (stator) which allows or better heat dissipation through the motor housing and into the atmosphere. This arrangement allows the motor to run cooler and more efficiently.

A PMDC motor can be a brushed type (commonly called a D.C. commutator motor³²) or a brushless type. The BLDC³³ motor has increased its market share because it has high efficiency, good power density and much less maintenance than the D.C. commutator motor. It does not use brushes or a mechanical commutator; rather, a controller electronically commutates the motor. Similar to a D.C. commutator motor, the BLDC motor uses permanent magnets to develop its field; the magnets are mounted on the rotor and the armature windings are mounted on the stator, where heat can be dissipated easily to the atmosphere, maintaining a cooler motor and promoting greater efficiency.