High Frequency Link Axial Flux Exciter

Notebook sketch of the axial flux transformer, showing cross section of windings.

I worked on this at OSU in 1995, as a brand new grad student. I thought I really had something. I, like all OSU power electronics grad students of that era, diligently studied the Brushless Doubly Fed Machine,(BDFM,) in all of its wonders and glory. This study began with an indoctrination that brushes (sliding electrical contacts) were completely disastrous for a number of reasons. Which is more or less true, in a greater or lesser degree, depending on application. With the evils of brushes firmly in mind, I was thinking a lot about brushless machines.

Hoping for a patent with all the fame and fortune, I brought my invention to the attention of the OSU Technology Transfer Office. I meticulously documented my work, built the thing up, and put on what I thought was a pretty good demo on Sept 11, 1995, witnessed by Don Amort and a few others.

An agent from the technology transfer office was assigned to work with me and do a prior art search. This was before USPTO was online. Unfortunately, our search turned up something pretty similar in US Pat. # 4612486, applied to a brushless motor. So we dropped it. I have a letter from them saying I don't have to assign them the invention. Theoretically, I could have pursued it myself.. but it really did look like I was "dependent" on the patent we found. Looking back, I am sure a good patent atty could have found many patentable improvements made by my invention over the existing art. Whether it would've been worth it, who knows? Anyways, 4612486 is dated Sept 16, 1986. So I suppose this technology is free and clear now. But Watch Out! Bosch seemed to think something a lot like this was worth patenting in May 2000, see US Patent 6333581 (from a 1998 German patent.)

I'd consider my demo a public disclosure, (Not sure Bosch would agree..) so my rights to patent what's here ran out a long time ago. If anyone actually applies any of it, and my work here has been helpful, I'd appreciate getting some credit. Heck, maybe you might want to pay me to make it work in your application! I think there are areas where this technology could be advantageous. Of course, rare earth magnets have got a lot cheaper since 1995.. so it's a lot less expensive to build a permanent magnet BLDC motor or generator... one of my main advantages was not requiring expensive field magnets.

I've decided to just put this whole thing in HTML. It's not that long, and I can type fast. That way it'll be searchable. The original paper description that went along with the disclosure form to OSU will be scanned and available as a PDF. The original has signatures of witnesses to my 1995 Demo. Dr. Alan Wallace (may his memory be a blessing,) Dr. René Spée, and Don Amort on some of these pages. I rescanned figures from my old lab notebook in this HTML document to get them in color.. and hopefully with a little better clarity than whatever photocopier I had access to in 1995 could do.

I couldn't find the original annotated photographs shown in the original description. Maybe the original was filed with OSU technology transfer office. I may try to dig around for negatives. I have photos close to these in my lab book, I scanned some here. In the original description, the figures and photos are attached at the end... in the web version here, I've stuck them where they're first referenced.

One funny thing that's not going to be readily apparent from the original description text. I built this with a spring loaded self-aligning plain bearing system. (which I thought was clever, and probably would've been patentable in this application) I did this partly because it was very cheap and simple, but mainly I did not have machining capability to align a separate bearing to support and align the transformer. The trouble was, what to use as bearing material between the two extremely hard ferrite core halves. I wound up with 10 mil Teflon sheet, which wore badly after a few running hours (although never actually failed.) I searched long and hard for the ideal bearing material. It turned out that graphite loaded plastics would've been ideal. Well, except for the fact that they'd be conductive! And, if you had sliding graphite contacts, you might have well have used them for brushes and slip rings!!!! Today I would build this with a decent ball bearing supporting the outside of the cores... not try to self-align with plain bearings like this. I did do drawings of proposed bearing systems back in '95, I just couldn't build them back then. It may be that some sort of greasy ceramic would work great for bearing material between the core-halves.. it certainly doesn't need much pressure. I am sure that slight beveling the surface edges on the pot core would have made massive improvements. (Pot core surfaces are ground perfectly flat, and have slots for wire lead-out holes. It's really a great tool for wearing out plain thrust bearings!

