Abstract:
A motor control system has a rotary member, a motor for imparting a torque to the rotary member, a motor driving circuit for supplying power to the motor, and a drive control circuit for controlling a motor current supplied to the motor. An upper limit value of the motor current supplied to the motor is set so as to be variable. At the start of the motor, the drive control means controls the motor current by using the upper limit value as a set value.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a motor control system and a vacuum pump equipped with the motor control system, and more particularly to a motor control system capable of shortening the starting time of a turbo molecular pump and a vacuum pump equipped with the motor control system. 
   2. Description of the Related Art 
   As a result of recent developments in electronics, there is a rapidly increasing demand for semiconductor devices, such as memories and integrated circuits. 
   Such semiconductor devices are manufactured by doping semiconductor substrates of a very high purity with impurities to impart electrical properties thereto, by stacking together semiconductor substrates with minute circuit patterns formed thereon, etc. 
   In order to avoid the influences of dust in the air, etc., such operations must be conducted in a chamber in a high vacuum state. To evacuate the chamber, a vacuum pump is generally used; in particular, a turbo molecular pump, which is a kind of vacuum pump, is widely used since it involves little residual gas, allows maintenance with ease, etc. Further, a semiconductor manufacturing process involves a number of steps in which various process gasses are caused to act on a semiconductor substrate, and, the turbo molecular pump is used not only to create a vacuum state in the chamber but also to evacuate such process gases from the chamber. 
   Further, in electron microscope equipment, a turbo molecular pump is used to create a high vacuum state in the chamber of the electron microscope in order to prevent refraction of the electron beam, etc. due to the presence of dust or the like. 
   Further, a turbo molecular pump is used in a movable simple vacuum chamber, or in order to place a flat panel display manufacturing apparatus in a vacuum state. 
   Such a turbo molecular pump is composed of a turbo molecular pump main body  100  for sucking and evacuating gas from the chamber of a semiconductor manufacturing apparatus or the like, and a control device  200  for controlling the turbo molecular pump main body. 
     FIG. 7  shows the construction of a turbo molecular pump. 
   In  FIG. 7 , the turbo molecular pump main body  100  has an inlet port  101  formed at the upper end of a round outer cylinder  127 . On the inner side of the outer cylinder  127 , there is provided a rotary member  103  in the periphery of which there are formed radially and in a number of stages a plurality of rotary vanes  102   a ,  102   b ,  102   c , . . . consisting of turbine blades for sucking and evacuating gases. 
   Mounted at the center of this rotary member  103  is a rotor shaft  113 , which is floatingly supported and position-controlled by, for example, a 5-axis control magnetic bearing. 
   An upper radial electromagnet  104  consists of four electromagnets arranged in pairs in the X- and Y-axes. In close proximity to and in correspondence with the upper radial electromagnet  104 , there is provided an upper radial sensor  107  consisting of four electromagnets. The upper radial sensor  107  detects a radial displacement of the rotor shaft  113 , and transmits a displacement signal to the control device  200 . 
   In the control device  200 , the upper radial electromagnet  104  is excitation-controlled through a compensation circuit with a PID adjustment function (not shown in the drawings) based on the displacement signal obtained through detection by the upper radial sensor  107 , thus adjusting the upper radial position of the rotor shaft  113 . 
   The rotor shaft  113  is formed of a high magnetic-permeability material (such as iron), and is attracted by the magnetic force of the upper radial electromagnet  104 . This adjustment is conducted independently in the X-axis direction and the Y-axis direction. 
   Further, a lower radial electromagnet  105  and a lower radial sensor  108  are arranged in the same way as the upper radial electromagnet  104  and the upper radial sensor  107 , adjusting the lower radial position of the rotor shaft  113  in the same manner as the upper radial position thereof. 
   Further, axial electromagnets  106 A and  106 B are arranged so as to sandwich from above and below a circular metal disc  111  provided in the lower portion of the rotor shaft  113 . The metal disc  111  is formed of a high magnetic-permeability material, such as iron. 
   Further, below the rotor shaft  113 , there is provided an axial sensor  109  for detecting an axial displacement signal of the rotor shaft  113 . An axial displacement obtained through detection by the axial sensor  109  is transmitted to the control device  200 . 
   Based on the displacement signal obtained through detection by the axial sensor  109 , the control device  200  excitation-controls the axial electromagnets  106 A and  106 B. At this time, the axial electromagnet  106 A attracts the metal disc  111  upwardly by magnetic force, and the axial electromagnet  106 B attracts the metal disc  111  downwardly. 
   In this way, the magnetic bearing appropriately adjusts the magnetic force applied to the rotor shaft  113 , thereby magnetically levitating the rotor shaft  113  and retaining it in a non-contact fashion. 
   Further, there is provided a motor  121 , which is a so-called brush-less motor. The motor  121  is equipped with an RPM detecting sensor, a motor current detecting sensor, a motor temperature detecting sensor, etc. described below, and, on the basis of detection signals from these sensors, the RPM, etc. of the rotor shaft  113  are controlled by the control device  200 . The construction of the control system for the motor  121  will be described in detail below. 
