LUT Energy Electricity | Energy | Environment 11.6.2

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Comparison of Ferrite Permanent Magnet Synchronous Machine with Induction Motor in Blower Application

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Some important factors which influent on the choice of the particular motor drive in different applications

PMSM IM

Economic aspect Reliability Factors which are improved by permanent magnets use

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Applications for rare-earth permanent magnets in 2008 and predicted applications for 2014

Isn’t it too much for rare-earth magnets which consist of limited materials?

It is advantageous to find the alternative solution for at least some of these applications. May be solution is near by us? Just look around

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− Automotive applications − Industrial and consumer motors − Industrial holding applications − Material handling − Computer peripherals − Speakers

Possible solution To reduce the price of the machine instead of neodymium magnets much cheaper and abundant ferrite permanent magnets can be used. Currently, the most usual applications for ferrite permanent magnets are:

Usually low power applications

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What are permanent ferrite magnets compared to rare earth magnets?

Weight-basis sales Ferritemagnets

Rare-earthmagnets

85 %

15 %

Characteristic NdFeB SmCo Ferrites

Remanence, Br [T] 1.44 1.12 0.41

Coercivity, Hc [kA/m] 1115 730 240

Energy density, (BH)max [kJ/m3] 400 240 32

Max cont. Temperature, Tmax [C°] 80 300 250

Resistivity, Ω [(ohm·m) 10-6] 1.1-1.7 0.65-0.9 10^9

Relative permeability, µ 1.04-1.1 1.04-1.12 1.1-1.3

Because of sharp rise since 2010, up to now the price of neodymium magnets is twenty to thirtyfold than the price of ferrite magnets

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Outer rotor give the possibility to increase the air gap radius without increasing the size of the machine

Fractional slot windings reduce Joule losses and the external size of the machine

Number of stator slots, Q 12 Number of rotor poles, 2p 10 Air gap flux density peak, Bpeak [T]

0.37

Machine constant, Cmec [kWs/m^3]

118

Fundamental winding factor, kw

0.924

Outer rotor fractional slot ferrite permanent magnet synchronous machine

Alternative which in some extent may combine the advantages of both IM (low price) and PMSM (efficiency)

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Outer rotor construction features

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It is possible to increase the electromagnetic torque and to improve the overall performance of the motor

Conductors can be inserted from outside to the slots of the armature – easier to wind

There is not the risk of the ejection of magnets from the rotor at high speed

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Fractional slot winding properties

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IM

PMSM

Capability to reduce the conductive material use compare with integer slot windings, and decrease copper losses, which are dominant losses in small and medium-power machines at low speeds Manufacturing is easier to implement as the end winding are not overlapping each other

With appropriate design , low cogging and ripple torques and an ability to achieve a significantly higher copper slot fill factor

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Properties of low-power asynchronous motor drives

Robust stand-alone construction

Automated production, cheap price

Speed control by Stand-alone frequency converter

Low efficiency in the smallest power ranges. Especially, increased rotor losses when supplied by a frequency converter

In low power range the highest possible efficiency is relatively low due to features of motor construction

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Construction and properties of PMSM with ferrite magnets and outer rotor Rugged construction of the motor

Excitation field is created by cheap permanent magnets

Hard, brittle and extremely low loss ferrites well suitable in blower application

Practically no rotor losses with ferrite

Higher efficiency than in IM drives because of synchronous running, tooth winding an practically no rotor losses

Simple motor control

The possibility of demagnetization of ferrite magnets should be taken into account

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Efficiency comparison PMSM losses Induction drive losses

Efficiency of the M2BA 132 SMB induction machine (which is in IE2 efficiency class) at nominal power 5.5 kW is 89 %. Plus additional losses in the motor caused by the frequency converter may decrease the machine efficiency by 2 %.

The efficiency of frequency converter controlled PMSM with ferrite magnets at nominal power 4.7 kW was measured to be 93,4 %

The performances of the machines at field weakening are not considered. Because, in the field weakening mode, the load torque should have a lower value with a higher speed. It is not the option of the blower drives as their torque rises in square with the speed

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Advantages of Integrated system

As it is seen in the figure, outer rotor construction has a liability to have fully integrated system with the blower and power electronics Factor Integration of power electronics

with the electrical machine Integration of a fan with the electrical machine

Separately composed system

Efficiency + + –

Accuracy + + –

Size + + –

Additional mounting + + –

Reliability + + –

Price depends on situation depends on situation depends on situation

Cable oscillations ± no effect –

Other electromagnetic compatibility (EMC) problems

± no effect –

Option of replacing parts of the construction

– – +

Operating in harsh environments – – +

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Conclusion

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Specification Induction machine PMSM

Number of slots per pole per phase, q

Integer Fractional (q ≤ 5)

End winding geometry

Overlapping winding, long end winding

Single tooth coil winding, short end winding

Efficiency Inherent efficiency limitation

Capability to reach very high efficiency

Coupling of fan blades

Additional mechanical units for coupling

Fan blades can be directly attached to the outer rotor (facilitating the use of the fully integrated system)

Power to volume ratio

Quite standardized value Higher than in the IM

Production process Well developed technological process

Slot tooth winding and permanent magnets located on the surface of the outer rotor may lead to the simplification of the production process

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Questions? Suggestions?

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Thank you for your attention

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References

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1. Juha Pyrhönen, Tapani Jokinen, and Valéria Hrabovcová, Design of Rotating Electrical Machines, New York : John Wiley & Sons, 2008. 2. ARPA-E Rare Earth and Critical Materials Workshop Breakout Session: Magnetics. Available at: http://arpa-e.energy.gov/Portals/0/Documents/ConferencesAndEvents/PastWorkshops/Breakout%20session%20-%20magnetics_lowres2.pdf 3. Ilya Petrov, Juha Pyrhönen, " Performance of low cost permanent magnet material in PM synchronous machines," Industrial Electronics, IEEE Transactions on, 2012 4. http://www.permanentmagnet.com/ceramic_magnet_ferrite_magnet.html 5. http://www.tdk.co.jp