SPIN a metal disc on a shaft in a magnetic field, and you create a dc voltage between the shaft and the edge of the disc. Add brushes that ride on the shaft and disc-edge, and you can send current to an external load. What could be simpler? But if it’s so simple, why have so-called homopolar generators remained primarily a laboratory curiosity for so many years?
Homopolar generators do indeed offer a number of advantages over conventional dc generators. They are simpler, since they do not have commutators, and they can produce extremely high current. On the other band, they suffer from high losses, making them impractical for continuous-duty applications. Homopolars are at their best in applications involving high-current pulses.
Recent advances have made homopolar generators commercially feasible, clearing the way for million-ampere generators that could revolutionize welding and other processes.
The one million-amp generator in the center is connected to the load on the left with numerous parallel bus bars. The generator is here being tested by the Center for Electro mechanics at the University of Texas (CEM.UT).
in brush, bearing, and generator design have made the generators cost-effective in a variety of processes, such as welding, that require high-current pulses.
Homopolar basics
Homopolar generators are inherently low-voltage, high-current devices because armature conductors are in parallel rather than in series as in commutator-type generators. Voltage output is determined by the equation
Thus, with maximum field flux density, rotor dimensions and speed determine voltage. Until recently, peripheral rotor speed in the area where brushes made contact had to be kept low to prevent short brush life. As a result, generated voltage was far below theoretical limits and brush voltage drop accounted for an appreciable portion of the voltage generated.
Homopolars that produce high current require many brushes. But constraints on rotor diameter have restricted the total number of brushes, thus limiting current ratings. These factors plus the excessive heat losses when operated continuously have, until recently, limited homopolar generators to 10 to 100 kA outputs. Moreover, brush life generally was poor.
Liquid-metal current collector systems have been used to avoid the high losses and short life of solid brushes. But these liquid-metal systems are expensive, difficult to maintain, and generally
do not perform well in strong magnetic fields.
Pulsed operation
High losses, however, are not a deterrent where homopolar generators produce intermittent pulses of current. Excess heat dissipates during the off cycles. And unlike other generators,
commutation limitations do not hamper homopolars. When operated in a pulse mode, homopolar generators can produce peak currents many times higher than when operating continuously. Total energy available in a pulse is a function of the energy stored in the rotor. Energy stored in a rotor is determined by the
Anatomy of a homopolar generator
In its simplest form, a homopolar generator consists of a metal disc mounted on a rotatable shaft A uniform magnetic field parallel to the shaft passes through the disc Rotating the shaft by external means creates a dc voltage between the shaft and the periphery dr* disc. Brushes residing on the periphery end on the shaft conduct current from these points to an external load circuit.
The disc can be viewed as an Infinite number of radial conductors, each cutting flux as the disc rotates. The voltage produced is proportional to the length of each conductor, the flux level, and the speed of rotation. For high current applications, brushes are located at multiple points around the periphery of the disc and shaft. Rotors in homopolar generators in some cases are cylindrical rather than disc-shaped. The field is then perpendicular to the cylinder surface. And in another design, the rotor is spool shaped, combining some of the characteristics of disc and drum types. But the principle of operation in all cases is identical.
The rotor of this 6.2 MJ homopolar generator is spool shaped because the geometry minimizes non-rotating back iron. Each half of the rotor generates 25 V. open circuit, at 6,245 rpm. The generator is rated 7 MA peak with the halves connected in series. Armature reaction is minimized by locating stator conductors close to the current-carrying surfaces of the rotor. Current flows only on or close to these surfaces because the skin effect prevents deeper penetration during the brief periods of current flow.
Equation E = 1/2J (.02 where E — kinetic energy, Joules,
stored in the rotor, J polar mass moment of inertia of rotor, kg•rnz, and w rotor angular velocity, rad/s.
Energy is stored in a rotor by first driving it, generally by external means, up to speed with the load disconnected. At this point drive power is disconnected. The generated output voltage is then switched across a low-impedance load. Load current builds rapidly to a very high level, then drops as the generator slows and stops. The slowing is due to the so-called Lorentz force, a dynamic braking action
Elementary homopolar generator Conduct, Disc m&aev 1984
resulting from the current and the magnetic field interacting. Pulse times typically are a few tenths of a second. Feasible peak currents can range from a few hundred thousand amperes up to a few million amperes.
Until the development of pulsed homopolar generators, systems requiring high-energy pulses were powered with banks of capacitors or batteries, but capacitor banks are many times larger than corresponding homopolar generators and store energy at voltages too high for most industrial pulsed-power processes.
Voltages available from battery banks are compatible with most industrial pulsed-power processes. But batteries have high internal resistance and correspondingly low efficiency.