   Formed on the rotor shaft  113  are the rotary vanes  102   a ,  102   b ,  102   c , . . . . There are arranged a plurality of stationary vanes  123   a ,  123   b ,  123   c , . . . , with a slight gap being between them and the rotary vanes  102   a ,  102   b ,  102   c , . . . . Further, in order to downwardly transfer through collision the molecules of the exhaust gas, the rotary vanes  102   a ,  102   b ,  102   c , . . . are respectively inclined by a predetermined angle with respect to planes perpendicular to the axis of the rotor shaft  113 . Similarly, the stationary vanes  123  are respectively inclined by a predetermined angle with respect to planes perpendicular to the axis of the rotor shaft  113 , and are arranged so as to protrude toward the interior of the outer cylinder  127  and in alternate stages with the rotary vanes  102 . 
   Further, one ends of the stationary vanes  123  are supported while being inserted into recesses between a plurality of stationary vane spacers  125   a ,  125   b ,  125   c , . . . stacked together. The stationary vane spacers  125  are ring-like members formed of a metal, such as aluminum, iron, stainless steel, or copper, or an alloy containing such metals as components. 
   Further, in the outer periphery of the stationary vane spacers  125 , the outer cylinder  127  is secured in position with a slight gap therebetween. At the bottom of the outer cylinder  127 , there is arranged a base portion  129 , and, between the stationary vane spacers  125  and the base portion  129 , there is arranged a threaded spacer  131 . In the portion of the base  129  which is below the threaded spacer  131 , there is formed an exhaust port  133 , which communicates with the exterior. 
   The threaded spacer  131  is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or an alloy containing such metals as components, and has on the inner peripheral surface thereof a plurality of spiral thread grooves  131   a . The direction of the spiral thread grooves  131   a  is determined such that, when the molecules of the exhaust gas move in the rotating direction of the rotary member  103 , these molecules are transferred toward the exhaust port  133 . 
   Further, in the lowermost portion of the rotary member  103  connected to the rotary vanes  102   a ,  102   b ,  102   c , . . . , there is provided a rotary vane  102   d , which extends vertically downwards. The outer peripheral surface of the rotary vane  102   d , is cylindrical, and protrudes toward the inner peripheral surface of the threaded spacer  131  so as to be in close proximity to the threaded spacer  131  with a predetermined gap therebetween. 
   Further, the base portion  129  is a disc-like member constituting the base portion of the turbo molecular pump main body  100 , and is generally formed of a metal, such as iron, aluminum, or stainless steel. The base portion  129  physically retains the turbo molecular pump main body  100 , and also functions as a heat conduction path, so that it is desirable to use a metal that is rigid and of high heat conductivity, such as iron, aluminum, or copper, for the turbo molecular pump main body  100 . 
   When, in this construction, the rotor shaft  113  is driven by the motor  121  and rotates together with the rotary vanes  102 , an exhaust gas from a chamber is sucked in through the inlet port  101  by the action of the rotary vanes  102  and the stationary vanes  123 . 
   Then, the exhaust gas sucked in through the inlet port  101  flows between the rotary vanes  102  and the stationary vanes  123 , and is transferred to the base portion  129 . At this time, the temperature of the rotary vanes  102  rises due to the friction heat generated when the exhaust gas comes into contact with the rotary vanes  102 , conduction of the heat generated in the motor  121 , etc, and this heat is transmitted to the stationary vanes  123  side by radiation or conduction due to the gas molecules, etc. of the exhaust gas. Further, the stationary vane spacers  125  are bonded together in the outer periphery, and transmit to the exterior the heat received by the stationary vanes  123  from the rotary vanes  102 , the friction heat generated when the exhaust gas comes into contact with the stationary vanes  123 , etc. 
   The exhaust gas transferred to the base portion  129  is sent to the exhaust port  133  while being guided by the thread grooves  131   a  of the threaded spacer  131 . 
   In the above-described example the threaded spacer  131  is arranged in the outer periphery of the rotary vane  102   d , and the thread grooves  131   a  are formed in the inner peripheral surface of the threaded spacer  131 . However, conversely to the above, the thread grooves may be formed in the outer peripheral surfaces of the rotary vane  102   d,  and a spacer with a cylindrical inner peripheral surface may be arranged in the periphery thereof. 
   Further, in order that the gas sucked in through the inlet port  101  may not enter the electrical section formed by the motor  121 , the lower radial electromagnet  105 , the lower radial sensor  108 , the upper radial electromagnet  104 , the upper radial sensor  107 , etc., the periphery of the electrical section is covered with a stator column  122 , and a predetermined pressure is maintained in the interior of the electrical section with a purge gas. 
   For this purpose, piping (not shown in the drawings) is arranged in the base portion  129 , and the purge gas is introduced through the piping. The purge gas thus introduced flows through the gaps between a protective bearing  120  and the rotor shaft  113 , between the rotor and stator of the motor  121 , and between the stator column  122  and the rotary vanes  102  before being transmitted to the exhaust port  133 . 
   Here, the turbo molecular pump main body  100  requires control based on individually adjusted specific parameters (e.g., the specification of the model and the properties corresponding to the model). To store these control parameters, the turbo molecular pump main body  100  has an electronic circuit portion  141 . 
