Transformer and Inductor Design Handbook Chapter 19

8/12/2019 Transformer and Inductor Design Handbook Chapter 19

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Ch apter 19

RotaryTransform er Design

yright 2004 by Marcel Dekker, Inc. All Rights Reserved.


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TableofContents

1. Introduction2. Basic Rotary Transformer3. Square Wave Technology4. Rotary Transformer Leakage Inductance5. Current-fed Sine Wave Converter Approach6. Rotary Transformer Design Constraints7. References



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IntroductionThere are many requirements to transfer signals an d power across rotary interfaces. Most things that us eslip rings or brushes can be replaced with a rotary transforme r. Science instruments, antennas an d solararrays are elements needing rotary power transfer for certain spacecraft (S/C) configurations, such as aspin, stabilized (S/C). Delivery of signals an d power ha s mainly been done by slip rings. There ar eproblems in using slip rings for long life and high reliabili ty: contact wear, noise, and contamination.Contact wear will lead to a condu ctive path to ground. This conduc tive path will generate noise and upsetthe original designed comm on-mode noise rejection. A simple slip ring assem bly and a rotary transformerare shown inFigure 19-1. High data rates an dpoor slip ring life forced th e Galileo (S/C) to replace th esignal interface with rotary transform ers. The use of a rotary transfor me r to transfer pow er on the Galileo(S/C) was contemplated, but it was thought th e impact on the (S/C) delivery was too great. The rotarytransformers on the Galileo (S/C) lasted th elifeof thespacecraft, from 1989to2003withoutaglitch.

Slip Rings Rotary TransformerConductive/TVTVTVTA PowerOutRingsPower OutRotor

StatorRotorThrough-Bore

Brushes PowerInThrough-BorePower In

Figure 19 1. Comparing aSlip Ring Assemblyand aRotary Transform er.Existing approaches to rotary power transfer use square wave converter technology. However, there areproblems caused by the inherent gap in a rotary transformer, coupled with the fast rate of change in thesquare wave voltage. Undue stress isplacedon thepower electronicsand theinterface becomes asourceofElectromagnetic Interference (EMI) that impacts th eoverallsystem'soperating integrity.

BasicRotary TransformerThe rotary transformer is essentially the same as a conventional transformer, except that the geometry isarranged so that th e primary an d secondary can be rotated, with respect to each other with negligiblechanges in the electrical characteristics. The m ost common of the rotary transform ers are the axial rotary



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transformer, shown inFigure 19-2,and the flat plane, (potcore type), rotary transformer, shown in Figure19-3. The power transfer is accomplished, electro-magnetically, across an airgap. There are no wearingcontacts, noise, orcontamination problems due tolubricationorwear debris.

Primary Winding Secondary Winding

Stator

PowerI n\ Outer BearingFlux Path

V

RotaryPlatform

Figure19-2. Pictorialof anAxial, type Rotary Transformer.Primary Winding

Power InFlux Path

Bearing

Ga p

RotaryPlatform

Stator Power OutSecondary Winding

Figure19 3. Pictorial of aFlat Plane, type Rotary Transformer.

Square Wave TechnologyThe ideal converter transformer would have a typical square B-H loop, as shown in Figure 19-4. Aconverter transformer is normally designed tohave aminimum of leakage inductance. The voltage spikesthat are normally seen on the primary of a square wave converter transformer ar e caused by the leakageinductance. To design a converter transformer to have aminimum of leakage inductance, th e primary an dsecondary must have a minimum of distance betwee n them. Minimizing th e leakage inductance will



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reduce th e need fo r power-wasting, snubber circuits. Although there ar e rotary power transformersdesigned withthe use ofsquare wave converter technology, theyarenot, without problems.

B tesla)

H oersted)

Figure 19 4.Typical, Transformer BHLoop.There are two basic problems not found in the normal transformer: (1) the inherent gap in a rotarytransformeris oneproblem,and (2) therequired spacing between primaryandsecondary that leadstolargeleakage inductanceis theother. These problems, along with asquare wave drive, ar ewhat leads to ahighloss, snubber circuit, and beccme a source of Electromagnetic Interference EMI) that impacts theadjoining systems operating integrity. The rotary transformer, becauseof its inherent gap,has a B-H loopsimilar to an inductor, as shown in Figure 19-5. Basically, the transformer t ransforms power, and theinductor stores energy in the gap. The rotary transformer does not have any of the traits of an idealtransformer. It is, more accurately,a trans-inductor havinga gap and a secondary, spaced away from theprimary.

B tesla)

H(oersted)Stored Energy

Figure 19 5.Typical, Rotary TransformerBHLoop.



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RotaryTransformer Leakage InductanceT he rotary transformer has aninherentgap andspacingof thepr imarya ndsecondary. The gap andspacingin the rotary transformer resu lt in a low primary magnetizing inductance. This low primary inductanceleads to a high magnetizing current. The leakage inductance, Lp, can be calculatedf orboth axial an d flatplane using Equation 19-1. The axial rotary transform er winding dimensions are shown in Figure 19-6.The flatplane rotary transformer winding dimensions ar eshowninFigure19-7.

47i MLT N;c - (l(T9), [henrys] [19-1]

^tntnr ^>

T ? ntnr

Through -B ore

a ,,55 ^ ^g ^ ^s^^^s^

_

b l-1+- 1

Ib2a = Winding length, cmb = Winding build, cmc = Space between windings,cm

Figure 19 6. Axial Rotary Transformer, Showing Winding Dimensions.