So.. without further ado, an oldy but goody project from back in the day, here is:

Description of
High Frequency Magnetic Link Brushless Excitation System for Synchronous Machine.

By Alex Faveluke, Oregon State University
09/20/95

Synchronous electrical machines are used as generators and motors in many applications. Synchronous machines operate on the principle of Faraday's Law. Faraday's Law states that a voltage is induced around a path if this path encloses a changing magnetic field. In the operation of most synchronous machines, magnetic flux is created by passing current through a "field winding" that electromagnetically creates two or more "poles" on the machine "rotor." Magnetic flux emanates from these poles. This rotor is rotated within a "stator" which holds coils of wire called the "stator winding" or "AC winding." When the rotor is turned the direction and amount of flux at any fixed location changes. The stator winding coils, being fixed, encircle changing flux, and by Faraday's law, electrical voltage is magnetically induced in these coils. AC power may be taken from these coils. In practical machines, iron is used to help focus and increase the amount of magnetic flux to useful levels.

Small synchronous electrical machines today rely on sliding electrical contacts between brushes on the stator and slip rings on the rotor to transmit power to the rotor field windings.

This brush and slip ring electrical contact is undesirable for many reasons. First, brushes wear down and must be replaced periodically on machines in medium to continuous duty use, causing down time and maintenance expense. The brush/slip ring contact is prone to contamination; grease, oil, or dust can cause the contact to fail, causing loss of power. The sliding electrical contact often sparks, rendering these machines unfit for service in explosive or hazardous environments.

The High Frequency Magnetic Link Brushless Excitation System provides a way to transmit power to synchronous machine rotor field windings without using any sliding electrical contacts. The invention consists of a high frequency switching power converter and a small transformer, wound in a commercially available ferrite part called a "pot core." One half of the pot core is attached to the stator, and does not rotate, while the other half is attached to the rotor and rotates with the machine shaft. The halves are centered and aligned with each other on a non-magnetic shaft (the prototype uses Delrin *, and separated by a thin disc of Teflon * or other non-conductive plain bearing material (See Figure 1.) The "secondary winding" is the power output winding and is wound in the rotor attached pot core half. The power converter changes electricity from a direct current source (such as a battery) to a high frequency alternating electrical current suitable for input into the transformer primary winding. A rectifier circuit is mounted on the rotor, and converts the high frequency electrical power from the transformer secondary winding into DC electrical current which flows in the rotor mounted field winding. The only means of energy transfer to the rotor is by the alternating magnetic field in this transformer, this is why the system uses no brushes or slip rings. Generator regulation is readily available by pulse width modulating the high frequency alternating current to the transformer primary. (See Figure 2 for electrical overview.)

Figure 1. Copy of notebook page 32. This is a basic drawing showing the arrangement of the transformer.

Figure 2. Copy of notebook page 48. This is a good basic explanation of the electrical operation of the system.

There are several alternatives available to a conventional brush fed field synchronous machine.

First, wherever possible, induction machines are used. Induction machines operate by inducing currents in short circuited rotor windings or heavy bars. Induction machines are especially suitable for use as motors. As generators, they must be connected to an existing outside alternating current system to create power, and the shaft must be driven past a certain speed, depending on the outside system.

Rotor mounted permanent magnets are used to create the field in some synchronous machines. This is the technique used in small, high performance motors called "brushless DC servomotors." Permanent magnet machines can be used for generator systems, but have the serious disadvantage of a non-controllable field . This means that the output voltage is dependent on the shaft speed, and there is no way to regulate the output voltage in a machine that must run at different speeds (such as an automobile alternator or a generator for use with wind applications.)

The Brushless Doubly Fed Machine being worked on here at Oregon State University is somewhat of a cross between an induction and a synchronous machine. In it there is an auxiliary stator winding called the "control winding" that is fed with a variable frequency power converter. Control of the Brushless Doubly Fed Machine requires relation of the main power winding electrical frequency, the control winding frequency and the shaft speed. All power to the rotor field is magnetically coupled to the control and power stator windings.

