Engineers at P.L. Porter Controls Inc. developed aircraft seat controllers that let passengers adjust their seats for comfort. The engineers customized a Series 8000 Lo- Cog motor with ball bearings to reduce friction and lessen power dissipation.

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.

Depending on the torque and speed, different factors affect dc motor power dissipations.

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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