PMSM at the Cryogenic Temperature

Liping Zheng

11/03/2003

University of Central Florida

Cryogenics

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

The properties of most materials change significantly with the temperature.

Many materials are unsuitable for cryogenic applications.

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

Previous PMSM Design

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

Stator : Laminated Silicon Steel

Permanent magnet: NdFeB

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)

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

SmCo does quite well at cryogenic temperature.

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

Magnet Properties

Material SmCo NdFeB

Density g/cm3 8.4 7.5

Compressive Stress Mpa 700~1000 1100

Thermal Conductivity W/(m.K) 10.5 9

Coefficient of Thermal Expansion

// 10-6 /K

I 10-6 /K

11

8

3

-5

Specific Heat J/(kg.K) 360 420

Electrical Resistivity µ.cm 60~90 150

Temp. Coefficient of Br %/K -0.035 -0.11

Temp. Coefficient of Hc %/K -0.047 -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

wirecopper RIP 2

S

LR wirewire

Electrical Resistivity of Cu

m 8107.1

m 8102.0

@ 300K

@ 77K

Copper Loss - AC

Skin effect can still be ignored Room temperature (300K)

Liquid nitrogen (77K)

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.

)(1.12

mm

)(108.5 7 S

)(37.02

mm

)(105 8 S

Stator Iron Loss

The iron loss for electrical steel:

Where Kh is the hysteresis coefficient

Kc is the classical eddy coefficient

Ke 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.

5.1max

2max

2max )()( fBKfBKfBkP echloss

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 20 40 60 80 100 120 140 160 1800.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Flu

x D

ensi

ty (

T)

Angle (deg)

Simulated Core Loss

Simulated Torque

Loss Estimation

Unit R. T 77K

Copper Loss W 75.5 7.2

Stator Iron Loss W 4.6 36

Rotor Loss W 3.8 1.0

Windage Loss W 8.4

Filter Loss W 11

Bearing Loss W 10

Total Loss W 114.3 73.6

%5.96)6.732000(2000 m

%95c%6.91 mc

Motor Efficiency:

Control Efficiency:

Total Efficiency: