US3832553A

US3832553A - Circuit for a rotary anode x-ray tube

Info

Publication number: US3832553A
Application number: US00369394A
Authority: US
Inventors: G Seifert; K Franke
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1971-10-27
Filing date: 1973-06-12
Publication date: 1974-08-27
Anticipated expiration: 1991-08-27

Abstract

A circuit is used for actuating the driving motor of a rotary anode of an X-ray tube. The driving motor is an a.c. asynchronous motor fed from a converter. The converter is provided with actuating means which increase the frequency of the feeding voltage during the starting of the rotary anode from an initial value to an end value so as to provide the shortest possible starting time. The device of the present invention is a component part of an X-ray apparatus feeding an X-ray tube.

Description

United States Patent Seifert et al.

[ Aug. 27, 1974 I CIRCUIT FOR A ROTARY ANODE X-RAY TUBE [75] Inventors: Gerd Seifert; Kurt Franke, both of Erlangen, Germany [73] Assignee: Siemens Aktiengesellschaft, Elangen,

Germany 22 Filed: June12, 1973 21 Appl. No.: 369,394

Related US. Application Data [63] Continuation-impart of Ser. No. 192,858, Oct. 27,

1971, abandoned.

8/1967 Wright, Jr 250/406 9/1969 Studtmann et al.... 3l8/231 X Primary ExaminerWilliam F. Lindquist Attorney, Agent, or Firm-Richards & Geier; V. Alexander Scher [57] ABSTRACT A circuit is used for actuating the driving motor of a rotary anode of an X-ray tube. The driving motor is an ac. asynchronous motor fed from a converter. The converter is provided with actuating means which increase the frequency of the feeding voltage during the starting of the rotary anode from an initial value to an end value so as to provide the shortest possible starting time. The device of the present inventionis a component part of an X-ray apparatus feeding an X-ray tube.

[56] References Cited UNITED STATES PATENTS 4 Claims 7 Drawing Figures 3,205,360 9/1965 Graves 250/406 l3 i2 if. control I oscmut I impulse. evice wider 6 fier onver ome tef PATENTED AUG 2 7 I974 sum 2 or 2 CIRCUIT FOR A ROTARY ANODE X-RAY TUBE The present application is a continuation-in-part of out copending patent application Ser. No. 192,858 filed Oct. 27, 1971, now abandoned.

This invention relates to a circuit for actuating a driving motor, consisting of an asynchronous motor, of a rotary anode of an X-ray tube, with at least one converter which changes the frequency of the feeding voltage relatively to the network frequency.

It is known to drive the rotary anode of an X-ray tube by-a one phase asychronous motor with an auxiliary winding, the motor being fed from a converter which supplies an outgoing voltage with a frequency of 150 Hz increased relatively to the network frequency. When going over from X-ray radioscopy to X-ray exposure the driving motor is switched on and the rotary anode runs after starting with a speed of about 8,500 rpm. The X-ray tube is comparatively highly charged due to this high rotary speed of the anode required by frequency changing.

The starting time after the end of which the anode reaches its final rotary speed when using a specific driving motor, depends in the known construction upon the voltage applied to the driving motor. Since this voltage cannot be raised at will due to technical insulation reasons, a reduction of the starting time is limited.

In order to move quickly from radioscopy to exposure it is desirable to keep the starting time of the rotary anode as small as possible. However, in known construction with acceptable expenditure it is not possible to lower the starting times below a lower end value of about 0.8 seconds.

An object of the present invention is to improve the circuit of the described type by making possible a very short starting time for the rotary anode with a comparatively small technical circuit expenditure, particularly for a comparatively low feeding voltage of the driving motor.

Other objects will become apparent in the course of the following specification.

In the accomplishment of the objectives of the present invention it was found desirable to provide the converter with actuating means which increase the frequency of the feed voltage during the starting time of the rotary anode corresponding to the course of the pull-out torque from an initial value to a second higher value and then decreasing to an end value which is intermediate the initial and second value so as to produce the shortest possible starting time. The present invention proceeds from the consideration that the greater is the starting moment of an asynchronous motor, the lower is the frequency of the feed voltage. The moment has its highest value during the starting period in each time period when the speed frequency and the number of rotations are so great that there is a pull-out torque. The present invention by changing the frequency of the feed voltage, produces an increase in the rotary moment of the driving motor during its starting, as compared to the prior art device wherein the frequency of the feed voltage remains constant during the entire starting period, namely, it has its final value. Thus the present invention provides a substantial shortening of the starting period.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawing showing by way of example only, a preferred embodiment of the inventive idea.

In the drawing:

FIG. I is a diagram showing the development of the rotary moment.

FIG. 2 is a block circuit of an embodiment of the present invention.

FIG. 3 is a diagram showing the construction of the converter.

FIG. 4 is a diagram illustrating the flow of voltages.

FIG. 5 is an example for the oscillator according to FIG. 2.

FIG. 6 is an example for the device 13 in FIG. 2.

FIG. 7 is the course of the voltage at the output of the device 13.

FIG. 1 illustrates the development of the rotary moment depending upon the r.p.m. of an asynchronous motor for different frequencies of the feed voltage.

