GB2100533A - Static system for controlling the speed of rotating anodes in X-ray tubes - Google Patents

Abstract

An arrangement is provided for controlling the speed of a rotating anode of an X-ray tube in which the anode forms part of the rotor of a single phase motor. The arrangement comprises a power supply circuit 4, which includes a single phase static transistor inverter and pulse width modulator, and a microprocessor-based control circuit 3. The control circuit supplies control signals to the pulse width modulator, in accordance with desired speed of the anode, to control the frequency, voltage, and waveform of the inverter output, which is supplied to the motor stator. The arrangement also controls braking which is effected by applying d.c. to the stator. <IMAGE>

Description

SPECIFICATION Static system for controlling the speed of rotating anodes in X-ray tubes The present invention refers to a static system for controlling the speed of rotating anodes (Rapid Starter TIRC) in order to accelerate the anode of the X-ray tube from 0 to 10,400 r.p.m., 0 to 3,400 r.p.m. and 3,400 to 10,400 r.p.m., in short periods of time, to maintain the speed acquired with a low dissipation and to brake it, utilizing new control and power technologies.

Distinct acceleration signals originating from the generator produce different operating sequences for the acceleration starter, maintenance and braking.

Three different types of X-ray tubes can be controlled simultaneously, whether a combination of three identical or three different tubes. Each type of tube requires different acceleration and braking times.

The system comprises a control of the power, frequency and voltage to supply a single-phase motor having two alternating current voltage windings, which windings of the stator are arranged about the X-ray tubes. The rotor constitutes the rotating anode whose dimensions and inertia vary depending on the type of power.

The power circuit is an inverter connected to a bridge of transistors which control the frequency and the power by means of voltage pulse modulation. The control circuit comprises the control step (control of waves) of transistors having a variable frequency by means of a microcomputer and static protecting systems.

The network supply voltage, single-phase or three-phase, 50 or 60 Hz, is converted to direct current voltage by means of rectification and filtration.

This direct current voltage is converted to alternating current voltage is square type waves, with or without pulse modulation which, through a transformer, feed, in the secondary circuit, the windings of the stator.

This system is controlled by a microcomputer, the functions of which are: interphase with the generator, control and generation of frequency, modulation control depending on various parameters, satisfactory acceleration control to protect the tube, and communication to the X-ray generator system of the operating status.

The present invention constitutes a new starting point to simplify and reduce the cost with respect to other conventional thyristor systems having a forced switching and/or transformers for adjusting the voltage.

Conventional systems presently use voltage stabilizers to adjust the output voltage of the system; in the power step it is constituted of forced switching thyristors and two or more voltage transformers to effect other accelerations having a different frequency, such as for example 180 Hz, 400 Hz or other intermediate and/or higher frequencies, depending on the type of X-ray tube.

By producing accelerations of the motor at the previously mentioned frequencies, a saturation is produced which is derived from the use of a voltage transformer at two different frequency levels. This saturation produces intensity points which will unnecessarily cause an over-dimensioning of the power module.

The braking sequence of the rotating anode is typically carried out by means of an auxiliary transformer with its equipment corresponding to a reduced voltage or, alternatively, a static variation system by means of controlled silicon rectifiers or a direct current direct current converter.

In the event of excess current or short-circuit, the only way to prevent a major failure is to completely disconnect the equipment, since normally the control system does not permit an automatic recovery of the equipment.

In other cases, in the power module, it is necessary to install various transistors parallel to the arm of the bridge of the transistor inverter, whereby the reliability of the system is seriously affected.

The object of the present invention is to proportion a static system for controlling the speed of rotating anodes, whose characteristics are summarized in the following points: a) to control the supply voltage to the motor by pulse width modulation techniques in the following sequence: - maintenance of the speed at 180 Hz, 10,400 r.p.m. approximately, with a constant pulse width modulation at a likewise constant modulation frequency, in order to prevent an unnecessary overheating of the motor after the acceleration period thereof. On the other hand, the alternative of the conventional system is to use an auxiliary transformer with its equipment corresponding to a reduced voltage or, alternatively, a static voltage variation system by means of silicon rectifiers controlled by a Graetz bridge or a direct current/direct current converter.

The described control system represents an important simplification, reduction in cost and increase in the reliability when compared with the previously mentioned conventional systems.

