REWORKING AN X-RAY TRANSFORMER AND CONTROLLER BOX

Basic information on design and function:

The so-called ‘high tension’ transformers from older x-ray generation equipment make useful power supplies for HV work, but they need some modifications to function properly for this type of work. We will limit this discussion to the medical type free standing single phase transformers and the first step is to determine its type. Small units may be down to around 100lbs. The average is around 250-300lbs. and 500, 1000, and 2000lb. units are not uncommon. Pictured below is a common style in the 450-500lb range (NOTE: that some of the pictures on this page were borrowed from another site for safety and instructional purposes only until we can produce similar ones of our own).

The very largest (800-2000lbs.) transformers are usually 3-phase generators. All units are DC generators with ½ of the peak voltage as a positive potential developed on the anode and a potential of the same value but opposite sign on the cathode (e.g. a 125kVP tranny will develop +62.5kV at the anode and -62.5kV at the cathode).

The spec. plate on the unit should give its peak output rating and its input demand. Very few will have peak outputs below 100 kV with average at 125 kV and 150 kV being the top end on medical units (though some industrial units may be up to 320 kV but at very low current ~5 ma). The output current of medical units averages around 300 ma., but I have seen them as low as 100 ma. and as high as 1300 ma. Most units are wired for 240V input but may also be 120V or 480V. Momentary current demand may be has high as 400 Amps., making inductive ballast a must to successfully run these transformers on residential power service.

Internally, there are usually either 2 high voltage step-up transformers:

or one transformer with 2 or more secondary coils.

These may exist in any of several types/configurations. The secondary windings may or may not be center-tapped to ground, making a difference in how useful the unit may be. Those that are so tapped will usually contain full-wave non-bridge rectification (1 diode or diode strip on each output leg of each secondary coil) – typically 2 coils and 4 diode strips as shown below:

Those that are not tapped will contain a full-wave bridge (4 diodes or diode strips) for each secondary coil – typically 2 coils and 8 diode strips as in the following case:

In addition, there will usually be 1 or more low voltage transformers to supply 24 volts to heat the filament of the x-ray tube.

The high voltage cables from the x-ray tube connect to the transformer through special receptacles inset in the transformer top.

The underside of each of these Federal Connector cups has 3 electrode connections - the cup on the anode side will have all 3 terminals shunted together and a single connection to one side of the high voltage. The cathode side will have 3 separate connections – 2 from the filament transformer(s) and one from the other side of the high voltage (filament common is also attached on post #3).

There may also be a second or third cup set and/or a manual or motorized selector for them.

Modifications:

All x-ray transformers must be opened and modified prior to use. The sealing screws or band must be removed and the internal structure raised from the oil.

Filament transformers should be completely removed or at the very least, disconnected and all wires to and from them clipped away completely. All three connections on the bottom of the cathode cup should be shunted together with only the high voltage lead still attached. The diodes should be carefully removed next and preserved for other use as they will have ratings from 50kV to 150kV PIV when used under oil.

The rewiring method depends on the number and nature of the transformers. By far, the majority of units will have one end of each secondary grounded to the frame and the meter leads will arise from these ground points. There will only be 1 HV lead coming from each secondary and going to a pair of diodes. THE METER LEADS MUST BE CUT AND SHORTED TOGETHER on these types of units. Once the diodes have been removed, one HV lead is extended to connect to the terminal on the bottom of the anode cup and the other extended to connect to the bottom of the cathode cup. This arrangement will allow approximately the same AC output as the original DC rating.

A much less common but very highly desirable configuration has 2 separate transformers that are not grounded to the frame and each secondary has 2 HV leads instead of 1. These connect to a full-wave diode bridge (4 diode strips) and the 2 leads from the other transformer connect to a separate bridge. Output leads from the high voltage secondaries of these types may be connected in series to give the twice full AC output, with proper phasing (CAUTION: pushing the output voltage too high can lead to internal arcing and destruction of the secondary). A much more useful aspect of this is that the rated output voltage may be achieved with ½ the input voltage, allowing the use of standard 120V service (with proper ballast) to power these 240 Volt units.

With the diodes removed, the high voltage leads will probably have to be extended to reach the terminals on the bottom of the output cups. Expensive high voltage wire may be used for this purpose. In the alternative, solid single conductor, jacketed THHN of AWG #10 (or #12 for smaller units) should be run into a length of ¼” ID PE or PP tubing equal to the length of the wire minus the stripped ends. Fully insulated quick connect terminals should crimped to the wire ends. NOTE: Wires must be suspended within the interior space in such a way that no arcing occurs.

When the internal modifications are complete, the structure is lowered carefully back into to oil and any lost oil replaced. Once the unit is sealed, power connections should be made and checked. Having disconnected or removed all non-essential wiring inside, only the input line connections (L1, L2 or P1, P2 or P, PC) and the mains ground terminal G are needed. Usually, 240V on either a 30 Amp or 60 Amp circuit will be available in a residential setting. WITH THE FUSE OUT OR THE BREAKER OFF, one side of the phase is connected to P1 and the other to P2 utilizing wire/cable of at least #8 AWG (use #4 for 60 Amp service). G is connected to earth ground.

