EVS28KINTEX, Korea, May 3-6, 2015

Rotating Transformer for a Wound Rotor Synchronous Motor

Jiyoung Lee1,2, Jongmoo Kim1, and Byoungchul Woo1

1Korea Electrotechnology Research Institute, Changwon, Korea2University of Science & Technology, Korea

jylee@keri.re.kr

Introduction

This paper presents a design of a rotary transformer to be used instead of brushes and slip-rings in 7.5kW-grade wound rotor synchronous motor for a propulsion system of electric vehicles.

The basic components of the rotary transformer are pot cores and adjacent windings, which are classified into two parts- primary and secondary as the principles of general transformers. And high frequency is used to reduce the overall volume.

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Introduction

There are five major design variables, which are (1) source frequency, (2) number of turns in primary winding, (3) inner diameter of winding window, (4) outer diameter of core, and (5) height of core. The design variables are optimized by Response Surface Methodology to efficiently transmit the required power. Factor experiments for the optimum design are performed by 2-dimensional axi-symmetry finite element analysis (FEA). In the analysis model for the FEA, the magnetic field is connected to external circuits. The primary circuit is external power source, and the secondary circuit is linked to field winding in the wound rotor synchronous motor. The designed rotary transformer is fabricated, and the characteristic are shown by both analytically and experimentally.

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Rotary Transformer for Exciter

Rotary Transformer for Exciter in Induction machine

[1] M.Ruviaro, F.Runcos, N.Sadowski, I.M.Borges, “Analysis and test results of a brushless doubly fed Induction machine with rotary transformer,” IEEE Trans. on Industrial Electronics 59(6), 2670-2677, 2012[2] http://www.scielo.br/scielo.php?pid=S2179-10742013000200013&script=sci_arttext#f1

Cutaway of a doubly-fed induction generator with a rotary transformer [1][2]

Laboratory scheme for load tests [2]

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Rotary Transformer for Exciter in Synchronous machine

InductionMachine

Rotor : AC field Rotor : DC field

SynchronousMachine

Rec

tifie

r

3ph-Rotary transformer 1~3ph-Rotary transformer

Rotary Transformer for Exciter

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Rotary transformer Configurations

Axial (left) and Pot core (right) rotating transformers [3][4]

[3] J.P.C.Smeets, L.Encica, E.A.Lomonova, “Comparison of winding topologies in a pot core rotating transformer,” IEEE Xplore, 2010[4] J.Legranger, G.Friedrich, S.Vivier, J.C.Mipo, “Comparison of two optimal rotary transformer deisgnsfor highly constrained applications,” IEEE Xplore, 2007

Winding topologies for the pot core rotating transformer, adjacent (left) and

coaxial (right) [3]

Conceptualconfigurationof objective model

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Design Specifications

Contents Values

Primary voltage 106.1 Vrms Primary circuit :Phase-shift DC-DC converterPrimary frequency 20~50 kHz

Secondary required voltage 66.6 Vrms

Secondary circuit :Full bridge rectifier

Secondary required current 3.0 Arms

Secondary power 200 W

Max rotating speed 10,000rpm

Diameter of axis 34 mm

Diameter of outmost housing 144 mm

Max axial length 78 mm

Air-gap length 1mm or less

Core material Mn-Zn soft ferrite

Cooling Natural cooling

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Analysis Model and Design Variables

Primary core

Secondary core

Primary coilSecondary coil

Axis of Sym

metry

Design variables(1) Cx2 (2) Cx4(3) Ch (=Ch1=Ch2)(4) Np (No. of turns in primary)(5) Freq (Primary frequency)

Axi-symmetric model for magnetic field analysis

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Analysis Model and Design Variables

External circuit

Independent variable 2Objective power:

200~300W

Independent variable 1

Independent variable 3

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Response Surface Methodology

è 43 DOE samples for 5 design variables

(DOE: design of experiment)

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Response Surface Methodology

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Response Surface Methodology

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Design Results

17

[Unit: mm]

20.537

39

6.5

6.5

3.2

air-gap = 0.5mm(Fill factor : 60% or less)

Axis of Sym

metry

[Variables]Freq=27.5 kHzNp = 40 turns ( Ns=26 turns)Cx1=17, Cx2=20.5, Cx4=39, Ch=6.5 (mm)

[Characteristics]CD1(current density of primary) =3.3 A/mm2

Crms1(current of primary) =2.8APin(power of primary) =231.7W Pload(power of secondary) =220.9WEff_sys(system efficiency) =95.3% Eff_mag(magnetic circuit efficiency) =98.5%

First RSM results

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Design Results

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17

21

42.5

10.5

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Axis of Sym

metry

[Unit: mm]

Second RSM resultsCore outer diameter 85mm

Core inner diameter 34mm

Housing inner diameter 32mm

Primary core height 10.5mm

Secondary core height 9mm

Air-gap 0.5mm

Core thickness 4mm

No. of turn in primary 28 turn

No. of turn in secondary 18 turn

Diameter of conductor 1.4 mm

Fill factor 40%

Primary frequency 30kHz

Power of sencondary 226 W

Efficiency 95.2 %

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Magnetic Characteristics

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Fabrications

(Primary & Secondary Ferrite core)

(Core + Coil)

(Rotary transformer + Power converter)

(Primary & Secondary coils)

(Core + Coil + Housing)

(Core + Coil + Housing à Epoxy molding)

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Experiments

Secondary power: 300W_max