Examples of Cascaded Rectifier Systems

Charlton and Hubbard built a cascaded generator of 10 stages for precision X-ray measurement work in the range 400 kV to 1.4 MV. The overall system was 30.5 feet high with ten stages. Each 140 kV stage was a full wave rectifier (using kenotrons) with a pair of 0.1 µF capacitors. It used the tertiary winding technique to pass the power to the stage above. The filament power was passed up the stack with same technique. The circuitry for two of the stages is shown in the following figure. Not shown in the schematic are reversing switches at each stage to reverse the polarity of the output.

Schematic of NBS 1400 kV generator

Each of the ten identical stages was contained in an oil filled Herkolite tank 50 inches in diameter and 28.55 inches tall. The high voltage winding, 3 kVA at 66 kV RMS, was insulated for 70 kV at the capacitor end and 140 kV at the tube end. The filament transformer had all 4 windings insulated for 150 kVDC. The top terminal was a metal spinning 100 inches in diameter and 48 inches tall. The corona current at 1.4 MV was only 0.16 mA. A 10.25 inch diameter tube was used as a conductor from the top of the stack to the top of the X-ray accelerator tube. 6 inch tubes were used for the taps.

The following table gives the load characteristics at 1.4 MV.

Load Current (mA)	Input V	Input A	kW	Power Factor	% efficiency
0.16	148	23.6	1.81	0.52
1.0	159	46.9	4.40	0.59	35
2.0	166	54	5.83	0.65	48
5.0	180.5	80	10.95	0.757	64
10.0	192	120	19.35	0.84	72.3
15.0	200	158	27.50	0.87	76.5

Another similar system was built by Koch and Sterzel in 1940 to reach voltages of about 900 kV. It used 4 stages, with capacitors of 0.22 µF and Siemens-Reiniger rectifiers rated at 320 kV max inverse voltage. The heaters were powered by generators driven by a shaft from the ground. This system used the scheme of highly insulated isolation transformers to send the power up the stack, and separate HV transformers feeding the rectifiers. The individual rectifiers were eventually replaced by a series of rectifiers all in a common vacuum stack which was continuously pumped. Each of these rectifiers had an internal resistance of 30-60 kOhm and a filament current of 9.5 A provided a saturation current of 100 mA. The system was also modified to allow its use as a 2MV Marx Generator, and for AC output, as well.

The Phillips company developed a technique of passing the heater current up the stack through capacitors at high frequencies.

Modern systems of this type tend to use air or ferrite cored transformers, and modern switching type power supplies to excite the bottom stage. And, of course, semiconductor rectifiers are almost universally used, since they eliminate the need to pass heater power up the stack. Vacuum tube rectifiers do have the advantage of better immunity to transients and momentary overloads. However, the cost difference of $10 for a 40 kV semiconductor rectifier and $100 for an equivalent vacuum tube becomes significant when 10 or 20 stages are contemplated.