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Design Considerations in Induction Motors for Ship Thruster Propulsion

Konstantina Nikolaou, Minos Beniakar, Charalampos Patsios and Antonios Kladas National Technical University of Athens, School of Electrical and Computer Engineering, Laboratory of Electrical Machines and Power Electronics, Zografou, Athens, Greece 1


Contents Design Considerations Application Details Application Specifications Alternative Topologies Stator Design Rotor Design Sensitivity Analysis Results Final Geometry Selection Equivalent Circuit Summary

Slide 3 Slide 4 Slide 5 Slide 6 Slide 7 Slide 8 Slide 9 Slide 10-22 Slide 23-24 Slide 25 Slide 26 2


Design considerations Basic Dimensioning using analytical formulasConsideration of space limitations and overload conditions

Winding configuration – Slot/pole combinations Sensitivity Analysis – Performance and efficiency indexes

Extraction of equivalent circuit parameters – Design validation

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Application details  In large size ships, the main propulsion system is assisted by auxiliary propulsion equipment composed of thruster propellers, used for low-speed, transversal, docking and station keeping maneuvers.  Thruster motors are typically induction machines connected to the on board grid.  The auxiliary propulsion systems are usually placed to the bow of the vessel.  For large ships and where enhanced maneuverability is required they are often fitted in pairs.

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Application specifications In the case examined the thruster propulsion system consists of two pairs of squirrel cage Induction motor thrusters of 148kW nominal power each. Dimensions and Specifications Voltage 400 V Frequency 50 Hz 282 A Nominal Current (In) Starting Current 5,8*In Nominal Torque (Tn) 720 Âą 2% Nm Maximum Torque 1,4*Tn Starting Torque 1,3*Tn Nominal Speed 1491 rpm Cooling Method IC 411 Pole number 4 Stator/rotor slot number 24/34 Stator/rotor outer diameter 300/175 mm 5


Alternative topologies Induction motors classification according to NEMA

Class

Starting Torque

Starting Current

Slip

Usual Applications

Α

normal

normal

normal

Funs, pumps (low inertia loads)

Î’

normal

lower

normal

Large funs and pumps (high inertia loads)

C

high

low

normal

compressors, conveyors

high

presses, mechanical drilling (high inertia applications)

D

high

low

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Stator design Selection of number of rotor slots: Ratio to stator slot number is important to avoid harmful synchronous torques and mechanical vibrations.

Qs 24

p=4 Acceptable number of rotor slots Qr 18, 30, 34, 38 Stator teeth height:

K is a coefficient valued between 2 and 6, D is the machine diameter, Q is the number of stator slots ,Bg is the desired value of the air gap flux density and Bts is the desired value of the flux density in the stator teeth area. 7


Rotor Design Class B

Class C

Class D

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Sensitivity Analysis Determination of the 2 parameters to be checked Determination of their minimum and maximum value and the number of steps between them From the minimum until the maximum value of the 2 parameters do:

Calculate stator and rotor Joule losses

Design the motor

Estimate efficiency

Analyze the motor/ Calculate the torque Calculate the magnetic flux linkage in stator and rotor teeth and body Calculate the current density in stator and rotor slots

Estimate iron losses

Have all the values of the two parameters been checked?

No

Yes

Extract 3D plots for the above measured quantinties 9


Results – Class B

 Flux density distribution at maximum torque before and after the sensitivity analysis.  The initial design resulted in an uneven distribution of the flux density. The stator iron was charged beyond the desired value of 1.8T.  After the sensitivity analysis the flux is more evenly distributed. 10


Results – Class B

ď ą Current density distribution at maximum torque before and after the sensitivity analysis. ď ą The change in the height and width of the stator and rotor slots can easily be observed. The stator teeth are widened and shortened. The rotor teeth are wider and slightly longer after the sensitivity analysis. 11


Results – Class B

Percentage of pole pitch (%)

Βst (T)

Stator Tooth/ Rotor Tooth 20 22 23 24 25

Brt (T)

Torque (Nm)

13 1.6742 1.637 1.6217 1.6093 1.6246

1.7845 1.7982 1.7843 1.7754 1.7762

1005.5 2003.6 1035 1074.5 1037.5

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Results – Class B The maximum torque increased by 23 % from 826 Nm and is 1070 Nm. The nominal slip has dropped from 0.0037 to 0.003.

Dimensions and Performance Indexes

Initial Design

Final Design

Nominal Torque Maximum Torque Nominal Slip Nominal Speed Stator teeth width Stator teeth height Rotor teeth width Rotor teeth height

530 Nm 826 Nm 0.0037 1494.45 rpm 18.7 mm 59.4 mm 17.1 mm 51.1 mm

763 Nm 1070 Nm 0.003 1495.5 rpm 32.0 mm 45.8 mm 18.1 mm 51.2 mm

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Results – Class C

 Flux density distribution at maximum torque before and after the sensitivity analysis.  The initial design resulted in an uneven distribution of the flux density. The stator iron was charged beyond the desired value of 1.8T.  After the sensitivity analysis the flux is more evenly distributed. 14


Results – Class C

ď ą Current density distribution at maximum torque before and after the sensitivity analysis. ď ą The change in the height and width of the stator and rotor slots can easily be observed. The stator teeth are widened and shortened. The rotor teeth are wider around both slots (top and bottom) after the sensitivity analysis.