- Maintenance of the speed at 60 Hz, 3,400 r.p.m. approximately, with a constant pulse width modulation at a likewise constant modulation frequency, in order to prevent an unnecessary overheating of the motor after the acceleration period thereof. On the other hand, the alternative of the conventional system is to use an auxiliary transformer with its equipment corresponding to a reduced voltage or, alternatively, a static voltage variation system by means of silicon rectifiers controlled by a Graetz bridge or a direct current/direct current converter.

The described control system represents an important simplification, reduction in cost and an increase in the reliability when compared with the previously mentioned conventional system.

b) To prevent saturation and its potential impact in the over-dimensioning of the transistor inverter, as well as the performance, referring to acceleration and losses of the transformer which supplies the motor. It likewise represents an important reduction in volume and cost.

c) To accelerate to 60 Hz, with a sinusoidal pulse width modulation of voltage.

The design of a single power transformer having fixed taps, calculated for 180 Hz, will produce a considerable limitation in the system due to the saturation at the frequency of 60 Hz. On the other hand, the alternative of the conventional system is to use an auxiliary transformer with its corresponding equipment for the operating frequency of 60 Hz.

d) To control the braking sequences from 10,400 r.p.m. to 3,400 r.p.m., from 10,400 r.p.m. to idle rotor, and from 3,400 r.p.m. to idle rotor, as follows: 10,4000 r.p.m. to 3,400 r.p.m.

Sinusoidal pulse width modulation of a 60 Hz voltage-wave. This system presents, among others, the advantage of being abie to adjust the braking time by means of mere changes in the software program of the microcomputer, without effecting hardware alterations or modifications.

10,400 r.p.m. to idle rotor Rectification of a 180 Hz square voltage wave with pulse width modulation, which is applied to the motor during the braking sequence.

3,400 r. p. m. to idle rotor Rectification of a 180 Hz square voltage wave with pulse width modulation which is applied to the motor during the braking sequence.

e) This is a dynamic system for limiting the intensity which permits functioning during an excess current or a short-circuit in the motor with a pulsating intensity, controlled with voltage pulse modulation and which prevents the power semi-conductors and fuses from being potentially damaged.

This system presents the advantage, when compared with conventional systems, of operating dynamically until the excess current level is reduced to a safety value, without having to partially or totally disconnect the equipment which could affect the functioning of the X-ray generator.

The most important advantages of this type of control, when comparedto conventional ones, are the following: a) It permits the elimination of the input autotransformer, necessary in all conventional equipments to adjust the network voltage shifts, furthermore presenting the important advantage of adjusting and compensating these variations dynamically.

b) The voltage applied to the motor by means of a network voltage detecting circuit, which once rectified and filtered, is converted to digital data, so that the microcomputer scans, compares and decides on the suitable voltage pulse width modulation in the acceleration systems at 180 Hz and 60 Hz, where the total acceleration time is critical for the life of the X-ray tube.

c) This system comprises a tube protection, by means of an indirect detection of the speed range in the specified time. The control system calculates the power developed during the acceleration sequence, by detecting the voltage and intensity applied to the motor which are integrated during the acceleration times and are compared with the permissible minimal values.

This system likewise permits a normal acceleration sequence to be discriminated from other different situations, such as acceleration under short-circuiting conditions where the voltage applied to the motor would be interrupted and modulated by the intensity limiting circuit, a fact detected by the previously mentioned control system.

d) The previously described braking control system represents an important simplification, reduction in cost and increase in the reliability with respect to the previously described conventional braking systems.

e) The equipment includes systems for protecting against short-circuits, loose cables or low voltages, to prevent the application of X-rays to the tube without being accelerated.

The equipment here described is a power transistor inverter having a digital control system.

The voltage at the output of the inverter is increased and through dephase condensers, is transmitted to the stator of the motor of the rotating anode.

In the drawings: Figure 1 is a block diagram of the static system for controlling the speed of rotating anodes in an X-ray tube.

Figure2 is a block diagram of the control of the system by a microcomputer.

Figure 3 is a simplified drawing of the power module, illustrating the transistor inverter.

Figure 4 illustrates the waveforms which reach the power transistors.

Figure 5 represents the conduction period of a power transistor (curve 2) and the pulse width modulation (curve 1). Curve 3 illustrates the square wave voltage of the transistor inverter and a sinusoidal voltage wave of the current in phase with the voltage.