   The electronic circuit portion  141  is formed by a semiconductor memory, such as EEP-ROM, an electronic component for the access thereto, such as a semiconductor device, a substrate  143  for the mounting thereof, etc. The electronic circuit portion  141  is accommodated in the lower portion near the center of the base portion  129  constituting the lower portion of the turbo molecular pump main body  100 , and is closed by a hermetic bottom cover  145 . 
   Incidentally, for enhanced reactivity, the process gas may be introduced into the chamber in a high temperature state. When it reaches a certain temperature by being cooled at the time of evacuation, such process gas may be solidified to precipitate a product in the exhaust system. Then, when such process gas is cooled and solidified in the turbo molecular pump main body  100 , it adheres to the inner wall of the turbo molecular pump main body  100  and is deposited thereon. For example, when SiCl 4  is used as the process gas in an Al etching apparatus, a solid product (e.g., AlCl 3 ) is precipitated when the apparatus is in a low vacuum state ( 760  [torr] to 10 −2  [torr]) and at lower temperature (approximately 20[° C.]), and adheres to and is deposited on the inner wall of the turbo molecular pump main body  100  as can be seen from a vapor pressure curve. 
   When precipitate of the process gas is deposited inside the turbo molecular pump main body  100 , the deposit narrows the pump flow path, which leads to a deterioration in the performance of the turbo molecular pump main body  100 . For example, the above-mentioned product is likely to solidify and adhere to the portion near the exhaust port, in particular, near the rotary vanes  102  and the threaded spacer  131 , where the temperature is low. 
   To solve this problem, there has been conventionally adopted a control system (hereinafter referred to as TMS (temperature management system)), in which a heater (not shown in the drawings), an annular water cooling tube  149 , etc. are wound around the outer periphery of the base portion  129  or the like, and in which a temperature sensor (e.g., a thermistor) (not shown in the drawings) is embedded, for example, in the base portion  129 , the heating by the heater and the cooling by the water cooling tube  149  being controlled based on a signal from the temperature sensor so as to maintain the base portion  129  at a fixed, high temperature (set temperature). 
   Here, a conventional motor control system will be described.  FIG. 8  shows the construction of the conventional motor control system. 
   In  FIG. 8 , a motor control system  300  is equipped with the motor  121  on the turbo molecular pump main body  100  side. 
   Further, the motor  121  is equipped with an RPM detecting sensor  124  on the stator side thereof. The RPM detecting sensor  124  is arranged so as to surround the rotor shaft  113 , and consists, for example, of a semiconductor Hall sensor. The RPM detecting sensor  124  detects the rotating magnetic flux density of the motor  121 , thereby detecting the RPM of the rotor shaft  113 . 
   Further, the motor  121  has on the stator side thereof three-phase motor windings  126 U,  126 V, and  126 W. These motor windings  126 U,  126 V, and  126 W are also arranged so as to surround the rotor shaft  113 . Further, the motor windings  126 U,  126 V, and  126 W are equipped with motor current detecting sensors  128  (only one of which is shown in the drawing), and the motor current detecting sensors  128  detect motor current Im flowing through the motor windings  126 U,  126 V, and  126 W. 
   Further, the motor windings  126 U,  126 V, and  126 W are connected to a motor driving circuit  220  on the control device  200  side. 
   A DC voltage is supplied to the motor driving circuit  220  from a power source  230  (In the drawing, the + side will be referred to as a positive pole  230   a , and the − side will be referred to as a negative pole  230   b ). The motor driving circuit  220  is equipped with inverter circuits  222  (only one of which is shown in the drawing) respectively corresponding to the motor windings  126 U,  126 V, and  126 W, and power is supplied to the motor windings  126 U,  126 V, and  126 W through the inverter circuits  222 . Each of these inverter circuits  222  is composed, for example, of two transistors  222   a  and  222   b  for one motor winding  126 U. 
   Further, a drive signal is input to the motor driving circuit  220  from a drive control circuit  210 . By this drive signal, the power supplied from the inverter circuits  222  to the motor windings  126 U,  126 V, and  126 W is controlled.  FIG. 9  is a block diagram showing the drive control circuit. 
   In  FIG. 9 , input to the drive control circuit  210  are detection signals from the RPM detecting sensor  124  and the motor current detecting sensors  128 . These detection signals are input to a comparator  212 . 
   Further, a command signal is input to the comparator  212  from a reference value setting circuit  214 , and this command signal is a signal indicating, for example, a reference RPM. 
   The comparator  212  compares, for example, the reference RPM indicated by the command signal with the detection signal from the RPM detecting sensor  124  to effect PID compensation, and, further, compares this output signal as a current command value with the detection signals from the motor current detecting sensors  128  to effect PID compensation. Thereafter, the comparator  212  outputs this output signal to a PWM control circuit  216 . 
   The comparator  212  is equipped with a current limiter circuit (not shown in the drawings), and this current limiter circuit performs control based on the comparison result output to the PWM control circuit  216  such that the motor current Im supplied to each of the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W does not exceed a fixed upper limit value. 
   Then, based on the comparison result from the comparator  212 , the PWM control circuit  216  performs pulse width control (PWM control) on the drive signal. 
   In this construction, the relationship between the motor current Im supplied to the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W and the rotating speed ω of the rotor shaft  113  is as expressed by Equation 1. 
   