Large synchronous machines often use a "brushless exciter system" that is based around an auxiliary generator mounted on the machine shaft. The field for this auxiliary generator is created from stationary poles with DC windings. The current from the armature winding of this auxiliary generator is rectified by a shaft mounted diode bridge and is passed to the field windings of the main machine. The output voltage of the main machine is controlled by changing the amount of current in the auxiliary generator field poles, thus changing its output voltage and how much current it delivers to the main machine field.

Induction Machine:
This High Frequency Magnetic Link Brushless Excitation System concerns excitation for synchronous machines. The field power is transmitted to the rotor magnetically, but it is done in an auxiliary, high frequency, axial mounted transformer. In an induction machine, the field is set up by the stator currents generating flux, which induces current directly into the field coils or bars. In the invention, the field is independently controlled from the main power winding. This makes it much more suitable as a generator because the main power winding does not have to be connected to a pre-existing alternation current system to generate power.

Permanent Magnet Machine:
The invention allows control over the level of field magnetization. The invention drives a conventional synchronous machine with an electrical winding to supply adjustable field magnetization. Permanent Magnet Machines have a fixed field excitation.

Brushless Doubly Fed Machine:
The invention uses an auxiliary, axial mounted transformer with high frequency currents. The energy transmission through this transformer is not dependent on the shaft speed of the machine. The invention is basically a direct drop-in replacement for the conventional synchronous machine slip rings and brushes system. The Brushless Doubly Fed Machine does not contain an axial mounted transformer; all power is induced to the rotor from stator mounted windings. The Brushless Doubly Fed Machine must have a frequency controlled inverter to drive the control winding. There is no need for accurate frequency and timing control in the High Frequency Magnetic Link system.

Large Generator Brushless Exciter Systems
The invention uses an axial mounted transformer with high frequency currents. The brushless exciter systems in present use an auxiliary generator, with stationary, stator mounted field poles. These systems are designed for operation at a set speed as the excitation is dependent on shaft speed. The invention gives complete control over the field current over the full speed range of the machine.

This system was prototyped and tested using a modified automotive alternator. This alternator was a rebuilt machine sold as a replacement part for a 1976 Dodge truck with a 318 cubic inch engine.

The stator circuits and windings were not modified. The stator circuit of this alternator consists of a three phase winding and a three phase diode bridge. The rotor field coil and the rotor pole structure were also left unmodified. (See Figure 2.)

The stator mounted brushes were removed. The rotor mounted slip ring assembly was also completely removed. Two Schottky diodes were glued to the rotor fan using an epoxy resin based glue. (See photograph 6.)

Photograph in lab book, showing diodes mounted with epoxy and shaft end work. This photo is almost exactly like photographs 5 and 6 accompanying the original description.

A hole was cut in the axial back of the case of the bearing running on the back end of the main shaft to pass the pot core mounting shaft and connecting wires.

This was extremely difficult for me becuase the back end bearing was sort of a needle bearing running in a drawn sheetmetal cup race. The cup was extremely hard and quite thin, so it was impractical to drill with the tools I had on hand. I got some sort of hole in it, then expanded this out to the needed size using a dremel grinder. It worked, but I did a lot of damage to the bearing. Worked fine for the demos, and still does, but makes a little noise.

A 1/4" diameter hole was drilled axially in the back of the alternator shaft to attach the potcore mounting shaft. This was done on a drill press, using the existing small hole in the shaft as the starting center. (See photographs 6 and 5.)

Two 1/8" diameter holes were drilled on either side of the centered 1/4" diameter hole. Holes were drilled radially in the shaft to meet these and form passages for the transformer secondary wiring. (See photograph 6.)