Inductive ballast should be connected in series with one side of the input phase. The ballast should be rated for current level it will see and on a 30 Amp circuit should have a 60Hz inductive reactance of at least 9 Ohms (27 Amps max) up to about 24 Ohms (1 Amp max). On 60 Amp service, the low end could safely be 4.5 Ohms reactance (53 Amp limit) if the transformer can handle that much current (as most can). Arc welders are often used for ballast since they allow for adjustment of the input current and their transformers have heavy high-current rated windings.

Finally, the output connections must be made very carefully to prevent injury, fire, or damage to the equipment. Connections to the transformer should be made with standard 75 kV x-ray cables in good condition with the Federal Plugs lubricated with non-conductive silicone grease and fully inserted and rotated until they lock. Then, the screw down lock rings must be tightened all the way down to prevent arcing to the case. Connection to the work can be made safely in 1 of 2 ways, each of which insures that the 3 conductors inside of the cable are securely coupled to one another to prevent the formation of loop subcircuits.

The first method is to simply use two Female Federal connectors removed from other equipment (such as the x-ray tube housing) into which to insert the Male Plugs from the ends of the cable. Once again, the three post terminals on the bottom of the cups are shunted together, for example with a circular wrap of #10 bare copper wire and then nuts tightened and the bare conductor attached securely to the work. Some method of securing the plugs inside the cups must be used as they will pull loose and arc destructively inside the cups if this is not done. I often use heavy thick elastic bands for this purpose.

Another method could be used if a permanent connection were needed. The 75kV cable is cut through at the appropriate length. The cable coverings, INCLUDING THE METAL MESH LAYER, are stripped back a minimum of 8 inches, cut off, and the cut ends wrapped with rubber splicing tape. This should be carefully done to avoid nicking the thick inner silicone rubber insulation layer. About 1-2 inches of this rubber insulation is then removed at the end of the cable to expose the 3 small inner conductors. These are then stripped of their coverings and soldered securely to each other (note that this may require burning off the thin insulation layer and sanding the inner conductors to bare copper to get a good bond with the solder). This then behaves a single conductor and is attached to the work. This method may also be used to produce terminal ends for ‘candlestick’ type electrodes.

Here is a reworked 100,000 Volt transformer by Continental X-ray Corp. powering the Jacob’s ladder in the front at about 116 kVAC. The higher voltage really gives it a snappy angry hiss far superior to the 12 kV NST farm employed previously. The power supply unit has a rotary tap switch, allowing section of voltage in increments over the range from 30 VAC to 156 VAC with 120 Volts in. A small doughnut shaped inductor with a reactance of about 8 Ohms is being used to control current.

REWORKING THE CONTROLLER BOX:

The controller boxes/panels for these transformers are exceptionally useful after a bit of rework also. Basically, they contain a large high-current autotransformer that is set, by means of high current rotary tap switches, to send a particular input voltage to the HV transformer. Unfortunately, they also contain a bunch of useless circuitry that amounts to an elaborate timer mechanism to control the duration of the pulse and other electronics for the motorized x-ray cassette holder (‘bucky’). There will be large resistors that are adjustable to control the milliamps of current output and to register this on a meter. Finally, there may be one to several relays or open contactors in the main power circuit.

The best approach is radical surgery. Simply put, to rip out all of the extraneous stuff, leaving the main power circuit and control mechanisms, and reconnect that into a continuous circuit. This is not nearly as hard as it sounds. With a power screwdriver and attachments and a pair of wire cutters, I have done these reworks in as little 20 minutes.

First, open the back (and front also if that is possible).

Locate the main autotransformer and follow the large wires coming out of it. Most will lead to the rotary tap switch(es) and those should all be kept intact.

The input power line to the transformer should also be identified and traced through any fuses or contactors to the main power switch and back to the autotransformer. Once this main power circuit and been identified and marked to prevent any of it being cut, everything else may then be removed, or conversely if this power supply is to be used elsewhere, this all may removed and everything else junked or salvaged for parts.

There are many useful parts in one of these units to delight electrical enthusiasts. The main KVP meter is usually just an AC or DC volt meter with a range of 0-120 or 0-240 volts. It may be removed or connected across the output to measure it. The milliamp meter is not of much use. There may be large stud type diodes, SCR’s or IGBT/dual SCR type modules rated at sometimes hundreds of Amps. Also may be found, many useful switches, huge power resistors/ potentiometers, relays, high current contactors, transformers, lights, heavy phenolic/acrylic terminal boards, brass screws/nuts, buzzers, and timers, etc. Any or all of this will certainly add to the spare parts bins.

L1, L2, and G from a very stout input cable (#8 AWG stranded conductors) are connected to the inputs terminals after placing a suitable inductor in series with one of the input conductors as ballast.

From here, large wires should proceed to the main power switch

and from there back to the ‘0’ wire tap of the autotransformer and either the highest output tap or a slightly lower tap to allow for some overvoltage (depending on transformer design, up 150% may be achieved before the frame saturates). There may also be line compensator terminals to connect to ‘the nearest to the input voltage.’ If there is a single rotary tap switch, its center lug is one side of the output (the ‘0’ line is the other).

If there are 2 such switches, the MAJOR KVP tap switch will set large voltage increments and the MINOR KVP will fine tune the output between major steps. In this case, the center wires from each of the tap switches are the P1 and P2 output lines and should go to the P1 and P2 terminals on the power board.

To these output terminals (P1, P2, G), is connected the main input cable to the high voltage transformer modified above.