Curve 1 is, for example, for a frequency of 50 Hz, curve 2 for a frequency of Hz and curve 3 for a frequency of about Hz. FIG. 1 shows clearly that the greater the initial rotary moment of an asynchronous motor, the lower is the frequency of the feed voltage.

Therefore, in accordance with the present invention the driving motor for the rotary anode of an X-ray tube, which is an asynchronous motor, is initially driven with a low frequency of, for example, 50 Hz and the frequency is continuously raised during the starting corresponding to the rise of the rotary speed of the driving motor until the desired end value is reached. In this manner the starting time can be greatly reduced as compared to a device wherein the driving motor is fed with a feed voltage of end frequency already at the time it is switched on.

A best starting time period is attained when the frequency of the feed voltage is increased corresponding to the development of the pull-out torque above the end value to a value which provides the starting of the asynchronous motor up to the reaching of the final rotary speed corresponding to the development of the pull-out torque, namely, with a pull-out torque corresponding to the prevailing feed frequency. FIG. 1 shows a curve 4 representing the rotary moment characteristic of the driving motor for that particular case. The curve 4 has a starting frequency of 50 Hz (curve I), a highest frequency of about 180 Hz corresponding to the development of the pull-out torque (curve 3) and an end frequency of 150 Hz (curve 2), of the feed voltage. FIG. 1 indicates with M1, M2 and M3 the pull out torques taking place at feed frequencies of 50 Hz, 150 Hz and 180 Hz. Thus the pull-out torque develops depending upon the feed frequency corresponding to the curve 4. The above-mentioned frequencies relate to a 50 Hz power system.

FIG. 2 illustrates a block circuit of an embodiment of the present invention. The driving motor for the rotary anode 26 of the X-ray tube 27 is a one phase asynchronous motor having a rotor 5, a main winding 6 and an auxiliary winding 7. The windings 6 and 7 are fed from converters 8 and 9 which are connected to the outlet of a rectifier 10. The rectifier 10 can be connected by for the windings 6 and 7, can be adjusted. For that purpose serves a driving oscillator 12 the frequency of which can be varied by a device 13 during the starting time period of the rotor 5 from an initial value to an end value. The oscillator 12 drives the converters 8 and 9 through an impulse divider 14.

FIGS. 3 and 4 show the main construction of the converters 8 and 9 and the development of voltages appearing at the windings 6 and 7. The converter 8 includes a pair of contacts and a pair of contacts 16 which connect the main winding 6 alternately to direct voltage. Similarly the converter 9 has

contact pairs

17 and 18 which connect the auxiliary winding 7 alternately to direct voltage, namely, to the outlet voltage of the rectifier 10. The converters 8 and 9 are reverse pole switches so that substantially rectangularly shaped voltages appear at the windings 6 and 7.

In order to start the motor, the voltage at the main winding 6, the development of which is shown by the curve 19 of FIG. 4, is phase shifted by 90 relatively to the voltage in the auxiliary winding 7, the development of which is shown by the curve 20 in FIG. 4. After the closing of the switch 11, namely, when going over from illumination to exposure, as shown in FIG. 1, the contact pair 17 is closed at the time moment 21 and the contact pair 18 opened and thereupon at the time moment 22 the contact pair 15 is closed and the contact pair 16 is opened, at the time moment 23 the contact pair 17 is opened and the contact pair 18 is closed, at the time moment 24 the contact pair 16 is closed and the contact pair 15 is opened and at the time period 25 again the contact pair 17 is closed and the contact pair 18 is opened, so that the described cycle is now periodically repeated.

The alternating actuation of contact pairs 15 to 18 is operated by the impulse divider 14 which supplies the outgoing impulses of the oscillator 12 alternately to the converters 8 and 9. The impulse divider 14 thus provides that the current flowing through the main winding 6 is shifted in phase relatively to the current flowing through the auxiliary winding 7.

As indicated in FIG. 4, the frequency of the oscillator 12 must be four times as great as the frequency of the feed voltage of the windings 6 and 7. Starting with the assumption that to produce a quick start of the rotor 5 the frequency of the feed voltage must be raised from 50 to 180 Hz and then lowered to the end frequency of I50 HZ, then the frequency of the outgoing voltage of the oscillator 12 must exceed the range between 200 Hz and 720 Hz. The switch device 13 thus increases continuously the frequency of the oscillator 12 after the closing of the switch 11 from 200 Hz to 720 Hz and then lowers it to 600 Hz. The function followed by the frequency change and the time period during which this takes place are fixed once and for all for a specific tube, namely, for a specific driving motor so as to produce the best possible starting time.

It was determined in actual practice, that the change of frequency of the feed voltage of the driving motor for the rotary anode, carried out in accordance with the present invention, makes it possible to shorten the starting time period to less than 0.5 seconds.

In the actual construction of the device shown in FIG. 2, for the contact pairs 15 to 18 are preferably used electronic switches, for example, thyristors which operate substantially without inertia and losses and with little wear.