     
       
         
           
             
               
                 E 
                 = 
                 
                   
                     L 
                     ⁡ 
                     
                       ( 
                       
                         ΔIm 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           t 
                         
                       
                       ) 
                     
                   
                   + 
                   
                     R 
                     · 
                     Im 
                   
                   + 
                   
                     K 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     ω 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 1 
               
             
           
         
       
     
   
   In Equation 1, L and R are the inductance component and the resistance component, respectively, of the motor windings  126 U,  126 V, and  126 W, and K is the counter electromotive force constant of the motor windings  126 U,  126 V, and  126 W; E is the drive voltage supplied from the inverter circuits  222 . 
   Further, the relationship of Equation 1 is also apparent from the equivalent circuit for the inverter circuits and the motor windings shown in  FIG. 10 . In  FIG. 10 , a power source  251  corresponds to the drive voltage E supplied from the inverter circuits  222 . An inductance  252  and a resistor  253  respectively correspond to the inductance component L and the resistance component R of the motor windings  126 U,  126 V, and  126 W. Further, an AC power source  254  corresponds to the counter electromotive force Kω generated in the motor windings  126 U,  126 V, and  126 W with the rotation of the rotor shaft  113 . 
   The drive voltage E supplied from the inverter circuits  222  is of a fixed value, and the inductance component L, the resistance component R, and the counter electromotive force constant K of the motor windings  126 U,  126 V, and  126 W are also values peculiar to the motor  121 . 
   Thus, the motor current Im that can be supplied to the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W is theoretically in conformity with Equation 1. In particular, the magnitude of the motor current Im when the rotating speed ω of the rotor shaft  113  is a rated RPM is referred to as the rated rotation current value Ir. 
   Here, when the rotor shaft  113  is to be rotated at the start of the turbo molecular pump, the RPM of the rotor shaft  113  and the reference RPM indicated by the command signal are compared with each other in the comparator  212 , and PID compensation is effected. Further, a current command value, which is the output result thereof, and the motor current Im flowing through the motor windings  126 U,  126 V, and  126 W are compared with each other to effect PID compensation. 
   Then, upon receiving this output result, the PWM control circuit  216  outputs a PWM-controlled drive signal to the inverter circuits  222  in the motor driving circuit  220 , whereby power is supplied to the motor windings  126 U,  126 V, and  126 W from the motor driving circuit  220 , and an AC voltage is generated in the motor windings  126 U,  126 V, and  126 W. Further, a torque corresponding to the motor current Im at this time is generated in the rotor shaft  113 . 
   At the start of the turbo molecular pump, a large current value is generated as the current command value in the comparator  212 ; however, the comparator  212  is equipped with a current limiter circuit, with the motor current Im supplied to the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W being controlled so as not to exceed an upper limit value set in this current limiter circuit. The upper limit value set in this current limiter circuit is the above-mentioned rated rotation current value Ir. 
   In this regard, at the start of the turbo molecular pump, the RPM of the rotor shaft  113  is less than the rated RPM, so that the counter electromotive force Kω is also less than the counter electromotive force Kω at rated rotation. Thus, at the start of the turbo molecular pump, according to the relationship of Equation 1, it is theoretically possible to supply a current value of not less than the rated rotation current value Ir, with the motor current being Im. However, to operate the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W reliably and safely in the range of the RPM of the rotor  113  from zero to the rated RPM, there is set, as the upper limit value set in the current limiter circuit, the rated rotation current value Ir, which is the motor current Im when the rotor shaft  113  is at rated RPM, to allow a leeway in terms of safety. 
   Here,  FIG. 11A  shows the relationship between the starting time of the turbo molecular pump and the RPM of the rotor shaft  113 , and  FIG. 11B  shows the relationship between the RPM of the rotor shaft  113  at this time and the motor current Im. The rated RPM of the rotor shaft  113  is approximately 37000 (rpm). 
   In  FIG. 11A , the turbo molecular pump is started at time  0 , and approximately 10 minutes thereafter, the RPM of the rotor shaft  113  attains the rated RPM. The time it takes the RPM of the rotor shaft  113  to reach the rated RPM is referred to as the starting time of the turbo molecular pump. 
   In  FIG. 11B , from immediately after the starting of the turbo molecular pump until the RPM of the rotor shaft  113  reaches a level near the rated RPM, the motor current Im remains fixed. This is due to the fact that the motor current Im supplied to the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W is controlled so as to be within a range not exceeding the upper limit value (the rated rotation current value Ir) set in the current limiter circuit. 
