A component-level DC transformer was developed in which no alternating currents or voltages are present. It operates by combining features of a homopolar motor and a homopolar generator, both DC devices, such that the output voltage of a DC power supply can be stepped up (or down) with a corresponding step down (or up) in current. The DC transformer should be scalable to low-megawatt levels, but is more suited to high-current than high-voltage applications.
This DC 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. It is a DC counterpart to the well-known AC transformer. The device is based on a combination of principles exhibited by homopolar motors and homopolar generators. Homopolar motors operate by flowing current in the presence of a magnetic field to generate torque. Homopolar generators operate by rotating a conductor in a magnetic field to generate a voltage, sometimes called a back EMF (electromotive force).
The rotary inertia of the DC transformer can be substantial, such that a sudden change in load resistance will change the output current, but the output voltage, input current, and input voltage will not change until the rotational speed begins to change. If the load 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.
A small-scale, non-optimized device was constructed to demonstrate the concept. The DC-DC transformer consists of a 3-in. (≈7.6-cm) and a 4-in. (≈10.2-cm) 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 in large aluminum cylinders and generate a magnetic field of about 0.2 Tesla across the disks. A highcurrent-capacity graphite brush was used to provide 30 A of current to the 3-in. disk with the return path passing through the system axle, 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 ±1.5 mV of voltage across the 3-in. disk, and about 30 ±1.5 mV, across the 4-in. disk. 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.
This work was done by Robert C. Youngquist, Stanley O. Starr, and Curtis M. Ihlefeld of Kennedy Space Center. For more information, please contact the Kennedy Space Center Technology Transfer Office at (321) 867-7171. KSC-13776