Obviously, in accordance with the present invention I the one phase asynchronous motor can be replaced by a three-phase asynchronous motor. In that case the two converters 8 and 9 are replaced by a converter providing an outgoing three-phase voltage and operated by an actuating device in such manner that its outgoing voltages passes through the desired frequency range during the starting of the motor.

A braking of the driving motor and of the rotary anode can be provided in a simple manner by operating the converters so that in at least one of the two converters a contact pair is closed during a few seconds while the other one remains open, so that the corresponding motor winding is subjected to direct voltage.

With reference to FIG. 1 there was described a change of the frequency of the feed voltage corresponding to the development of the pull-out torque (curve 4). This produces the best starting time.

FIG. 5 shows an example of the circuit connections of the operating oscillator 12. The oscillator 12 includes a field effect transistor 28 the operative stretch of which includes a condenser 29 and which is provided with feed voltage through

resistances

30 and 31. The transistor 28 operates jointly with the condenser 29 as an oscillator. The swingings of this oscillator are transmitted through a transistor 32 serving as an amplifier which is connected with two

resistances

33 and 34 for coupling to the transistor 28 and the voltage source, to an impulse distributor 14 through the line 35. The oscillator 12 along with the transistor 28 and the condenser 29 constitute a voltage-frequency converter. The outgoing frequency depends upon the voltage at the location 36 which is supplied through a coupling resistance 37. The timely running of this voltage is selected corresponding to the desired course in time of the outgoing frequency of the oscillator 12. Therefore the voltage at the location 36 initially rises according to FIG. 5 and then drops to an end value which corresponds to the final r.p.m. of the rotary anode.

FIG. 5 shows diagrammatically the course of the voltage in the line 35. It is apparent that the frequency starts by increasing with time.

As shown in FIG. 5, the actuating device 13 produces voltage at the location 36. It constitutes a function generator with a condenser 38, diodes 39 to 35, voltage dividers 46 to 52, a charging resistance 53 and a time member 54 (FIG. 6).

The outgoing voltage in the line 55 proceeds according to FIG. 7. When the main switch 56 is closed, the diode 39 switches during the time period t1 the poten' tial of the point 57 to the line 55. During this time period the condenser 38 is being charged and the potential at the location 58 reaches after the expiration of the time period [1 the potential at the location 57, so that now the diode 40 becomes conductive. After the expiration of the time period t1 the potential in the line 55 now rises corresponding to the charging of the condenser 38 through the resistance 53. During this rise the diodes 41 to 45 become conductive one after the other and switch the potential taken from the corresponding voltage dividers upon the condenser 38. The charging of the condenser 38 takes place accordingly in stages as indicated in FIG. 7. After the expiration of the time period [2 the time member 54 closes the switch 59 and switches the potential taken from the voltage divider 47 upon the condenser. The measurements of the voltage divider 47 are such that the voltage at the condenser 38 again drops somewhat, as shown in FIG. 7. I

By suitably dimensioning the parts of the function generator shown in FIG. 6, particularly the voltage dividers 46 to 52 and the charging resistance 53, the desired run of voltage at the location 36 (FIG. 5) can be attained. The feed frequency of the motors Sto 7 proceeds then corresponding to this voltage during the start of the rotary anode. By suitably dimensioning the function generator 13 it is thus possible to vary the feed frequency of the motors 5 to 7 during the starting of the rotary anode and thus also the starting of the motor, so

that the smallest possible starting time is produced.

What is claimed is:

1. In combination with a feed voltage supplying a.c. source, an X-ray tube having a rotary anode, an asynchronous a.c. motor driving said anode, at least one frequency converter having an outlet connected with said motor and an inlet connected with said source for supplying the motor with an adjustable frequency feed voltage, and means connected with said converter for increasing the frequency of the feed voltage during the starting of the anode corresponding to the rotary anode torque from an initial frequency value to a second higher frequency value and then decreasing to an end frequency value which is intermediate the initial and second value.

2. A device in accordance with claim 1, wherein said motor is a one phase motor with a main winding and an auxiliary winding, said device having two converters, one of said converters being connected with said main winding and the other one of said converters being connected with said auxiliary winding, said means comprising a device shifting the feed voltage for said auxiliary winding relatively to the feed voltage for the main winding to the extent of 3. A device in accordance with claim 2, wherein each frequency converter comprises a reverse pole switch, said converters having rectifier means connected with said source and supplying direct voltage to said reverse pole switches, and a common oscillator connected with said reverse pole switches and operating them with variable frequency.

4. A device in accordance with claim 3, comprising an impulse divider located between said oscillator and said converters and supplying the outgoing impulses of said oscillator to each of said converters to change its switching frequency.

Claims

2. A device in accordance with claim 1, wherein said motor is a one phase motor with a main winding and an auxiliary winding, said device having two converters, one of said converters being connected with said main winding and the other one of said converters being connected with said auxiliary winding, said means comprising a device shifting the feed voltage for said auxiliary winding relatively to the feed voltage for the main winding to the extent of 90*.

3. A device in accordance with claim 2, wherein each frequency converter comprises a reverse pole switch, said converters having rectifier means connected with said source and supplying direct voltage to said reverse pole switches, and a common oscillator connected with said reverse pole switches and operating them with variable frequency.