   Thereafter, when the RPM of the rotor shaft  113  reaches a level near the rated RPM, the motor current Im is reduced in accordance with a signal that has under gone PID compensation. During this period, the drive torque imparted to the rotor shaft  113  may be small, so that the quantity of electricity supplied from the motor driving circuit  220  to the motor windings  126 U,  126 V, and  126 W is reduced or the supply is stopped, with the result that the motor current Im is smaller than that immediately after the starting of the turbo molecular pump. 
   Incidentally, in the conventional motor control system  300 , at the start of the turbo molecular pump, a current value Ir at rated rotation is set as the upper limit of the motor current Im thereof, so that no torque in excess of the current value Ir at rated rotation is imparted to the rotor shaft  113 , and the starting of the turbo molecular pump cannot be said to be quick enough. 
   The shipment of the turbo molecular pump involves a process in which the starting and stopping of the turbo molecular pump is repeated in order to inspect and evaluate the same; since the starting of the turbo molecular pump is slow, there is a fear of the production period for the turbo molecular pump becoming rather long. 
   Further, also when, in a case in which the turbo molecular pump is used for a movable simple vacuum chamber, the turbo molecular pump is re-started, due to its slow starting, there is a fear of the vacuum degree in the chamber being reduced, resulting in a reduction in the service life of the material and measurement filament. 
   Further, from now on, a flat panel display will tend to increase in size, and the turbo molecular pump will also tend to increase in size in order to improve the vacuum performance of the manufacturing apparatus for such a large flat panel display. Thus, in a turbo molecular pump equipped with the conventional motor control system  300 , there is a fear of its starting becoming still slower. 
   SUMMARY OF THE INVENTION 
   The present invention is made in view of the above problems in the prior art, and an object thereof is to provide a motor control system capable of shortening the starting time of a turbo molecular pump and a vacuum pump equipped with the motor control system. 
   Therefore, the present invention relates to a motor control system including: a rotary member; a motor for imparting a torque to the rotary member; a motor driving circuit for supplying power to the motor; and drive control means for controlling a motor current supplied to the motor, characterized in that the motor current is controlled by using as a set value a value of a motor current Im calculated by Equation 1. 
   The motor current Im calculated by Equation 1 is a theoretical motor current value obtained from the motor and an equivalent circuit for the motor driving circuit. The motor current supplied to the motor is controlled by using this calculated value of the motor current Im as a set value. To control the motor current within the safety range for the motor and the motor driving circuit, it is desirable to determine a leeway in Equation 1. This leeway is obtained in advance by, for example, experiment or calculation. 
   This makes it possible to increase the motor current within the safety range for the motor and the motor driving circuit, thus making it possible to shorten the motor starting time. 
   The set value of the motor current is an upper limit value or a command value. 
   Further, the present invention relates to a motor control system including: a rotary member; a motor for imparting a torque to the rotary member; a motor driving circuit for supplying power to the motor; temperature detecting means for detecting one or both of a temperature of the motor and a temperature of the motor driving circuit; a characteristic curve defining one or both of a relationship between the temperature of the motor and a current value at continuous rating of the motor and a relationship between the temperature of the motor driving circuit and a current value at continuous rating of the motor driving circuit; set value setting means for, in accordance with the characteristic curve and based on the temperature detected by the temperature detecting means, setting the current value at continuous rating at the temperature as the set value of the motor current supplied to the motor; and drive control means for controlling the motor current such that the motor current does not exceed the set value set by the set value setting means. 
   In the drive control means, the motor current is controlled so as not to exceed the set value set by the set value setting means. At this time, in the set value setting means, in accordance with the characteristic curve and based on the temperature detected by the temperature detecting means, the current value at continuous rating of the motor and the motor driving circuit at this temperature is set as the set value. 
   Accordingly, the motor current is controlled within the range of the current value at continuous rating of the motor and the motor driving circuit. Thus, it is possible to prevent excessive supply of motor current to the motor and the motor driving circuit. 
   This makes it possible to increase the motor current while preventing breakdown of the motor and the motor driving circuit, thereby making it possible to shorten the motor starting time. 
   Further, the present invention relates to a motor control system including: a rotary member; a motor for imparting a torque to the rotary member; a motor driving circuit for supplying power to the motor; temperature detecting means for detecting one or both of a temperature of the motor and a temperature of the motor driving circuit; a characteristic curve defining one or both of a relationship between the temperature of the motor and a current value at short-term rating of the motor and a relationship between the temperature of the motor driving circuit and a current value at short-term rating of the motor driving-circuit; set value setting means for, in accordance with the characteristic curve and based on the temperature detected by the temperature detecting means, setting the current value at short-term rating at the temperature as the set value of the motor current supplied to the motor; and drive control means for controlling the motor current such that the motor current does not exceed the set value set by the set value setting means. 
