Power Transfer System

This invention relates to a rotary work table, turret of wheel carrying RF heating coils which are located at a series of stations about the periphery of the rotating work carrier. Each RF heater has to be supplied with power at a prescribed time during the rotation of work carrier whereby work will be performed in a curing, baking, sealing, processing, heating or a similar operation. This necessitates a commutation device for carrying RF energy at low voltage high current and frequencies in the radio wave range (450 kHz). In the past such commutation was accomplished by switching devices which include a large set of bus bars, slip rings or commutating bars that were mounted on the rotating member and caused to pass stationary contacts carried by a RF power supply. The problem with such arrangements was the tendency to arc at the point where contact or switching occurred. The transfer of high current on an intermittent basis caused care and safety problems with respect to the operation of such machinery. Another approach was to carry the power source with the rotary member, but this too was unacceptable because such power supplies are large in size and require numerous connection which complicate that approach.

In order to build a compact, reliable and simple arrangement for transferring RF energy from a fixed power supply to a rotating member and overcome the problems of the prior approaches, a shunted power supply loop was conceived and connected to a stationary RF power supply. The loop consists of a pair of contact strips which are connected to the power supply and held in parallel spaced relation. The middle of each strip receives the power while the end of each is shunted to the other. The loop is designed to follow the shape of the circumference of the rotating work carrying member whereby contact brushes can periodically engage or disengage the loop without arcing. The power received by the brushes is proportional to the distance on the loop i.e., from the loop shunt. That is to say that, the power available at the shunt portions of the loop is not transmitted since there is no potential difference between the contact strips which form that portion of the loop. More particularly, the contact strip connected to one power supply terminal and the contact strip connected to the other supply terminal have no potential difference at the shunts. As the contact brushes first engage the loop moving from the shunt to the point of the power connections the potential difference increases in accordance with the distance traversed. Consequently, the load attached to the brushes receives the power in an ever increasing fashion from the contact strips such that there is no sudden connection to the high power supply, which would cause contact arcing thus shortening brush and contact strip life or sparks. The load only receives varying potential difference which increases as the brushes move from the shunted portion of the loop where contact is first made to the center of the loop where the power connections enter the loop. Similarly, the power diminishes as the brushes continue across the loop from the power connections to the terminating shunt. At the shunts the potential difference across the contact strips is zero.

The foregoing system for transferring power from a stationary RF power supply to a rotating member can be accomplished overcoming the problems of the prior techniques.

It is an object of this invention to provide a low cost power transfer device for connecting RF energy from a stationary power supply to a moving set of contact brushes.

It is a further object of this invention to provide a power supply transfer means which is reliable, safe and simple to construct.

It is still a further object of this invention to provide a transfer loop having central power supply inputs connected to elongated legs of the loop which extend in both directions therefrom and are shunted at their ends thus forming a varying potential difference of the voltage in the legs of the loop which is a function of the distance from the power connections or the proximity of either shunt.

FIG. 1 is a perspective view showing the preferred embodiment of a power supply and an associated loop with shunted ends.

FIG. 2 is an enlarged cross-sectional view shown partially schematic for a contact brush arrangement to be used in connection with the loop and the cross-section is taken along section 2--2 through the loop in FIG. 1, and

FIG. 3 is a voltage versus position graph depicting the varying potential difference between the legs of the loop in FIG. 1 at various locations from end to end along the loop.

In FIG. 1 a power supply such as an induction heating generator 10 and output transformer is shown for providing RF energy at low voltage (200 to 1000 open circuit voltage), at high frequency (300 to 600 kHz) and at high current (75 to 200 amps). The generator 10 transmits power along a pair of upper and lower supply bus connections 11 and 12 to the loop generally designated 13 and being comprised of two semi-circular loop busses 13a (upper) and 13b (lower), FIG. 1. The loop busses 13a and 13b are disposed in parallel spaced relation to one another. The busses 11 or 12 and the loop 13 is fashioned from a suitable conductor such as copper and may be hollow whereby coolant can be circulated therethrough in order to maintain the operating temperature at a reasonable level notwithstanding the fact that high currents are being transmitted. At the ends of the respective loop busses 13a and 13b are shorting plates 14 which act to provide a shunted path for the ends of each loop. The shorting plates 14 are also made of suitable metal conductors such as copper and can include cooling passages. However, the plates 14 shown in FIG. 1 are not so equipt and are merely affixed by bolts 15 which are designed to hold the shorting plates 14 to the ends of the loop busses 13a and 13b in intimmate contact engagement.

The busses 13a and b are shown as spaced apart semi-circular rings juxtaposed to each other and are connected to the bus connectors 11 and 12 at the center of their semi-circular form; the shorting plates 14 are located at the ends of the busses. It is that arrangement which varies the potential difference between the loop busses 13a and 13b depending upon position along the busses from the center to the ends where the shorting plates 14 are connected. The maximum potential difference is at the center where the supply busses 11 and 12 join the loop busses 13a and b.