PrimarySecondary

Through-Bore

Statora = Winding length, cmb = Winding build, cmc = Space between winding s, cm

Air GapFigure 19 7.Flat Plane Rotary Transformer, Showing W inding Dimen sions.



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Current fed Sine Wave Converter ApproachThe current-fed, sine wave converter topology is a good candidate to power th e rotary transformer. Thedesign would be a current-fed, push-pull, tuned tank converter requiring a gapped transformer. Acomparison between a standard, square wave converter, shown in Figure 19-8,and acurrent-fed, sine waveconverter, is shown in Figure 19-9. Using the rotary transformer in this topology, the energ y that is storedin the rotary gap that causes so much trouble in the standard square wave driving a rotary transformer, isrecovered an d is used in the tank circuit. There would not be any need of power-was ting snubbers usingtherotary transformer approach. Se eChapter 18.

CR1 L In4

6 2

C R 2

C2_ C R 3 k s 1

fSnubber CircuitFigure 19 8.Typical, Voltage-fed, Square wave Converter Circuit with Snubbers.

L I 2 1 Tuned Tank Circuit-L ,

CR3

C2C R 4 v

O

Figure 19 9. Typical,Current-fed, Resonant Converter Circuit.Thecurrent-fed sine wave converter requires aresonant , LC, tank circuit to operate properly. The primaryof the rotary transform er would be the ideal inductor, because of the inherent gap of the rotary transform er.There are several advantag es to incorporating the resonant tank circuit into the rotary transformer. First, itminimizes th enumber of components in thepower stage. Secondly, the output of the inverter is a naturalsine wave,asshown inFigure 19-10,an dusually requires no additional filtering. Thirdly, energy stored inthe gap of the transformer isreleasedwhen either power sw itch is turned o ff. This energ y is com mutated inthe resonant tank circuit. Thisprovides the capa bili ty for direct exchange of power b etween the tank circuit



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and the load. There is not a noticeable drive torque in a rotary transformer. The tuning or tank capacitormustbe ofhighquality, stable,a ndwith lo wESR.

Voltage Across Secondary

Horizontal =10 usec/cm igure19 10. Current-fed Converter, Secondary SineWaveSecondary Voltage.

RotaryTransformer Design Con straintsT he rotary transformer requirements pose some unusual design constraints compared to the usualtransformer design. The first is the relatively large gap in the mag netic circuit. This gap size depends onthe eccentric dimension and the tolerance of the rotating shaft. The gap results in a low primarymagnetizing inductance. Secondly, th e large space separating primary an d secondary windings results inan unusually highprimary-to-secondary leakage inductance. Thirdly, th e large through-bore requirementresults in an inefficient utilization of the core material and copper, due to the fixed mean-length turn. Thislarge diameter results in requiring more copper area for the same regulation. Finally, the core has to bemore robust thanthenormal t ransformer because of thestructural requirement. Se e Figure 19-11.

Stator PrimarySecondaryRotor

Air Gapt

Winding Separation

PrimaryStator

Through-Bore

Air Gap

SecondaryRotor

Through-Bore

Axia l Flat Face-to-FaceFigure 19 11. Geometries of theBasic Type R otary Transformers.



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Rotary transformer dimensions ar eusually governedby the mechanical interface,inparticular th erelativelylargegap and the large through-bore, resultingin a long Mean Length Turn (MLT). T he rotary transformeris not an ideal magnetic assembly. A toroidal core is an ideal magnetic assembly. Manufacturersuse testdata, taken from toroidal cores, to present magn etic material characteristics. The mag netic flux in atoroidal core travels through aconstant core cross-section,Ac,throughoutthe whole Magnetic Path Length,MPL, asshown in Figure 19-12, and provides ideal magn etic characteristics. It can be seen that the corecross-section throughout th e rotary transformers, shown inFigure 19-13a ndFigure 19-14,does notprovideconstant flux density or an ideal magn etic assembly. The rotary transform ers for the Galileo spacecraftwere about 10 cm in diameter, and manu factured byCM I (Ref 4.)

Magnetic Path Length, M PLIron Area,Ac

Perspective V iewFigure 19 12. Typical Perspective Viewof aToroidal Core.

Magnetic Path Length A View Stator Rotor

A View Primary I Secondary Perspective View

Figure 19 13. Open Viewof a Flat Plane, Type Rotary Transformers.Magnetic Path Length AView, Stator

Prim ary SecondaryA View I Perspective ViewFigure 19 14. Open Viewof anAxia lTypeRotary Transformers.



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References1. E. Landsman, Rotary Transform erDesign. Massachusetts Institute Technology,PCSC-70Record,

pp. 139-1522. L. Brow n, Rotary Transform er Utilization in a Spin StabilizedSpacecraft Powe r System. General

Electric,pp373-376.3. S.Marx, AKilowatt Rotary Power Transformer. Philco-FordCorp., IEEE Transactions on

Aerospaceand Electronic Systems V ol. AES-7, No. 6 Novem ber 1971.4. Ceramic Mag netics, Inc. 16Law D riveFairfield, NJ 07006. Tel. (973) 227-4222.

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Transformer and Inductor Design Handbook Chapter 19