   The characteristic curve defines the relationship between the temperature of the motor and the motor driving circuit and the current value at short-term rating of these components. 
   Further, in the set value setting means, in accordance with the characteristic curve and based on the temperature detected by the temperature detecting means, the current value at short-term rating of the motor and the motor driving circuit at this temperature is set as the set value. Thus, it is possible to control the motor current within a range not less than the current value at continuous rating. 
   Thus, it is possible to further shorten the motor starting time. Further, since the motor current is controlled so as not to exceed the permissible value at short-term rating of the motor and the motor driving circuit, it is possible to prevent their breakdown in a critical state. 
   Further, the present invention provides a vacuum pump equipped with a motor control system, characterized in that the vacuum pump is installed in target equipment and sucks a predetermined gas from the target equipment. 
   The motor control system of the present invention is mounted in a vacuum pump. 
   Accordingly, the process before shipment, which requires the vacuum pump to be started and stopped repeatedly, can be conducted quickly, thereby making it possible to shorten the production period for the vacuum pump. 
   Further, also when the vacuum pump is used in a movable simple vacuum chamber, the starting time for restarting the vacuum pump can be shortened, which makes it possible to regain the requisite vacuum degree inside the chamber promptly. Thus, it possible to increase the service life of the material and measurement filament in the chamber. 
   Further, even when the vacuum pump is increased in size to be in conformity with manufacturing apparatus for a large flat panel display, it is possible to shorten its starting time. Thus, it is possible to shorten the start-up time of the manufacturing apparatus for the large flat panel display. Further, when applied to a vacuum pump of conventional size, the present invention helps to shorten the start-up time of a semiconductor manufacturing apparatus, an electron microscope, etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagram showing the configuration of a motor control system according to an embodiment of the present invention; 
       FIG. 2  is a block diagram (No.  1 ) showing a drive control circuit; 
       FIGS. 3A and 3B  are graphs showing the relationship between turbo molecular pump starting time, rotor shaft RPM, and motor current; 
       FIG. 4  is a block diagram (No.  2 ) showing a drive control circuit; 
       FIG. 5  is a block diagram (No.  3 ) showing a drive control circuit; 
       FIG. 6  is a time chart showing an example of how motor current is controlled; 
       FIG. 7  is a diagram showing the construction of a conventional turbo molecular pump; 
       FIG. 8  is a diagram showing the construction of a conventional motor control system; 
       FIG. 9  is a block diagram showing a drive control circuit; 
       FIG. 10  is a diagram showing an equivalent circuit for an inverter circuit and a motor winding; and 
       FIGS. 11A and 11B  are graphs showing the relationship between turbo molecular pump starting time, rotor shaft RPM, and motor current. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will now be described. 
     FIG. 1  shows the configuration of a motor control system according to an embodiment of the present invention. The components that are the same as those of  FIG. 8  are indicated by the same symbols, and a description thereof will be omitted. 
   In  FIG. 1 , a motor control system  600  includes a turbo molecular pump main body  400 , in which motor windings  126 U,  126 V, and  126 W are equipped with a motor temperature sensor  424 . The motor temperature sensor  424  consists, for example, of a thermistor, which is adapted to detect the temperature of the motor windings  126 U,  126 V, and  126 W. 
   Further, the motor control system includes a control device  500 , in which also the inverter circuits  222  are equipped with an inverter temperature sensor  524 . The inverter temperature sensor  524  also consists, for example, of a thermistor, which is adapted to detect the temperature of transistors  222   a ,  222   b , etc. constituting each inverter circuit  222 . 
   Detection signals from the motor temperature sensor  424  and the inverter temperature sensor  524  are input to a drive control circuit  510  described below. 
     FIG. 2  is a block diagram showing this drive control circuit. The components that are the same as those of  FIG. 9  are indicated by the same symbols, and a description thereof will be omitted. 
   In  FIG. 2 , the drive control circuit  510  includes a comparator  512 , to which there are input the detection signals from the motor temperature sensor  424  and the inverter temperature sensor  524 . Further, as in the case of the conventional comparator  212 , input to the comparator  512  are a command signal from the reference value setting circuit  214  and detection signals from the RPM detecting sensor  124  and the motor current detecting sensor  128 . 
   Further, like the conventional comparator  212 , the comparator  512  compares, for example, a reference RPM indicated by the command signal with the detection signal from the RPM detecting sensor  124  to effect PID compensation, and then, further, compares this output signal as the current command value with the detection signal from the motor current detecting sensor  128  to effect PID compensation. 