Capacitor and battery banks, moreover, typically cost from $0.20 to $1 per Joule of stored energy. In contrast, homopolar generators now cost from 5 to 10c per Joule.
Homopolar generators, however, are modeled in equivalent circuits as low-voltage capacitors. Relatively small homopolar generators store energy equivalent to that stored in large capacitors charged to several thousand volts. The equivalent capacitance of a homopolar generator is determined with the equation 2EV
where C = capacitance in farads, E = stored energy in Joules, and V — peak open-circuit voltage, V.
New developments
Until recently, peak pulse current was generally limited to a few hundred-thousand amperes. This level assured reasonable brush life and avoided bearing
hew varied from 57% to 126% but worth some loss in ductility. Metals having high thermal and electrical conductivity, however, are difficult to weld. Some dissimilar meals joined successfully but others, like copper-aluminum, have been unsuccessful.
A variety of parts have been welded experimentally with a 10 MJ generator at CEM-UT. These include 6-in., Schedule 80, carbon-steel pipe (8.5 in.’ of welded surface), 4-in. Schedule 80, 304 stainless pipe (4.4 in.’ welded surface); carbon-steel rails, pipe flanges, and steel plates.
Pipes and rails are butt-welded easily with little upset. Parts having a high aspect ratio can be a problem, however, due to the difficulty in maintaining alignment.
Two pieces of metal to be pulse welded are held lightly together by a fixture. The fixture can automatically adjust pressure between the two work pieces during welding. One work piece is connected to the positive terminal of the generator, the other to the negative. At the start of a pulse, heat concentrates at the interface between the two pieces because of the constriction resistance in this area. As the interface heats to forging temperature, the fixture automatically increases pressure between the two, forging the pieces together.
Weld current at the start of a weld is controlled by adjusting field excitation and by initiating the pulse at a preselected rotor speed. The shape of a pulse is controlled by varying the field excitation during the pulse.
A compact generator
The prototype of a new type homopolar generator has recently been built and tested at CEM-UT and is commercially available from OIME Inc. Rated 1 MA at 50 V, open circuit, it stores 6.2 MJ of energy at 6,245 rpm. This is equivalent to a 4,960-F capacitor. A capacitor this size would fill a very large room, but the new generator is only about 25 in. long and 33 in. in diameter.
The generator is brought to speed in about two minutes by two 70-hp, positive displacement, bent-axis piston motors operating at 5,000 psi. The motors free wheel during generator discharge. Generator bearings are a rolling-element type. The generator contains1,160 brushes. Its maximum speed is determined not by brush life but by stress in the solid alloy-steel rotor and by bearing speed limitations.
The brushes lift off the rotor during power up, eliminating brush loss and wear during this portion of each pulse cycle. The brushes are set onto the rotor pneumatically to initiate a pulse cycle, closing the circuit to the load. Voltage typically drops to about 40 V by the time current has risen to 1 MA.
The 6.2 MJ stored in the machine is only the equivalent of that stored in a Sears Diehard auto battery. But the generator has an internal resistance of only 7.5 ufl and an inductance of only 30 nH. The low resistance permits the extremely high current peaks, and the low inductance gives extremely fast rise times. A bank of batteries probably could not match these parameters.
Future possibilities
It seems likely that pulse welding will be widely used in the future for large-area welding (> 5 in.2). Examples are oil field and
gas-transmission pipe, boiler tubing, railroad rails, structural shapes, and heavy plate. But homopolar generators may also be applied to a variety of other tasks such as ingot heating, metal forming, nuclear fusion, and the launching of high-speed projectiles.
Results of recent experiments carried out at CEM-UT, for example, indicates that high-current pulses are suitable for heating metal billets for forging, bending, or shaping operations. The process appears to be more costly than induction heating for non-magnetic steel.
High-current pulses may also be used to create high pressure for metal forming. In this process a pulse is applied to a coil which in turn produces eddy currents in a work piece. Reaction forces perform the work. The process can be used with both ferrous and non-ferrous materials. Alternately, the large reaction torque that results from short-circuiting a homopolar generator may be used for mechanical metal forming.
Nuclear fusion and exotic weapons such as particle beams and high-velocity projectiles all require high-energy electric pulses. The speed of a projectile powered by chemical propellants, for example, is limited to a maximum speed for a shock wave, about 5,000 ft/s. But projectiles launched by high current pulses have no theoretical limit short of the speed of light.
The 6.2 MJ compact homopolar generator described here is now in commercial production, and a 15 MJ version is being designed by OIME Inc. An experimental 60 MJ facility planned for CEM-UT will consist of six 10-MJ machines. Connected in series the generators will provide 1 MA at 600 V; connected in parallel