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Major Challenges in Small Machines – Taming the Hidden Energy Workhorse. Joint Seminar – Power and Energy Systems and Grainger Center for Electric Machinery and Electromechanics. P. T. Krein Director, Grainger Center Department of Electrical and Computer Engineering
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Major Challenges in Small Machines – Taming the Hidden Energy Workhorse Joint Seminar – Power and Energy Systems and Grainger Center for Electric Machinery and Electromechanics P. T. Krein Director, Grainger Center Department of Electrical and Computer Engineering University of Illinois at Urbana-Champaign
Outline • Small motors in context • How many, how much energy, technology. • Why research trends are having less impact for small motors • Future promise • Moving away from brush machines • Materials and manufacturing issues • Reluctance and induction machines • Polyphase motors in single-phase uses • Automotive considerations • Miniaturization
Small Motors in Context • The vast majority of motors are “small,” less than 10 HP (7.5 kW). In shear numbers, most of these are 500 W and below. • Motors are everywhere. I counted at least 16 in use in my own office (notthose stored away or for demo.) • Try to estimate the count for atypical household. Don’t forget • Fans, pumps, tape players, appliances, clocks, disk drives, VCRs, cars (up to 100 motors each), hair dryers, …
Small Motors -- Context • Perhaps an average American household has 300 motors. • With 100 million households,this is 30 billion small motors– about 100 per person. • If this is 20% of the worldtotal, that means 150 billiontotal motors. • If the number grows to 100 per person world wide over the next fifty years, the total number of small motors will exceed one trillion.
Small Motors -- Context • Now some more perspective. If we could save an average of 10 W per machine, the total savings would reach 10,000 gigawatts – equivalent to 10,000 large power plants. • Even motor-related savings of just 100 W per household would save 10 gigawatts in the U.S., right now. • The issues would be more obvious if life-cycle cost were a factor. • But manufacturing cost is the more significant issue for consumer motors.
Small Motors -- Context • Brush-type dc machines and “universal” series machines are common in home appliances. • The rotors must be wound;simplestator windings support low cost. • “Shaded-pole” induction machinesare common for fans and clocks. • Larger appliances use capacitor-start single-phase induction motors. • Nearly all these technologies were selected for the purposes about 50 years ago.
Small Motors -- Context • Considerations included • Low cost of components • Ease of manufacturing • Simple single-phase operation • They did not include • Energy efficiency • High reliability • Standardization of physical packages • Modern motors for computers, printers, office machines have pushed technology a little (stepper and PM machines).
Small Motors -- Context • Some detailed elements: • Thick lamination plates with simple geometries • Ample clearances • Low-cost wrapped and welded cases • Cheap bearings • Molded plastic parts • Extra winding space for simple machine winding • Common designs date to the 1950s or earlier • What has changed? – Everything!
Research Trends – Why Little Impact? • In small motors, the technology today is in the manufacturing process rather than the machine. • Machine changes carry a high burden in manufacturing. • Lack of clear cost/benefit tradeoffs works against wide innovation in motors. • But life-cycle analysis will probably push in the other direction. • At the same time, precision needs are growing.
Research Trends – Why Little Impact? • Rotor windings • A major drawback in medium and large machines • Wound-rotor machines are disappearing in the medium range • In small machines, the rotor can be easier to wind than the stator (machine windings) • Small brush machines can be cheap to make, even though they do not last • Simple reluctance motors stillrequire wound stators, withprocess complexity
Research Trends – Why Little Impact? • Electronic Controls • Cost pressure precludes many types of sensors • In most cases, the motor is just supposed to start and run • No extra cost margin available to add electronics • “Just plug it in” was afundamental basis formotor design.
Research Trends – Why Little Impact? • Automotive • The automotive market is probably the largest consumer of dc motors • 12 V designs are several decades old • Many manufacturers have no electromechanics engineers on staff! • Strong incentive tomaintain old designs.