In FIG. 1, the surfaces that face inwardly toward the center of the semi-circular arc are surfaces used for contacting the rotary brushes carried by a work table 16. The work table 16 has a central axis 16a about which it rotates and table 16 carries a number of induction heating coils 17 each of which are identical in design and are mounted to move with the table 16. The heating coils 17 and their contact brushes will be described in detail later in this specification. Suffice it to say for the present that, as they rotate along with table 16 and they engage with the loop busses 13a and 13b so that power is provided by each loop bus to a side of each coil 17.

FIG. 3 shows a voltage versus position graph having the variation of potential difference along the loop busses (on the verticle axis) and the distance or position along the loop bus from the left side shorting plate to the right side shorting plate (on the horizontal axis). As is apparent from FIG. 3, the potential difference of the voltage measured between the loop busses 13a and b, increases as location becomes closes to the center power supply connections, increasing to a maximum at the point of connection and conversely the potential difference diminishes between the loop busses 13a and b as the location is nearer to the shorting plates 14. It can be appreciated that the power available to a work coil 17 as it connects with the loop busses 13a and b near a shorting plate 14 will not be transferred with arcing because the potential difference at the shorting plate 14 is zero. Consequently, the contact brushes of the work coil 17 as they engage or disengage the loop busses 13a and b will not arc because the power transferred to the work coil at that point is zero and increases as a function of the distance traversed along the loop busses 13a and b toward the contact supply busses 11 and 12 in accordance with the graph of FIG. 3.

FIG. 2 is a cross-sectional view of a contact brush arrangement for work coil 17. More particularly, the loop busses 13a and b include passages for a cooling fluid labelled 18 and 19 respectively and such passages 18 and 19 commonly carry water. The cross-section of the loop bus 13 is generally rectangular, but any suitable shape which is convenient to manufacture can be used, for example, these busses could be manufactured out of any conventional tubing and those skilled in the art would appreciate that the contact brushes would be shaped to fit with complimentary surface on the bus. The smaller inwardly disposed face of the rectangular loop bus cross-section is designated 20 and 21 for the loop bus 13a and 13b respectively. These faces 20 and 21 form are inwardly with respect to the semi-circle formed by the loop bus 13 and are generally vertical surfaces (see FIG. 1). It is these faces 20 and 21 which are used to supply the power to the work coil 17. In FIG. 2, the work coil 17 is shown connected to a pair of movable contact brushes 22 and 23 designed to ride against surfaces 20 and 21 respectively. Each brush 22 or 23 has a contact face 22a or 23a which is shaped and positioned to bear against surfaces 20 or 21 respectively. The brushes are mounted for movement and are biased toward surfaces 20 and 21 and as shown schematically in FIG. 2. Non-metallic springs 24 are positioned between the work table 16 and the contact brushes 22 and 23. Each contact brush is carried by a suitable support and connector bus 25 which permits the contact brush 22 or 23 to slide therein toward the faces 20 or 21 respectively of the loop busses 13a or 13b due to the urging of its spring 24. The connector busses 25 are each connected to a leg of a work coil 17.

Thus, power is transmitted from loop bus 13a through surface 20 to the brush 22 via surface 22a, then to the connector bus 25 which carries it to one side of work coil 17 and therethrough to the other side of work coil 17 across the other connector bus 25 through brush 23 and its contact face 23a which engages with surface 21 of the loop bus 13b for applying a working load to the generator 10. The table 16 rotates and a series of work coils 17 are consecutively brought into engagement with the loop bus 13 one right after the other, and the power transmitted to each work coil 17 builds in accordance with the motion of the table 16 as it rotates about its pivot 16a. Thus, as the coils 17 approach the connecting supply bus 11 and 12 the power transmitted to each work coil 17 increases to a maximum and as the work table 16 moves that particular work coil 17 past the center of the loop bus 13. The power transmitted will begin to diminish and the coil 17 will continue to move toward the end of the loop bus which is away from the center.

Those skilled in the art will appreciate that the concept is broader than the particular loop configuration shown. More particularly, the invention in its broadest application is an appreciation of the fact that the potential differences between the contact strips of a commutating device can be varied from a minimum to a maximum whereby contact between the moving contact brushes and the fixed contact strips can be made at the site of the minimum voltage potential difference thus permitting connection without arcing. Similarly, the disconnection can be made at a site where potential difference is at a minimum. Applications with linear arrangements or even one-half of a loop are also to be covered in the claims which follow as well as different cross-sectional configurations for the contact strips, shunts and contact brushes. It is, therefore, desired that the claims which follow cover the entire spectrum of physical arrangements and motions between the the respective parts which could be used to take advantage of the disclosed invention.