Figure 6 represents the transformation of a sinusoidal modulation given by the microcomputer (curve 4).

Curve 5 illustrates the obtention of the sinusoidal wave resulting from the intersection of a delta wave with a sinusoidal wave.

Curves 6, 7, 8, 9 and 10 illustrate the conduction waves of the four transistors of the bridge of the power module with or without modulation.

Figure 7, at point 11, represents the excess current detecting system.

Curves 12, 13 and 14 illustrate the arrangement of the prior circuit with the excess current detected by the shunt.

Figures 8, 9, 10, 11, 12 and 13 represent diverse waveforms of the wave of the current and the voltage of the transistor inverter as a result of tests.

The static system for controlling the speed of rotating anodes is comprised of the following fundamental elements, in accordance with the block diagram of figure 1: 1. Input conditioner 2. Optical isolators 3. Control circuit by microcomputer 4. Power module 5. Adaptation transformer 6. Condenser and brake selector 7. Output selector 1.Input conditioners This circuit is comprised of circuits C-MOS having a high noise immunity which constitutes the interphase between the optic isolators and the microcomputer. The type of tube is binary codified to inform the microcomputer of the necessary acceleration and braking times (point A) and on the other hand, the system transmits an exposure permission signal to the X-ray generator (point B).

2.- Opticisolators An assembly of optic isolators receives the signals from the X-ray generator through the input conditioners, which have different sequences of speed, timings, information of the type of tube, etc.

The optical couplers accept alternating or direct current voltage signals to be connected to any X-ray generator, this being completely isolated electrically from the rotating anode accelerator and transmits this information to the microcomputer control circuit.

3.- Microcomputer control circuit The microcomputer complies with the following functions: - It receives the signals from the X-ray generator - It determines the operating frequency of the inverter - It proportions sinusoidal modulations - It controls the suitable modulation in each case.

- It modifies the modulation in accordance with the operating voltage - It controls that the current in the cables of the motor is adequate.

- It indicates the correct functioning of itself and indicates failures in the circuits - It sets the time of acceleration and braking and the specific sequences of the generator.

- It activates relays for operating the power step and for indicating to the X-ray generator correct functioning.

Figure 2 illustrates in detail the main parts of this circuit, which comprises: - Input buffers and tube codes (block 8) which pick up the signals from the optical couplers and transmit this information to the microcomputer.

- A microcomputer control (block 9) which controls the complete system.

The microcomputer used includes a programming memory of 1K x 8 and has 27 possible inputs/outputs.

This circuit receives information from the generator and sets the acceleration times for different types of tube, frequency, modulation, protection verifying signals, and signals for relay activation and signals to the generator and failure information.

The frequency given by the microcomputer depends on the speed selected. For a slow speed, the conduction signals of the transistors have a sinusoidal modulation to prevent saturation in the transformer 5 and increases the performance.

To accelerate the anode, in a very short period of time, a high power, 4 Kwfor high speed and 1.8 Kwfor low speed, is applied.

Once the microcomputer is accelerated, it transmits modulation signals which lower the conduction times of the transistors, avoiding unnecessary heatings of the motor of the anode, reduce the applied power and maintain the speed ofthe motor.

When the tube should be braked, the microcomputer transmits modulation signals to brake the anode from 10,400 to 3,400 or a complete braking.

- A control logic (block 10), comprised of a series of logic circuits TTL, sets the inputs of the microcomputer to activate the conduction of the bridge of the power transistors. This block also includes the current limiting protection circuits, recutting of conduction waves and formation of the modulation.

- A voltage detector (block 11) which supervises the input voltage to the power module and detects the minimal supply voltage value below which functioning of the power is prohibited. This circuit is provided with a hysteresis to prevent fluctuations.

4.-Power module Includes the power inverter comprised of a bridge of transistors and diodes, a voltage step-up transformer and dephase condensers for the motor and a braking circuit consisting of a rectifier and a relay.

Three relays select the output for three possible tubes.

The transistor inverter comprises four Motorola power transistors MJ-10016 arranged as indicated in the scheme of Figure 3.

The switching and the square wave voltage is produced by trigerring simultaneously two transistors of the same diagonal alternately, as illustrated in Figure 3.

Thetriggering signals which reach the powertransistorsare indicated in Figure 4.

The basic conditions for the functioning of the transistors of the power inverter are: 1. Triggering on of the transistors of the same diagonal is simultaneous.

2. Triggering off of two transistors of the same diagonal takes place firstly in that situated above and 100 s thereafter that situated below, due to the pulse width of the frequency generator (master frequency).

3. In the logic circuits, these waves are recut in the 80 s increasing edge to prevent two transistors from being triggered simultaneousiy in the same vertical, which could occur due to a delay in the power circuit fundamentally.

The inverter, besides generating a square wave alternating voltage, controls the conduction period of the transistors and modulates the pulse width by means of the transistors Q32 and Q13. The transistors Q41 and 012 do not have modulation and remain in conductionforl80 , according to curves 1 and 2 of Figure 5.

Modulation and conduction in a diagonal.

Thus, the power supplied to the motor is controlled in: Acceleration to 3,400 r.p.m.

Maintenance at 3,400 r.p.m.

Maintenance at 10,400 r.p.m.

Braking Only in the acceleration to 180 Hz (10,400 r.p.m.) is the power maximum and, therefore, no modulation exists.

The power supplied by the inverter during acceleration to 180 Hz is that obtained by one square wave voltage and one sinusoidal current wave in phase with the voltage (curve 3 of Figure 5).

The peak intensity of the current wave is 280 A, equivalent to 20 Arms and the power P=V.I.= 320x20 = 6,400 KW.

For 60 Hz and due to saturation problems in the transformer, there has been selected a sinusoidal modulation given by the microcomputer and which consists in 9 pulses having a variable width as illustrated in the curve 4 of Figure 6.

The obtention of the sinusoidal wave in pulses is the result of the intersection of a delta wave with a sinusoidal wave in the curve 5 of Figure 6, where the amplitude of the sinusoidal wave has been selected to obtain a rms voltage value of 175 volts in the primary of the transformer, according to the optimum experimental results.

A similar waveform, but with a lesser equivalent amplitude, is used for the braking from 10,400 r.p.m. to 3,400 r.p.m. The effective voltage in this case in the primary of the transformer is equivalent to 120 volts.

Figure 6, curves 7 to 10 illustrate the conduction waves of the four transistors of the power bridge with or without modulation.

The intensity limiting circuit recuts the conduction waves to limit the current in the power transistors and to protect with a wide safety margin the semi-conductors, especially the power transistors, curve 6 being the schronization frequency wave.

This circuit should be sufficiently rapid to prevent, in case of short-circuit, the current from raising above 35 amps, leaving a safety margin of 20 A (maximum limit for the MJ-10016 75A).

Figure 7, scheme 11, schematically illustrates the excess current detection circuit.

A Schmitt gate is triggered with the excess current detected in a shunt. The effects caused by the cut in conduction is illustrated in Figure 6, and with the waveforms of the curves.

5.-Adaptation transformer The adaptation transformer steps-up the voltage in a transformation ratio of 1.4. The output of the transformer is applied to the following block.

6.- Condenser and brake selector (Block 6) The secondary of the transformer 5 requires dephase condensers for the acceleration of the motor. The different operating frequencies require the change of condensers, by means of selecting relays. The relay selecting control takes place by the microcomputer which controls the complete system.

To brake the anode by applying direct current to the windings of the stator, the output of the transformer 5 is rectified and applied to the motor. A power contactor applies the secondary of the transformer to a rectifying bridge.

7.-Output selectors (Block 7) The output of the anode accelerator can be applied by selection of the X-ray generator to three possible tubes.

Within this block there is included a current detector for each supply cabie to the motor, comprised of small current transformers which transmit the signal to the microcomputer. The function of the current detectors is that of preventing, in the case of open cable, short circuiting or failure in any part of the equipment which produces an insufficient acceleration current to the motor, power from being supplied to the X-ray tube which would destroy or damage the anode.

Figures 8, 9, 10, 11, 12 and 13 illustrate the results obtained with a static control system for controlling the speed of rotating anodes in X-ray tubes.

Figure 8 represents the current and voltage of the output of the power module when accelerating to 180 Hz (10,400 r.p.m.).

Voltage: 200 volts/division Current: 10 Amps/division Time: 2 milliseconds/division Figure 9 indicates the voltage and the current of the output of the power module when maintaining the speed at 10,400 r.p.m., modulated wave of 180 Hz, effective voltage 75 V, peak voltage 320.