   Further, this comparator  512  is equipped, instead of the current limiter circuit provided in the conventional comparator  212 , with a current adjustment circuit. In this current adjustment circuit, the upper limit value of the motor current Im is set so as to be variable in accordance with a characteristic curve shown below. 
   Here, the comparator  512  is equipped with a characteristic curve defining the relationship between the temperatures of the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  and the current value at continuous rating of the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  at that temperature. 
   This characteristic curve is obtained in advance by table making through experiment or by calculation through modeling of the drive control circuit  510 , the motor  121 , etc. Further, this characteristic curve may be set so as to be fixed inside the control device  500 , etc. or so as to allow rewriting from the outside. 
   In accordance with this characteristic curve and based on the temperatures detected by the motor temperature sensor  424  and the inverter temperature sensor  524 , the comparator  512  sets, in the current adjustment circuit, the lower one of the current values at continuous rating of the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  at that temperature as the upper limit value of the motor current Im. Thus, the upper limit value set in the current adjustment circuit is a current value at continuous rating satisfying both the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 . 
   In order to prevent errors, etc. due to a delay in terms of time between the comparing operation in the comparator  512  and the temperatures detected by the motor temperature sensor  424  and the inverter temperature sensor  524 , the comparator  512  may be equipped with a feed forward control circuit (not shown in the drawings). 
   In this construction, the motor current Im at the start of the turbo molecular pump of the present invention is larger than the motor current Im in the prior art. The reason for this will be explained below. 
   In the conventional motor control system  300 , the current value Ir at rated rotation is constantly set as the upper limit value of the motor current Im set in the current limiter circuit. 
   In contrast, in the motor control system  600  of the present invention, the current value Ir at rated rotation is not used as the upper limit value of the motor current Im. According to the relationship of Equation 1 as described above, at the start of the turbo molecular pump, it is theoretically possible to provide a motor current Im not less than the rated rotation current value Ir. 
   Thus, in the motor control system  600  of the present invention, it is possible for the motor current Im to be larger than in the prior art. 
   However, in thus increasing the motor current Im, it is necessary to prevent breakdown of the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  due to excessive supply of the motor current Im. 
   In view of this, in the motor control system  600  of the present invention, there are provided the motor temperature sensor  424  and the inverter temperature sensor  524 , which respectively detect the temperatures of the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W. Then, in the comparator  512 , the upper limit value of the motor current Im is set based on the temperatures of the inverter circuits  222  and the motor windings  126 U,  126 V, and  126 W. 
   Thus, in the present invention, the motor current Im is controlled in accordance with the characteristic curve and within the range of the current value at continuous rating satisfying both the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 . Thus, it is possible to prevent excessive supply of the motor current Im. 
   Here, based on the above operation,  FIG. 3A  shows the relationship between the starting time of the turbo molecular pump and the RPM of the rotor shaft  113 , and  FIG. 3B  shows the relationship between the RPM of the rotor shaft  113  at this time and the motor current Im. The components that are the same as those of  FIGS. 11A and 11B  are indicated by the same symbols, and a description thereof will be omitted. 
   In  FIG. 3B , the motor current Im at the start of the turbo molecular pump is larger than the motor current Im in the prior art. This is due to the fact that the current value Ir at rated rotation in the prior art is not used as the upper limit value of the motor current Im and that, theoretically, it is possible to supply a motor current Im according to the relationship of Equation 1. 
   On the other hand, the motor current Im immediately after the start of the turbo molecular pump (RPM: 0) is approximately 9 A, which is smaller than the value of the motor current Im calculated by Equation 1. This is because the upper limit value of the motor current Im set in the current adjustment circuit in the comparator  512  is the current value at continuous rating, which satisfies both the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 . 
   As a result, a motor current Im larger than the motor current Im in the prior art is supplied, within the range of the current value at continuous rating, to the motor windings  126 U,  126 V, and  126 W, so that the drive torque imparted to the rotor shaft  113  increases, and, as shown in  FIG. 3A , it is possible to shorten the starting time of the turbo molecular pump. 
   Accordingly, the process before shipment, which requires the turbo molecular pump to be started and stopped repeatedly, can be conducted quickly, thereby making it possible to shorten the production period for the turbo molecular pump. 
   Further, also when the turbo molecular pump is used in a movable simple vacuum chamber, the starting time for restarting the turbo molecular pump can be shortened, which makes it possible to regain the requisite vacuum degree inside the chamber promptly. Thus, it possible to increase the service life of the material and measurement filament in the chamber. 
