Taming the Hidden Energy Workhorse - University...

Major Challenges in Small Machines – Taming the Hidden

Energy Workhorse

P. T. KreinDirector, Grainger Center

Department of Electrical and Computer EngineeringUniversity of Illinois at Urbana-Champaign

Joint Seminar –Power and Energy Systems and

Grainger Center for Electric Machinery and Electromechanics

Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign

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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


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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, …


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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.


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• 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.


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• 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.


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• 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).


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• 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!


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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.


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• 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 still

require wound stators, withprocess complexity


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• 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 a

fundamental basis formotor design.


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• 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 to

maintain old designs.


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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


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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.


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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


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• 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!


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• 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


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Reluctance and Induction Machines

• Reluctance motors: spatial variation of inductance to generate torque


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• 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


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Reluctance Machines

• An axial lamination orientation is better for non-salient stators


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Induction Machines

• Construction fundamentally similar to reluctance machine.

• Aluminum rotor bars for current and torque.

• Single-phase machines not easy to optimize


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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.


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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.


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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.


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Polyphase Motors in Single-Phase

• We can treat the three-phase input as two unique inputs.

• Two inputs sixty degrees apart.

AA'

B

C B'

C'


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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.


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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.


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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.


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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.

Date post:	15-Mar-2020
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