All this work was unfortunately before I knew anything about, or had access to, any sort of a lathe. It's very tricky to drill holes in round things without a lathe, but, where there's a will, there's a way. Luckily for me, the centering hole in the back of the shaft was fairly deep... so the .25 drill at least got a decent start in the right direction.

The transformer windings were wound first around half a standard bobbin mounted on a bolt held in a vice. While thus wound, they were saturated in catalyzed (mixed, hardening) epoxy resin. While the resin was firm but not brittle, they were separated from the bobbin and bolt, and fit tot he ferrite core halves. This assembly was then filled with catalyzed epoxy resin and clamped gently with wax paper, cardboard, and a smooth metal plate to ensure the windings did not protrude beyond the face of the core halves. After the epoxy had hardened, the core faces were cut down a small amount with a progression of find sand papers to remove any unwanted build up of the epoxy. On the primary core, the winding is 20 turns of 17 strands of #35, with center tap. On the secondary core, the winding is 40 turns of 8 strands #35. The stranding was to improve high frequency performance, and will probably not be necessary.

To make the pot core mounting shaft, a section of 1/4" diameter Delrin rod was chucked into a drill press and cut down with a file to fit the transformer halves center holes. The section which the primary rotates about was cut down for a slip fit to the pot core center hole, while the section on to which the secondary is pressed was left a few thousandths larger. A washer shaped disk was cut from 0.011" thick Teflon sheet to form a separator between the two transformer core halves. (See photograph 4.)

Photograph 4: An exploded view of the transformer assembly. Not shown is the machine frame and main shaft bearing, which are located between the secondary pot core and the main shaft. All wiring is disconnected in this photograph.

Three mounting holes were drilled and tapped for a #4-40 screw in the back of the alternator frame. These were to mount the three stand-offs that support the spring attachment assembly. (See photograph 3 for spring attachment location)

Photograph 3: A close look at the transformer and thrust spring assembly. Not visible is the bushing around a stand off that slips into the primary side pot core wiring outlet slot to prevent rotation. A good view of the pot core wiring outlet slot is in photograph 4.

The transformer halves were fitted to the pot core mounting shaft. The secondary half was hooked to wires running to the field winding and rectifier bridge. The connections to these wires were secured with epoxy resin between the secondary core half and the alternator frame. (See photograph 4.)

The spring attachment assembly was made from a small triangular piece of .065" thick aluminum plate. Three 1/8" inch holes were drilled to fit #4-40 screws for attachment to the standoffs projecting from the alternator frame. A 1/4" x 20 threaded hole was drilled and tapped in the center of this plate, into which was screwed a 1/4 x 20 screw. A small bushing cut from Delrin rod was cut to closely fit the steel spring that was used to gently hold the core halves together. An additional small bushing was pressed over one of the standoffs to loosely key into the primary transformer half unused wire outlet to prevent rotation. (See Photograph 3. Rotational restriction busing is not visible. See Figure 5 for axial view of pot core half, and location of "retainer rod" to restrict rotational movement.)

The power converter/regulator was built on to a small piece of prototyping "pad per hole" board. This was firmly mounted to a small heat sink, which was screwed to the alternator frame. Wires were run to the transformer and to the alternator ground (frame) and power connections. (See Figure 3.)

For operational testing the machine was bolted into a test rig and coupled by a V-belt to a DC motor as a drive. A 12 volt battery is connected, with a current meter in series and a volt meter across the terminals. The machine has been run up around 2500 RPM. In the prototype, voltage output was set to 13.9-14V, which was demonstrated holding constant with fluctuation of shaft speed. (See photograph 1)

Photograph 1: The test rig, showing mounting of alternator and drive motor. Also visible is the battery and the meters used to monitor and display the system voltage and current flow of the system while in operation. Not shown is a piece of 1/2" thick plywood that was secured to the front of the mounting as a cover for the V-belt and pulleys.

In mass production, the spring mounting supporting the primary side transformer half would be die cast into the machine housing. Shafts could be made in an automatic screw machine, and separator disks could be punched from sheet stock. It would also be interesting to explore the possibility of just Teflon coating the bearing face of one of the pot core halves.