   Further, even when the turbo molecular pump is increased in size to be in conformity with manufacturing apparatus for a large flat panel display, it is possible to shorten its starting time. Thus, it is possible to shorten the start-up time of the manufacturing apparatus for the large flat panel display. Further, when applied to a turbo molecular pump of conventional size, the present invention helps to shorten the start-up time of a semiconductor manufacturing apparatus, an electron microscope, etc. 
   In addition, in increasing the motor current Im to shorten the starting time of the turbo molecular pump, the motor current Im is controlled in the current adjustment circuit of the comparator  512  so as to be within the range of the current value at continuous rating as indicated by the characteristic curve, whereby making it possible to prevent breakdown of these components. 
   While in the above description of the present invention the comparator  512  sets, as the upper limit value of the motor current Im, the current value at continuous rating satisfying both of the components at the temperature in accordance with the characteristic curve and based on the temperatures detected by the motor temperature sensor  424  and the inverter temperature sensor  524 , this should not be construed restrictively. 
   For example, as shown in  FIG. 4 , instead of detecting the temperatures of the motor windings  126 U,  126 V, and  126 W and of the inverter circuits  222 , it is also possible to set the upper limit value of the motor current Im through calculation based on Equation 1 mentioned above. 
   Further, in this case, in order to prevent breakdown of the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 , a safety value of the motor current Im satisfying both of the components may be obtained previously by experiment or calculation. Then, in the comparator  512 , a function indicating a leeway based on this safety value is added to Equation 1, and the motor current Im is calculated by this equation. As a result, this calculated motor current Im is adopted as the upper limit value of the motor current Im set in the current adjustment circuit. 
   Due to this arrangement, there is no need to provide the motor temperature sensor  424 , the inverter temperature sensor  524 , etc., so that it is possible to achieve a reduction in parts cost. Further, since the motor current Im can be controlled within the range of the safety value of the motor current Im obtained through experiment, it is possible to shorten the starting time of the turbo molecular pump within a range of reliability reflecting the various factors regarding the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 . 
   Further, while in the above description of the present invention the motor current Im is controlled within the range of the current value at continuous rating satisfying both of the components, the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 , this should not be construed restrictively. 
   If the current value at continuous rating of one of the two categories of components: the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 , is obviously larger than the current, value at continuous rating of the other, it is also possible to control the upper limit value of the motor current Im within the range of the current value at continuous rating solely with respect to the components of the lower current value. 
   Further, while in the above description of the present invention the motor current Im is controlled within the range of the current value at continuous rating, this should not be construed restrictively. 
   For example, as shown in  FIG. 5 , if for a short period, the motor current Im may be controlled within a range beyond the current value at continuous rating, that is, within the range of the current value at short-term rating. 
   In this case, the comparator  512  is equipped, instead of the characteristic curve showing the relationship of the current value at continuous rating as mentioned above, with a characteristic curve indicating the relationship of the current values at short-term rating of the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  with respect to the respective temperatures thereof. 
   In the comparator  512 , a permissible time is first selected, and a characteristic curve corresponding to this permissible time is selected. As a result, in accordance with the characteristic curve thus selected, the comparator  512  sets the upper limit value of the motor current Im based on the temperatures detected by the motor temperature sensor  424  and the inverter temperature sensor  524 . 
     FIG. 6  is a time chart showing an example of how the motor current Im is controlled in this case. 
   In  FIG. 6 , two current values IL and IH are supplied to the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  as the motor current Im. 
   Here, the current value IL constitutes the current value at continuous rating. The current value IH constitutes the current value at short-term rating of the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  at the temperatures at that time, in accordance with the characteristic curve corresponding to the permissible time tH determined by the comparator  512 . 
   As shown in  FIG. 6 , by alternately supplying the current value IL and the current value IH to the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222  as the motor current Im, it is possible to increase the average value of the motor current Im. 
   By performing such control on the motor current Im at the start of the turbo molecular pump, it is possible to further shorten the starting time thereof. Further, in supplying the current value IH, the current value IH is controlled based on the characteristic curve at short-term rating with respect to the motor windings  126 U,  126 V, and  126 W and the inverter circuits  222 , so that it is possible to prevent breakdown of these components in a critical state.