A component level dc transformer is described in which no alternating currents or voltages are present. It operates by combining features of a homopolar motor and a homopolar generator, both de devices, such that the output voltage of a de power supply can be stepped up (or down) with a corresponding step down (or up) in current. The basic theory for this device is developed, performance predictions are made, and the results from a small prototype are presented. Based on demonstrated technology in the literature, this de transformer should be scalable to low megawatt levels, but it is more suited to high current than high voltage applications. Significant development would be required before it could achieve the kilovolt levels needed for de power transmission

Ihlefeld, Curtis M.

Starr, Stanley O.

Youngquist, Robert C.

NASA Technical Reports Server

A DC Transformer 
Robert C. Youngquist1, Curtis M. Ihlefeld, and Stanley 0 . Starr 
Abstract 
A component level de transformer is described in which no alternating currents or voltages are 
present. It operates by combining features of a homopolar motor and a homopolar generator, 
both de devices, such that the output voltage of a de power supply can be stepped up (or down) 
with a corresponding step down (or up) in current. The basic theory for this device is developed, 
performance predictions are made, and the results from a small prototype are presented. Based 
on demonstrated technology in the literature, this de transformer should be scalable to low 
megawatt levels, but it is more suited to high current than high voltage applications. Significant 
development would be required before it could achieve the kilovolt levels needed for de power 
transmission. 
Keywords: DC Transformer, Homopolar Generator, Homopolar Motor, Homopolar Machine, 
DC Power 
This research has not been published in any other journal and has not been presented at a 
conference. Please consider this manuscript for publication as a Letter. 
All authors work for NASA in the Kennedy Space Center Applied Physics Laboratory. 
This work was supported by the NASA Office of the ChiefTechnologist. 
]-Corresponding Author, mailing address: Mail-Stop NE-L5, Kennedy Space Center, Fl. 32899. 
Voice-(321)867-1829, Fax-(321)867-1177, email- Robert.C.Youngquist@nasa.gov 
https://ntrs.nasa.gov/search.jsp?R=20130011800 2019-08-31T00:31:48+00:00Z
A DC Transformer 
Introduction 
This letter presents an electro-mechanical component that transforms de power, allowing de 
v·oltages or currents to be stepped up or down. This de transformer is a fundamental element, 
comparable to an ac transformer; it does not use internal electrical components, does not rely on 
switching or commutation, and does not require additional power to operate. As the theory 
section will show, this device is a de counterpart to the well known ac transformer. 
DC transformers are commonly referenced in the literature as being synonymous with de 
converters [1,2,3,4] , utilizing internal switching to create an ac waveform that can be stepped up 
or down with inductors or an ac transformer, and then switched back to a new de voltage. Yet, 
this nomenclature is relatively new. A generation ago de transformers were regarded as a 
fundamental component, with no internal elements, and were considered to be fictional until a 
superconducting version appeared [5] in the 1960s. In this letter a true component level de 
transformer is presented, not a de converter. 
The device presented here is based on a combination of principles exhibited by hornopolar 
motors [6,7] and homopolar generators [8,9]. The underlying idea does not appear to be in the 
recent literature however a patent was found describing a "homopolar transformer" [ 1 0] , which 
appears to be related to the present concept, although no analysis or results are given in the 
patent and the design is more intricate than that developed below. Consequently, the authors 
believe that the de transformer described in this letter is worth consideration not only for its 
novelty, but for possible further development. There is a need for high power de transformers 
[ 1 ,2] and this device should be capable of handling megawatt de power levels by extending 
engineering already developed for homopolar motors and generators. 
Theory 
The simplest, isolated, form of the de transfonner consists of two conductive disks, radius a1 and 
a2 resp., that are spinning at frequency, m, in a uniform magnetic field, B, as shown in Figure 
1. Voltage leads are connected to sliding contacts on the disk edges and the returns are connected 
to the two electrically isolated axels. If the voltages do not need to be isolated a simpler, single 
disk form exists, as shown in Figure 2, where the contacts are located on the disk face. It will be 
assumed below that ~ .ret = V2.ret = 0 so that the analysis will apply to either configuration. 
Load 
Power Supply ~ ,.....----+--
Sliding 
Contacts Ro 
Figure 1. The simplest isolatedform of the de transformer consists of two mechanically 
connected spinning disks located in a un(form magneticfield. 
\'i 
v; 
'i a I 
B 
'21 a2 
Figure 2. The simplestform for the de transformer is composed of a single spinning disk in a 
magnetic field with two sliding contacts on the disk face . 
. The operation of this device is based on a combination ofhomopolar motor and homopolar 
generator concepts. Homopolar motors operate by flowing current in the presence of a magnetic 
field to generate torque [ 11]. The total torque, r, is the sum ofthe torques generated by each disk 
current and is given by 
{1) 
where / 1 and / 2 are the currents in each disk. In the steady state, ideal case where the rotational 
velocity is constant and there is no friction, the total torque is zero and equation (1) reduces to 
{2) 
showing that the disk radii scale the current ratios similar to the coil winding number in an ac 
transformer. 
Homopolar generators 9perate by rotating a conductor in a magnetic field to generate a voltage, 
i.e. a back emf [II]. In the configurations shown in Figure 1 and 2 voltages, v; and V2 
respectively, are generated at the disk radii , a1 and a2 according to 
and 1 j V2 = - OJai_B 2 
{3) 
where OJ is the rotational velocity of the two disk system. The last relation in equation (3) shows 
how the disk radii scale the ratio of the voltages on the two disks. Combining equations (2) and 
(3) yields the fundamental equation for the operation of an ideal de transformer; 
{4) 