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Results – Class C

Percentage of the pole pitch (%)

Βst (T)

Stator tooth/ Rotor slot 21 22 23 24 25

Brt (T)

Torque(Nm)

16 1.7655 1.7294 1.7039 1.6875 1.6986

1.5415 1.5391 1.5398 1.534 1.5271

1024.4 1043 1062.7 1083.7 1063.3

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Results – Class C The maximum torque increased by 26 % from 792 Nm and is 1073 Nm. The nominal slip has dropped from 0.0044 to 0.0038.

Dimensions and Performance Indexes Nominal Torque Maximum Torque Nominal Slip Nominal Speed Stator teeth width Stator teeth height Rotor top slot diameter Rotor teeth (bottom slot) width Rotor teeth (bottom slot) height

Initial Design 524 Nm 792 Nm 0.0044 1493.4 rpm 18.7 mm 59.4 mm 16.1 mm 19.3 mm 34.6 mm

Final Design 814 Nm 1073 Nm 0.0038 1494.3 rpm 32.0 mm 45.8 mm 11.1 mm 19.5 mm 32 mm

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Results – Class D

 Flux density distribution at maximum torque before and after the sensitivity analysis.  The initial design resulted in an uneven distribution of the flux density. The stator iron was charged beyond the desired value of 1.8T.  After the sensitivity analysis the flux is more evenly distributed. 18


Results – Class D Rotor teeth flux density as function of the stator teeth width and height

Stator and rotor iron losses as a function of the stator teeth width and height 19


Results – Class D

ď ą Current density distribution at maximum torque before and after the sensitivity analysis. ď ą The change in the height and width of the stator and rotor slots can easily be observed. The stator teeth are widened and shortened. The rotor teeth are wider after the sensitivity analysis. 20


Results – Class D

Percentage of the pole pitch (%)

Βst (T)

Stator tooth/ Rotor slot 20 23 24 25

Jrt_t (A/mm2)

Torque(Nm)

16 1.4878 1.4176 1.4218 1.4539

6.3968 6.4903 6.5001 6.4937

1005.9 1003.5 1000.9 1007.6

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Results – Class D The maximum torque increased by 19 % from 826 Nm and is 1020 Nm. The nominal slip has the same value of 0.006, which meets the speed requirements.

Dimensions and Performance Indexes

Initial Design

Final Design

Nominal Torque Maximum Torque Nominal Slip Nominal Speed Stator teeth width Stator teeth height Rotor slot diameter

537 Nm 826 Nm 0.012 1491 rpm 18.7 mm 59.4 mm 19.2 mm

749 Nm 1019 Nm 0.012 1491 rpm 32.0 mm 45.8 mm 19.5 mm

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Torque - Slip Results

 ΝΕΜΑ design C motor has higher torque capabilities.  NEMA design D motor has higher slip at maximum torque. 23


Starting Torque Results- Final Selection

ď ą Only NEMA design D motor is able to produce starting torque 1,3*Tn at starting current 5,8*In 24


Equivalent Circuit ď ą The deviation between the two curves is due to the use of a simple equivalent circuit which does not take into consideration the skewing of the rotor slots and the endrings. ď ą The estimation of the parameters is done using the least-squares method.

Lm= 0.005449 H R2= 0.01657504 Ohm L1= 0.00151579 H 25


Summary  An optimization procedure comprising an extended sensitivity analysis concerning the shape of both stator and rotor teeth of an IM was proposed.  The procedure was applied to NEMA designs B, C and D to determine the optimal design for an induction motor used in ship thruster propulsion.  The final design was acquired after a series of analyses using the finite element method.  The results were validated using a simple per-phase motor equivalent circuit.  The design procedure proved to be suitable and adequately efficient.  The final design fully aligns with the defined restrictions and specifications of the application. 26


ACKNOWLEDGMENT THE

WORK PRESENTED IN THIS PAPER HAS BEEN DEVELOPED WITHIN THE

THALES-DEFKALION PROJECT. THIS RESEARCH HAS BEEN COFINANCED BY THE EUROPEAN UNION (EUROPEAN SOCIAL FUND – ESF) AND GREEK NATIONAL FUNDS THROUGH THE OPERATIONAL PROGRAM "EDUCATION AND LIFELONG LEARNING" OF THE NATIONAL STRATEGIC REFERENCE FRAMEWORK (NSRF) - RESEARCH FUNDING PROGRAM: THALES: REINFORCEMENT OF THE INTERDISCIPLINARY AND/OR INTER-INSTITUTIONAL RESEARCH AND INNOVATION.

FRAMEWORK OF THE


Thank you for your attention

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1. Design Considerations in Induction Motors for Ship Thruster Propulsion  
1. Design Considerations in Induction Motors for Ship Thruster Propulsion  
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