Regardless of the application, even small reductions in dc motor losses, such as bearing friction, can have significant gains in overall process efficiency, motor life, and cost effectiveness. For example, a ball bearing may add about 20% to a motor's cost but result in 5% higher efficiency and 75% percent longer life. Even if it means augmenting a motor's frictional torque by a mere 1/10 th of one oz-in., it can quickly pay for itself.
Plugging the leaks
As a general rule, a motor's application dictates the nature (mechanical or electrical) and the magnitude of energy loss. The losses can be static or speed sensitive, which are mechanical in origin; and load sensitive, which is electrical in nature.
Static dissipation. These losses are usually a frictional component that affects smaller more than larger motors. In a dc brush motor bearing friction, and to a lesser extent brush contact, are the two primary contributors.
For bearings, particularly on small motors, the friction (and noise) usually results from overloading, dented races, ring distortion, misalignment, and inadequate lubrication.
For brush contact, power is lost from the heat created as current flows through the brush and commutator system. When the brush glides over the conductive surface of the commutator, the action forms a film that is essential for proper brush lubrication. But the brush-film union has properties similar to a dielectric material, so resistive dissipation may be relatively high compared to winding resistance. While winding resistance varies over a large range, brush-film resistance may be as much as 35% greater in some cases.
Speed-sensitive losses. Added together, the losses of eddy currents, hysteresis, and short-circuit currents are velocity dependent factors that oppose torque. The constant and velocity dependent terms that comprise torque may be defined as cogging and viscous damping. Often, motor manufacturers combine and quantify eddy current and hysteresis dissipation into a single term – damping. For the purpose of this discussion, however, they remain separate to distinguish their varying causes and characteristics.
Eddy currents, which are proportional to the speed of the motor, are generated by magnetic fields circulating around a conductor that in turn sets up currents, creating an opposing magnetic field. In the case of permanent magnet (PM) motors, the iron in the stator or armature undergoes the magnetic field change that causes eddy current flow.
As the eddy currents circulate, they can measurably warm the motor, particularly in high-speed operation. Moreover, combined with hysteresis effects, they limit the maximum speed attainable from the armature or stator, whether slotted or slotless. However, they are significantly reduced in the latter. This is not to say that eddy currents are present in all PM motors: Non-ferrous armature motors, which require less power to produce higher rotational speeds, are generally immune to this dissipation anomaly.
Hysteresis (iron loss) is also associated with fast motor speeds. During motor operation and speed shifts, all parts of the motor's armature undergo a magnetization change as the armature rotates in the magnetic field, causing the magnetic boundaries to shift. As the motor speeds up, resistance to this boundary shift generates heat.
These losses are an inherent trait of the type of steel used in the armature. They are usually linked with eddy current losses under the generic badge "iron loss."
A possible cause of iron-related power losses is lamination orientation. During armature assembly, the laminations may be magnetically oriented in the same direction when pressed onto the armature. Spinning the laminations before pressing them varies their orientations and alleviates this condition.
Short-circuit currents do not necessarily suggest a fault in motor design. Rather, they are a normal current that can be created, albeit briefly, in a commutated armature coil. When the brush makes contact with two commutator segments, it can result in a short circuit between the end of the coil and the two segments.
As motor speed increases, these currents create a drag on the armature, which can limit a motor's maximum speed. Having brushes commutate coils in regions of low magnetic flux minimizes heat losses. Another solution is to use three-bar contact. Here, both brushes together contact no more than three commutator bars at one time. Four-bar contact, on the other hand, involves each brush momentarily touching two commutator bars at a time, which can cause short-circuit currents.
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Load-sensitive losses. Brush-commuted dc motors are more likely to be affected by these losses than brushless, especially slotless designs. In either case, major load-sensitive thermal dissipation is often associated with winding resistance.
In dc motors, voltage determines speed and current determines torque. The motor's torque constant is expressed as torque per unit of current while motor speed is quantified as speed per unit of voltage. Increased torque requires more current to the motor, causing greater power losses.
Building a more efficient system
Factors that strongly effect motor efficiency include bearings, lubricants, and the motion profile.
Bearings. Both brushless and brushcommutated motors use either sleeve or ball bearings. Sleeve bearings are a costconscious design that supports a stainless steel shaft within a sintered, bronze, oilimpregnated bearing. With a large area of contact, these bearings often contribute large frictional losses. Single or doubleshielded ball bearings, on the other hand, provide lower friction and ample reserve lubrication, and they maintain their alignment. If properly installed, they are selfaligning and have a smaller area of contact. And because they reduce friction torque, they cut heat losses. However, they can be more costly than sleeve bearings.