For a larger, more powerful machine, a small ball or roller bearing may be used instead of the plastic positioning shaft and thrust washer combination. This bearing would have to provide both radial and thrust positioning but would only have to support a very light load. It would turn about a small brass or bronze shaft pressed into the machine main shaft. (See Figures 5 and 6.)

Figure 5 is at the top of this web page.

Figure 6: Copy of notebook page 51. It shows an alternate method of attaching the primary pot core half to an auxiliary bearing. Also notes the problem of axial location, which is solvable but does create a few additional steps in manufacturing. One possible method of accurately locating these pot core halves axially on the shaft would be to machine the mounting shaft to have a step in the diameter for the bearing and the secondary pot core half.

The main advantage offered by the invention over present practice is the elimination of brushes and slip rings with the preservation of conventional brushed synchronous machine features such as voltage regulation and ease of power generation. The elimination of brushes will allow use of the controllable field machine in previously "off limit" areas such as explosive or dirty environments, and in applications where brush service and replacement is prohibitively costly. Another potential advantage comes from the fact that because a transformer is used, the available voltage for the field coil is not limited to the system voltage. This may allow machine designers to build more efficient or cheaper machines by optimization of the field voltage. In addition, this excitation system may cost less to manufacture than the conventional brush and slip ring system. All the alignment critical machine work is done axially to the shaft, and can be accomplished by one shaft press fit to a hole that is already partially drilled in the present version of most machines.

Note: Delrin and Teflon are trademarks of DuPont.

The disclosure form from OSU asked for potential manufacturers, and their addresses. So here was my list. They specifically asked for examples of companies with operations in Oregon.

Kohler Power Systems     (generator manufacturer)
Kohler, WI 53044

Marathon Electric     (motor, generator and drive manufacturer)
PO Box 8003
Wausau, WI 54402-8003


Baylor Generator/Motor Group     (brushless generators)
Sugar Land, TX 77478


General Electric     (generators, motors, and controls)
3135 Easton TPKE
Fairfield, CT 06431


GM AC-Delco     (automotive electronics)
3031 W. Grand Blvd.
Detroit, MI 48202


Electric Power Conditioning (    may be interested in building a wind generator)
1895 NW 9th
Corvallis, OR 97330


Sure Power Industries     (automotive/truck electronics)
10189 SW Avery
Tualatin, OR 97006


Zman Magnetics     (custom transformers and magnetic components)
1600 NW 167th Place
Beaverton, OR 97006

There are hundreds of companies building generators and power systems. Any one of these could possibly be interested in manufacturing or using the brushless excitation system in their machines.

Commercial possibilities include the use of the system in wind power systems, emergency generators, marine generators, submerged generators, high performance motors, generators and alternators for explosive environments (such as needed in fire trucks or mining equipment.) automobile and truck alternators, as well as many other applications.

I was firmly in sales mode here. I listed a lot of what I could think of that I though was exciting. I don't know that fire trucks must use brushless alternators. Where I was really hoping this would go was in million mile running alternators for over the road trucks... Freightliner... I don't know why I left them off the list of companies.. perhaps I hadn't thought of this before I submitted the disclosure.. I still think this might have some application in submerged generators... the plain bearing is a natural for running in fluid.

Price range depends on the method of implementation. The electronics for the prototype system (an alternator) cost around $13 retail, which is also about the retail price for a conventional regulator that is replaced by the system. I have not looked at cost issues beyond this. I believe that in some systems the market would be willing to pay a premium to gain the benefits of brushless excitation.

Quantity depends on the application. A small wind system for battery charging might draw orders of a couple thousand a year. If the large automobile manufacturing companies decide to use this system, they could eventually be building millions.

That's the end of the description. If you read this far, you're interested! Give me a call. I've still got the prototype.

Here is a scan of the original description of 1995.