PMSM at the Cryogenic Temperature

Liping Zheng 11/03/2003 University of Central Florida

Cryogenics

 

The properties of most materials change significantly with the temperature.



Cryogenics is generally defined when temperature is less than 120K.

Many materials are unsuitable for cryogenic applications.



Our motor will work at 77K (liquid nitrogen).

Previous PMSM Design

Stator : Laminated Silicon Steel Permanent magnet: NdFeB Litz-wire: 1.78 mm x 2.27 mm 50 strands @ AWG 30 Gap : 0.5 mm Stator Di: 25.5 mm Do: 38 mm Length: 25.4 mm Shaft diameter: 16 mm

Some Considerations



The PMSM need to operate at both room temperature and 77K. We consider:

    Thermal stress PM stability Winding loss Stator core loss 

Some modifications will be made after the above consideration.

Permanent Magnet

 

NdFeB (neodymium -iron-boron ) SmCo (samarium cobalt)

  SmCo does quite well at cryogenic temperature.

NdFeB does well above 135K (-138 ºC). But it undergoes a spin reorientation below 135K.

Stanley R. Trout, “Using permanent magnets at low temperature,” Arnold TECHNotes.

Magnet Properties

Material Density Compressive Stress Thermal Conductivity Coefficient of Thermal Expansion Specific Heat Electrical Resistivity Temp. Coefficient of Br Temp. Coefficient of Hc g/cm 3 Mpa W/(m.K) // 10 -6 /K I 10 -6 /K J/(kg.K) µ

W

.cm

%/K %/K SmCo 8.4

700~1000 10.5

11 8 360 60~90 -0.035

-0.047

NdFeB 7.5

1100 9 3 -5 420 150 -0.11

-0.55

Thermal Expansion of Titanium: 4.8~5.6 x 10-6 /K Stainless steel: 8.9~9.6 x 10-6 /K

Copper Loss- DC



DC resistance

 Electrical resistivity reduces with temperature due to reduced phonon electron scattering.



Residue resistivity ratio (RRR)

 Resistivity at 300K (room temperature) / resistivity at 4.2K (liquid helium).  The value showing the purity of a sample.



Copper loss

P copper



I

2

R wire R wire

 

L wire S

Electrical Resistivity of Cu

  1 .

7  10  8 W 

m

@ 300K   0 .

2  10  8 W 

m

@ 77K

Copper Loss - AC



Skin effect can still be ignored

 Room temperature (300K)   5 .

8  10 7 (

S

)   2   1 .

1 (

mm

)  Liquid nitrogen (77K)   5  10 8 (

S

)   2   0 .

37 (

mm

)   Litz-wire : 50 @ AWG 30 (D=0.01 in or 0.25 mm)

Proximity effect due to rotating flux can also be ignored because of the smaller size Litz-wire.

Stator Iron Loss



The iron loss for electrical steel:

P loss



k h

2

B

max

f



K c

(

B

max

f

) 2 

K e

(

B

max

f

) 1 .

5  Where

K h K c

is the hysteresis coefficient is the classical eddy coefficient

K e

is the excess eddy current coefficient.

F

is the frequency.

Eddy current losses are proportional to electrical conductivity.

 At low temperature, stator core loss will increase due to increased electrical conductivity.

Modified PMSM

    Permanent magnet: NdFeB -> SmCo Wire size: 50 -> 20strands@AWG30 Winding structure: 5 ->6 turns/phase/pole Gap length: 0.5 -> 1.0 mm  Shaft thickness: 0.5mm -> 1.5mm

Simulated Flux

Flux Density Distribution Flux Lines

Airgap Normal Flux

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0 20 40 60 80 100 Angle (deg) 120 140 160 180

Simulated Core Loss

Simulated Torque

Loss Estimation

Copper Loss Stator Iron Loss Rotor Loss Windage Loss Filter Loss Bearing Loss Total Loss

Motor Efficiency: Control Efficiency: Total Efficiency:

Unit W W W W W W R. T 75.5

4.6

3.8

8.4 11 10 77K 7.2

36 1.0 W 114.3



m

 2000 ( 2000 

c

   95 % 

c



m

 

73.6

73 .

6 )  91 .

6 % 96 .

5 %