Use of ball-bearing designs promote lower friction torque and help curtail friction losses while increasing motor capacity. For example, a brush-commutated motor with a standard sleeve bearing has a friction torque rating of 0.35 oz-in. When compared to a similar motor equipped with ball bearings, the friction- torque value drops more than a quarter, to 0.14 oz-in. In this case, by designing a motor with ball bearings for 1 oz-in. continuous operating torque, the motor's operating capacity jumps by a factor of 0.2 – a 20% increase. This value will vary on a case-by-case basis, but as a general rule, a motor with ball bearings will have a greater operating capacity than the same motor with sleeve bearings.
Ball bearings also ensure proper concentricity in systems using incre - mental optical encoders. In such cases, sleeve bearings may negatively affect concentricity, increasing the potential for wobbling. Ball bearings are essential in these systems for correct encoder accuracy, and to lower power losses.
Lubricants. These fluids can reduce power dissipation in dc gear - motors because they provide a film that reduces friction between the gears. A lubricant with the lowest viscosity and that won't separate between the gears is the most suitable. When designers use a higher viscos - ity than necessary, power is lost as the gears work to overcome the internal friction of the lubricant. However, if the viscosity is not high enough, the lubricant can separate, creating greater gear friction and more power losses.
Operating temperature and sani - tation and safety are particularly im - portant considerations. In food preparation equipment, for example, industry sanitation regulations limit the choice of lubricant. Non-toxic lubricants ensure safety in case of fluid separation due to a higher than normal operating temperature or simple aging and potential leaking.
Motion profile. S-shaped profiles permit smoother control of accelera - tion and deceleration whereas trape - zoidal profiles change velocity in a linear fashion until it reaches its tar - get. S-curve profiles minimize jerk and reduce the possibility of the motor overheating.
Watch your speed . . . and applied voltage
You may also improve efficiency by operating motors at an applied voltage lower than the rated, catalog value. Because the motor runs at lower speeds, it generates less heat, increas - ing overall efficiency. The drop in speed also adds about 20% to brush life, increases gear life, reduces audible noise, and decreases EMI.
These benefits vary from motor to motor. For a particular application and selected group of motors, com - pare the motor speed as well as the current and gearhead reduction ratio at identical operating points – which is possible through motor simulation spreadsheets. Usually, the lowest speed indicates the motor that offers the longest gear and brush life, and the lowest heat dissipation. This guideline applies to motors operating at speeds above 1,000 rpm. At lower speeds, dust from motor brushes can quickly accumulate in commutator slots and electrically short across two terminals. Ideally, motor speeds should be as low as possible for life and efficiency bene - fits while high enough for centrifugal force to remove the dust from the commutator slots.
For example, consider an applica - tion that requires a motor supplying 10 oz-in. of torque at 300-rpm output speed. Three gearmotors are found to meet the requirements. The first gear - motor has a gearhead reduction ratio of 30.9:1, rated at 24 V, and runs at an applied voltage of 22.6 V. The second and third gearmotors both have a gearhead reduction ratio of 6.3:1. The rated and applied voltages for the sec - ond motor are 24 Vand 6.9 V, respec - tively. The third motor is rated at 48 V and runs at 13.8 V.
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Of the three motors, the first one appears to be an obvious choice be - cause the application comes close to rated specifications – running a 24-V motor at 22.6 V to meet the operating requirement. However, simulation shows that the motor is the least effi - cient. With voltages reduced from rated levels, the second and third mo - tors offer similar speeds, gear life, and efficiency. However, because the third motor operates at half the current, its brushes last longer. By contrast, the higher current in the second motor reduces EMI. Thus, the solution is to select one of the two motors for either long brush life or low EMI.
The pros and cons of slotless, brushless designs
Brushless and brush-commutated motors offer different advantages. Conventional wisdom holds that brushless motors provide high speed and fast acceleration, generate less audible noise and electromagnetic interference, and require low maintenance. Brushcommutated motors, on the other hand, afford smooth operation and greater economy. However, recent improvements in brushless motor technology and manufacturing are challenging conventional wisdom and allowing designers to select brushless motors that combine the traditional advantages of both types.
One of the keys to smooth brushless performance and a lower heat signature centers on the slotless stator and refinements to the lamination processes.
In a traditional brushless motor, copper wires are wound through slots in a laminated steel core -- a method that increases cogging and eddy currents. As magnets pass by the lamination shoes, they have a greater attraction to the iron at the top of the laminations than to the air gap between the shoes. This uneven magnetic pull causes cogging, which affects motor vibration and noise.
In a slotless, brushless design, copper wires are wound against the laminations and held with adhesive. This design yields smooth rotation, virtually eliminates cogging, and significantly reduces damping losses.
In both slotted and slotless motors, eddy currents are induced as the magnets and laminated iron pass each other, resulting in damping losses. These currents are weaker in a slotless motor because the distance between the laminated iron and magnets is greater than in a slotted motor. With low damping losses, slotless, brushless motors achieve more efficient operation.
Richard Green is senior engineer at Pittman, Harleysville, Pa.