which is the same power conservation relationship produced when analyzing an ideal ac 
transformer. In addition, like an ac transducer, the input and output connections can be 
interchanged turning, for example, a voltage step up device into a voltage step down device. 
The figures show a power supply with voltage, V; , impedance, R;, and a load Ro . One purpose 
of a transformer is to match load impedance to power supply impedance in order to maximize 
power delivery. In the case of the de transformer shown in Figure 1 the power supply sees an 
impedance given by v; I 11, which can be rewritten as 
showing the significant role that the disk radii play in converting the load impedance. 
The analysis above can be generalized to include friction by the addition of a frictional torque, 
T.r, acting to decelerate the disks yielding a modified version of equation ( 1) 
_, 
(5) 
Making the steady state assumption, where the total torque is set equal to zero, this expression 
can be simplified significantly by the incorporation of a frictional current, lr defined by 
Ir = 2T1 I (a~ B). This allows equation (5) to be rewritten as 
and permits the rest of the analysis to proceed as above yielding ~11. 1 = V2J2 • Physically this 
states that friction requires a current penalty, but after paying this the device operates ideally. 
Note that the frictional torque and the frictional current are not necessarily constant, for example, 
they can vary with rotational speed in the case of air resistance. 
In the presence of friction the induced voltage equations (3) are unaffected. Combining these 
with equation (5) yields 
which states that the total mechanical power entering the two-disk system is equal to the 
incoming electrical power minus the outgoing electrical power minus the power loss to friction. 
In other words, the device operates by using differences in electrical power to ramp up or down 
mechanically stored energy after paying a frictional penalty. Energy lost to resistance in the 
device could also be explicitly handled, but it can be lumped in with the energy lost in the power 
supply and the load resistances. 
The rotary inertia of this de transformer can be substantial, such that a sudden change in load 
resistance will change the output current, but the output voltage and input current will not change 
until the rotational speed begins to change. If the load impedance decreases and more current 
flows out of the transformer, rotational energy will be converted into electrical energy until a 
new, slower, steady state, rotational speed is reached. This high frequency filtering effect may be 
advantageous to protecting the power source. 
There are alternative configurations for this de transformer. For example, the device could utilize 
cylindrical conductors and radial magnetic fields instead of disks, a common approach in the 
literature and the equations for this cylindrical version would now be functions of the radii and 
the length of the cylinders. Alternatively, the two rotating disks or cylinders could be coupled 
using belts or gears yielding a wide variety of devices and performance characteristics. 
Prototype Device 
Significant engineering has gone into developing large scale homopolar generators and motors 
[ 6-1 0], but a de-de converter using these has not been demonstrated. So, we decided to construct 
a small scale, non-optimized, device--a toy compared to systems in the literature--to 
demonstrate the concept. Figure 3 shows our de-de transformer, consisting of one 3-inch and one 
4-inch diameter aluminum disk spinning on a common axle in a magnetic field generated by a 
pair of fixed rare earth magnets. The magnets are located inside of the large aluminum cylinders 
and generate an approximately uniform magnetic field of about 0.2 Tesla across the disks. 
A high current capacity graphite brush was used to provide 30 Amps of current to the 3-inch disk 
(left disk in the Figure) with the return path passing through the system axel, across the bearings, 
and down the center pedestal. This current caused the two-disk structure to accelerate up to a 
17.5 Hz rotational speed generating about 16 m V ± 1.5 m V of voltage across the 3-inch disk 
and about 30 m V ± 1.5 m V across the 4-inch disk (the right disk in Figure 3). Higher speeds 
were not possible due to friction in the sliding contacts and by slight imperfections in the disks 
causing the sliding contacts to lose contact at higher rotational speeds, but this result does 
demonstrate a voltage increase approximately equal to the ratio of the radii squared 
( 4 I 3)2 = 30m V I 16m V as predicted by Equation 3 above. 
Figure 3. A small scale device was constructed to demonstrate the de-de transformer concept 
using a pair of conducting disks spinning in a magnetic field. 
The voltage from the 4-inch disk was fed across a low impedance load generating a current that 
peaked at about 40 mAmps, but due to variations in the sliding contacts the expected increase in 
the input current could not be measured-the variations in the frictiona,l current, I 1 , were greater 
than the ideal device input current. Even so, power was generated and delivered, albeit only 
about 1 mW, with about 400 mW being used to overcome friction. Increased scaling would be 
necessary to create an efficient, high power, device, but this "tqy" device has demonstrated the 
basic operation of the concept, by shifting voltage up and delivering power. 
Conclusions 
A true de transformer has been described and demonstrated, but its utility is unclear. These 
devices cannot compete with low power semiconductor based de converters, however, they can 
be scaled to high power, where the primary need is for high voltage converters (kilovolts) for de 
power transmission [2]. The difficulty is that homopolar generators are more suited to high 
current, rather than high voltage, applications and have only been demonstrated up to 750 volts 
[8]. Even so, with engineering this may be extended and they may find a role where their de 
conversion potential coupled with their power filtering/storage capability may solve a need in the 
rapidly growing area of high de power transmission. 
Acknowledgements 
The authors acknowledge funding from the NASA Office of the Chief Technologist. We also 
acknowledge Robert B. Cox for fabrication and assembly of the prototype device. 
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A DC Transformer

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