Core-less motor

7/29/2019 Core-less motor

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A coreless motor

Saurabh Kumar,12304EN004M.Tech(MD) Sem-2,IIT BHU.


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Iron loss due to ferromagnetic core.

Torque pulsations due to non uniform magnetic field.

Moderate efficiency ( 70-80 %).

Low power density ( 131W/kg in IM and 222W/kg in SM)


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PM motors has core accompanying the armature which

encounter a very high frequency flux cutting (frequency

proportional to speed of motor), causing high eddy

current loss . (as in PMDC).

P= kf2 ( f=NP/120).

The maximum flux density encountered is around 0.6 Twhich is not sufficient to create a high power density

machine.


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YES, we can do this with the help of

Halbach array


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A Halbach array is a special arrangement of permanent magnets

that augments the magnetic field on one side of the array whilecancelling the field to near zero on the other side.

Fig1: Basic Process of Halbach Array generation.


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Fig2: Flux density plot of NdFeB- (52MGOe) magnet


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For rotary disc machine applications, this array can be

made in a circular form as shown below:

F ig3: Linear array Fig4: Circular disc array

The field decreases somewhat where the ends of the magnet

array join on itself, but this decrease is not more than 5%.


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The peak value of the magnetic flux density at the active

surface of Halbach array is given by :

For example: taking a medium grade NdFeB magnet ( Br=

1.25T, =262, nM

=4, hM=0.006m, we get Bmo = 0.9T which

is sufficient for a high power density machine.


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Fig5: Exploded view of disc type PMBM Fig6: Multidisc configuration


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The tangential torque developed can be given by the

following equation:

There is a flexibility in controlling maximum possible torque

by controlling Dout and Din. However there is a mechanicallimit up to which Dout can be increased. In this case we can

go for multidisc arrangement.


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Fig7: Speed torque characteristics similar to sync. motor


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Operation wise it is similar to sync. Motor, so speed

below base speed is achieved by frequency control.

Speed above the base speed can be achieved by flux

weakening.

Halbach discs give a static field. However discs can be

nested, and by rotating one disc relative to the other,

cancellation of the field and adjustment of the direction

can be achieved. As the outside field of a disc is quite low,the relative rotation does not require big forces.


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Fig8: Efficiency and PF as function of current

%=92.5,p.f.=0.94 (at rated current)


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Power density is 82% higher (444.25 W/kg versus 243.55

W/kg)


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High efficiency coreless disc-type PMBMs are excellent

machines for application where efficiency is the primary

demand and high power density is required.

Some applications are:

Airborne apparatus (lightweight construction), solar

powered vehicles, solar powered boats or even solarpowered aircrafts. Single disk motors are used for servo

drive systems.


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Some torque pulsations due to quasi sinusoidal flux very

near to the array.

Fig9: Magnet Arc Shaping

Magnet Arc Shaping Technique (MAST) can be applied to solve

the above-mentioned problem. In this technique, the Laplaces

equation is used to determine the magnet shapes for

producing the required flux distributions.


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The experimental results, analysis and the FEM simulationshow the advantages of coreless disk type PMBMs in terms

of performance and mass of active materials. A 10 kW coreless motor is about 45% lighter and has 82%

higher power density than its laminated counterpart.Owing to lack of ferromagnetic cores, the hysteresis and

eddy current losses do not exist. High cost of manufacturing limits commercial applications

of coreless PMBMs to small size machines, special servodrives, airborne apparatus (lightweight construction) and

drives where ripple free torque application is required.

The high cost of manufacturing is temporarilyagainst the mass production of coreless motors.


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[1.] J. C. Mallinson, One-sided Fluxes - A Magnetic Curiosity?, IEEE

TRANSACTIONS ON MAGNETICS, VOL. MAG-9, NO. 4, DECEMBER 1973, PP. 678-682. [2.] Richard F Post, Hallback Array DC Motor/ Generator, Patent U. S. 5705902,

Jan 6, 1998.

[3.] Jacek F. Gieras Advancements in Electrical Machinery, 1st ed., Springer,Illinois, 2008.

[4.]Jacek F. Gieras, Performance Analysis of a Coreless Permanent MagnetBrushless Motor Industry Applications Conference ,VOL 4., OCTOBER 2002, PP.2477-2482.

[5.] Jae-Seok Choi and Jeonghoon Yoo, Design of a Halbach Magnet Array Based onOptimization Techniques, IEEE TRANSACTIONS ON MAGNETICS, VOL.44, NO. 10,OCTOBER 2008, PP. 2361-2366.

[6.]Min-Fu Hsieh and Yu-Sheng Hsu, An Investigation on Influence of Magnet ArcShaping Upon Back Electromotive Force Waveforms for Design of Permanent-

Magnet Brushless Motors, IEEE TRANSACTIONS ON MAGNETICS, VOL.41, NO. 10,OCTOBER 2005, PP. 3949-3951.

[7.] Video, Helios aircraft( NASA), http://www.youtube.com/watch?v=TlFlRURUW3k


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Core-less motor