Voltage: 200 volts/division Current: 2 Amps/division Time: 1 millisecond/division Figure 10 illustrates the acceleration to 60 Hz (3,400 r.p.m.), with a sinusoidal modulation. The current can be seen as sinusoidal wave.

Voltage: 200 volts/division Current: 1 O 10 Amps/division Time: 5 milliseconds/division Figure 11 illustrates the waveforms when maintaining the speed at 60 Hz (3,400 r.p.m.). Effective voltage 75 volts approximately.

Voltage: 200 volts/division Current: 2 amps/division Time: 2 milliseconds/division Figure 12 illustrates the waveforms during braking, using d.c. with modulation, from 3,400 or 10,400 r.p.m.

to complete braking.

Voltage: 440 volts Current: 2 amps/division Time: 1 millisecond/division Figure 13 represents the waveforms during braking from 10,400 to 3,400 r.p.m., with a sinusoidal modulation.

Voltage: 300 volts/division Current: 5 amps/division Time: 10 milliseconds/division

Claims (22)

1. An arrangement for controlling the speed of a rotating anode of an X-ray tube in which the anode forms the rotor of a single phase motor, comprising a power supply circuit, including a single phase static transistor inverter and pulse width modulation means for controlling the output voltage, waveform, and frequency of the inverter to be supplied to the motor stator, and a microprocessor-based control circuit for supplying control signals to the pulse width modulation means to control the motor.

2. An arrangement for controlling the speed of a rotating anode of an X-ray tube, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

3. An X-ray apparatus including an arrangement as claimed in claim 1 or 2.

4. Static system for controlling the speed of rotating anodes in X-ray tubes, characterised by a system for controlling the power, frequency and voltage to supply a single-phase motor having two alternating current voltage windings which constitute the stator and are arranged about the X-ray tube, so that the rotor of the motor is, in turn, the rotating anode of said tube, and whose dimensions and inertia vary depending on the type and the power, mainly comprised in combination of: a) A power system in the range of from 6 to 10 KW, which firstly converts the alternating current voltage, single-phase or three-phase, 50 or 60 Hz, to direct current filtered through a rectifier and a condenser; this direct current voltage is converted to alternating current through a mono-phase transistor inverter, to square type waves, with or without pulse modulation which, through a transformer supply, in the secondary circuit, the windings of the stator of the motor.

b) A microcomputer control system which constitutes the main element of the control area, which complies with the following main functions: - Interphase with the generator - Control and adjustment of frequency - Modulation control.

- Satisfactory acceleration control for protecting the tube.

- Communicating to the X-ray generator the operating status.

5. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 4, characterised in that the supply voltage to the motor is controlled by pulse width modulation techniques, sinusoidally, in the acceleration technque to 50/60 Hz, with a modulation frequency adjustable by software program, typically in the range of 1.5:2 KHz.

6. Static system for controlling the speed of rotating anodes of an X-ray tube according to claim 5, characterised in that two or more voltage transformers are eliminated to effect other accelerations at different frequencies, such as 180 Hz, 400 Hz or other intermediate and/or higher frequencies, depending on the type of X-ray tube.

7. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 5, characterised in that the saturation which is derived from the use of a voltage transformer having two different frequency levels is prevented, such as those produced when accelerating the rotating anode to 180 Hz, 400 Hz or other intermediate and/or greater frequencies; and then, if necessary, effecting accelerations at 50 Hz or 60 Hz with lower-power techniques.

8. Static system for controlling the speed of rotating anodes in X-ray tubes according to claims 5 and 7, characterised in that an unnecessary overdimensioning in the power transistor inverter, due to the intensity points derived from the saturation, is prevented.

9. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 5, characterised by an improvement in performance in the acceleration to 50/60 Hz and a reduction in losses, in the range of 30%, in the voltage transformer, when saturation is prevented.

10. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 4, characterised by controlling the supply voltage of the motor by constant pulse width modulation techniques ("on/off"), with the same power inverter as in the preceding claims, at the speed maintenance sequence of 60 Hz to 400 Hz, 3,400 r.p.m. to 20,000 r.p.m. typically and even with intermediate or higher frequencies, in order to prevent an overheating of the motor after the acceleration period thereof, contrary to the conventional technique of using an auxiliary transformer with its equipment corresponding to a reduced voltage or, alternatively, a static voltage connection system by silicon rectifiers controlled by Graetz bridge or d.c./d.c. converter.

11. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 4, characterised by controlling the braking sequence of the alternating current motor from freqencies higher than 400 Hz to 60 Hz or intermediate frequencies up to 60 Hz, by sinusoidal pulse width modulation, to adjust the braking time by mere changes in the software program of the microcomputer, without effecting alterations or modifications of the control electronic circuit.

12. Static system for controlling the speed of rotating anodes in X-ray tubes according to claims 4 and 11, characterised by controlling the braking sequence from frequencies higher than 400 hz to idle rotor, by the rectification of a square voltage wave with pulse width modulation and whose braking time is adjustable by a software program in the microcomputer, contrary to the conventional technique of using an auxiliary transformer with its equipment corresponding to a reduced voltage or, alternatively, a static variation system by controlled silicon rectifiers or a d.c./d.c. converter.

13. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 4, characterised by a dynamic limitation in intensity, which permits operation during an excess current or a short-circuit in the motor, producing a pulsating modulated intensity which prevents a potential damage of the power semi-conductors and fuses, and which does not require a disconnection of the equipment, thereby permitting an automatic recovery when the excess current or short-circuit disappears, all of which represents an important advantage when compared to conventional systems which, on the other hand, operate by completely disconnecting the equipment when an excess current or a short circuit is produced, in deterioration of the functioning of the X-ray generator.

14. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 13, characterised in that the hysteresis levels are adjustable depending on the type of power transistor used in the transistor inverter, wherefore the triggering and inhibition levels in the upper transistors of the inverter can be adjusted.

15. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 4, characterised in that it incorporates a microcomputer control circuit which automatically varies the voltage pulse width by the detection of the network voltage which, once rectified and filtered, is converted to digital detector, wherefore the microcomputer scans, compares and decides on the adequate voltage pulse width modulation, in acceleration techniques comprised typically at frequencies of 400 Hz to 60 Hz, likewise including intermediate and/or higher frequency ranges than said frequencies, where the total acceleration time is critical for the life of the X-ray tube.

16. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 15, characterised in that the input autotransformer, necessary in all conventional equipments to adjust the network voltage shifts, is eliminated.

17. Static system for controlling the speed of rotating anodes in X-ray tubes according to claims 15 and 16, characterised in that the network voltage shifts are adjusted and compensated dynamically with a closed digital loop, controlled by a microcomputer and by automatic adjustment of the voltage pulse width.

18. Static system for controlling the speed of rotating anodes in X-ray tubes according to claims 4 and 17, characterised by an X-ray tube protecting system, by the indirect detection of the speed range in the specified time, without using speed transducers, where the control of the protecting system calculates the power developed during the acceleration sequence, by the detection of the voltage and the intensity applied to the motor, which are integrated during the acceleration time, by means of the microcomputer of the control system which compares this integration power with permissible minimal values, depending on the type of tube which appears in the software program.

19. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 18, characterised in that it discriminates a normal acceleration sequence from other different situations which could be produced, such as the acceleration under short-circuiting conditions, where the voltage applied to the motor would be recut and modulated by the intensity limiting circuit.

20. Static system for controlling the speed of rotating anodes in X-ray tubes according to claim 4, characterised in that it incorporates a transistor power inverter, in bridge connection, single-phase, with the amplification transistors thereof (drivers) controlled by control logic, whose cutting, saturation and triggering circuits guarantee a high reliability in operation, by the use of a single power transistor at the arm of the bridge (a total number of 4), in comparison with other conventional systems which use various parallel power transistors due to the limitation in the control technique applicable to highly reliable systems.

21. Static system for controlling the speed of rotating anodes in X-ray tubes according to claims 4 to 19, characterised in that the proposed system proportions an importantsimiplification in power and control circuits, a reduction in space and volume in the range of 40%, a reduction in cost in the range of 50%, when compared to other conventional forced-switching thyristor systems, where reactances, transformers and additional switching condensers are necessary with respect to the transistor power inverter described.

22. Static system for controlling the speed of rotating anodes in X-ray tubes according to the preceding claims 4 to 21, characterised by proportioning an increase in reliability in the range of 30%, in mean failure time of components and in the protection area of the tube, relative to the dynamic intensity limiting circuit and, on the other hand, the indirect detection of the speed developed during the acceleration sequence.