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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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