Future Promise • Moving away from brushes • A low-value, high-wear component • Brushes limit speed, reliability, and aspects of function • They spark and create noise and vibration • But only brushes lead to a direct-connection dc motor • Ac machines have better power-to-weight ratio and are more reliable • General ac use requires inverters
Moving Away from Brushes • At medium sizes (1 to 100 HP) dc machines are more expensive than ac machines. • In industrial applications, conventional dc motors are vanishing fast. • The dynamic performance of many ac designs exceeds that of conventional dc • Electronics and sensing are more complicated with ac machines – but not necessarily more costly.
Materials and Manufacturing Issues • Permanent magnet materials have been one of the most significant developments • SmCo and NdFeB have near-ideal demagnetization characteristics and support very strong fixed fields • But practical problems remain for mounting and attachments
Materials and Manufacturing Issues • Typical small motor construction: • Simple stack of thick laminations • Machine windings with ample excess space • Paper insulation • Welded mounts and stator cases • Low level of standardization • A tendency to classify similar technologies in widely disparate groups based on construction • Compare to other modern products!
Materials and Manufacturing Issues • Today, thin laminations, narrow air gaps, better cases, and a higher level of standardization can be done without extra cost. • The combination of motors and electronics opens a huge new suite of possibilities. • Plastic and composite insulation, cases, and mounts
Reluctance and Induction Machines • Reluctance motors: spatial variation of inductance to generate torque
Reluctance and Induction Machines • This is simple, and can be used for open-loop or closed-loop stepping operation • Electronics (the drive switches and any sensing) and inherent • Open questions: • Vibration mitigation • “Optimal” geometries • Use of damper bars or embedded magnets
Reluctance Machines • An axial lamination orientation is better for non-salient stators
Induction Machines • Construction fundamentally similar to reluctance machine. • Aluminum rotor bars for current and torque. • Single-phase machines not easy to optimize
Induction Motors • A time delay (and spatial phase shift) is needed to establish a travelling flux. • This is essential for starting. • Alternatives: • Capacitors • Shaded poles (inductive delay) • Split-phase (a second winding with inductive delay, physically like shaded pole) • In general, these alternatives do not make ideal use of the materials.
Induction Motors • A possible alternative is the “inverter-dedicated” induction motor, for which an electronic circuit produces the waveforms for the travelling wave. • This can support the useof conventional polyphasemachines for single-phase applications.
Polyphase Motors in Single-Phase • Once we decide to rely on power electronics, the motor design and technology can be decoupled from the source characteristics. • The classical concept is to operate a three-phase machine indirectly from a single-phase source. • Many schemes have been proposed for this operation. • The most formal uses an ac-dc converter, followed by a dc-ac converter and the motor. • Less formal ones try to reconstruct phases.
Polyphase Motors in Single-Phase • We can treat the three-phase input as two unique inputs. • Two inputs sixty degrees apart.
Automotive Considerations • A 42 V bus structure could make inverter-driven motors feasible. • However, reliability issues change in the context of automotive: lifetime is short, and if cost can be traded against life, lower cost is better. • It is hard to re-tool 12 V designs for 42 V, so ac motors might be more logical.
Miniaturization • Miniaturization comes about in two contexts: • MEMS • Small motors that use miniaturization process techniques • A key consideration concerns fundamental aspects of electromagnetic forces. • Magnetic forces are volume forces. • Force density is J x B. Both J and B are limited by materials, and in fact ultimately must fall with scale.
Miniaturization • Electric forces are surface forces. • Force density is related to E². • The available forces actually rise when the scale is small enough. • At a size scale of about 1 mm, electric forces provide density similar to magnetic forces. • Consider the size range above 1 mm. • Here, a variety of new processes and materials might be of help.
Conclusion • Small motors are often neglected from a research perspective. • Present designs are often 50 years old or more. • There is ample room to apply fundamental motor design methods. • But the real promising areas relate to electronic controls and to newer materials.