BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for controlling the speed of an alternating current (AC) motor with a frequency changer. 
     The term AC motor utilized herein includes a synchronous motor and an induction motor, and as a frequency changer may be used a current type inverter, a voltage type inverter, a cycloconverter or the like. 
     FIG. 1 shows an example of a prior art control apparatus of an AC motor which comprises an AC source 11, a rectifier 12, a DC reactor 13, an inverter 14, an induction motor 15, a speed reference setter 16 comprising a variable resistor which sets a desired speed reference an input limiter 17, a frequency controller 18, an output voltage controller 19, an output voltage detector 20, an input current detector 21 in the form of a current transformer, and an input voltage detector 22 which are connected as shown. The control apparatus shown in FIG. 1 is used to control the speed of the induction motor 15 by using a current type inverter in which the AC input from the source 11 is rectified by the rectifier 12, the output DC thereof being smoothed by the DC reactor 13 and then inverted by the inverter 14 into an AC power supplied to the induction motor 15. The AC power supplied to the induction motor 15 is controlled by an output voltage frequency reference signal e 2  obtained by comparing a reference signal e 1  set by the speed reference setter 16 with an input voltage signal in the input limiter 17. The primary frequency of the voltage supplied to the induction motor 15 is determined by varying a gate signal supplied to thyristors constituting the inverter 14 from the frequency controller 18 according to the output voltage frequency reference signal e 2 . On the other hand, the primary voltage supplied to the induction motor 15 is controlled by controlling the gate signals applied to the thyristors constituting the rectifier 12 from the output voltage controller 19 according to the output voltage frequency reference signal e 2 . Thus, the primary voltage detected by the output voltage detector 20 is compared with the output voltage frequency reference signal e 2  with a comparator 23 in a closed voltage controlling loop, while current is controlled by a minor closed current controlling loop including input current detector 21 so that the output current of the rectifier 12 would not exceed a predetermined value. More particularly, when the output current of the rectifier is caused to increase by a short circuit or an overload of the motor 15, the output current is limited to be less than the predetermined value regardless of the output voltage frequency reference signal e 2  by decreasing the output voltage of the rectifier 12. In other words, within a predetermined output current, the primary voltage and the primary frequency of the induction motor 15 are controlled in a predetermined manner according to the output voltage frequency signal e 2 , whereas when the output current tends to increase beyond the predetermined value, the primary voltage is decreased to limit the output current. 
     The variation in the output voltage of the AC source 11 is detected by the input voltage detector 22. Thus, when the operation of the source 11 is stopped momentarily, the voltage detector 22 momentarily stops the operation of the frequency converter. Such operation mode is illustrated in FIG. 2, in which curve (a) shows the voltage of the AC source 11, (b) a running signal, (c) the number of revolutions of the induction motor 15 and (d) the output voltage frequency reference signal e 2 . Suppose now that the voltage of the source 11 decreases below the predetermined value at a time t 1  and that the voltage resumes the predetermined value at a time t 2 , then during an interval Tc between t 1  and t 2 , the operations of the rectifier 12 and the inverter 14 would be interrupted, and during this interval the number of revolutions of the induction motor 15 decreases by ΔN. For this reason, the output voltage frequency reference signal e 2  is also decreased by ΔE during this interval to a value corresponding to the number of revolutions of the induction motor. This is caused by the torque T-primary current characteristic I of the motor 15 shown in FIG. 3, in which the abscissa represents the number of revolutions N and the ordinate the torque T and the primary current I of the induction motor. When the primary frequency of the induction motor 15 is F 1 , the torque is designated by T 1 , the primary current by I 1 , and the synchronous speed by N 01 , whereas when the primary frequency is F 2 , the torque is designated by T 2 , the primary current by I 2 , and the synchronous speed by N 02 , where F 1  &lt;F 2 . When the primary current is limited to I 21  for the primary frequency F 2  the torque T is the same as T 2  between a speed range of N 21  -N 22 , but in other speed ranges the generated torque decreases by an amount shown by hatched portions. 
     Suppose now that the motor 15 running with the primary frequency F 2  up to time t 1 , stops running during the interval Tc and restarts at time t 2  with the primary frequency F 2 . Under these conditions, where the speed of the motor 15 is higher than N 21  immediately prior to time t 2 , it is easy to return the motor speed to that prior to time t 1 , whereas when the speed has been reduced to a value less than N 21  the output current of the inverter 14 (i.e., rectifier 12) would be limited to I 21  with the result that the torque T of the motor decreases, thus making it impossible to resume the number of revolutions before time t 1 . Accordingly, when the primary frequency is decreased to F 1  from F 2  corresponding to the decrement ΔN in the speed, it would be possible to make the torque to be T 1  at time t 2  to begin to accelerate the motor at time t 2  to resume the original speed provided that the synchronous speed satisfies a relation N 02  -N 01  ≧ΔN. 
     From the foregoing description, it will be noted that where the source voltage momentarily disappears, the motor can restart to resume the original speed, but unless the output frequency reference signal e 2  is also decreased by ΔE corresponding to the variation ΔN in the motor speed during the interval Tc, the output current of the inverter would be limited to I 21  which decreases the torque T making it difficult to resume the original speed. 
     Consequently, how to determine the width of variation ΔN in the speed presents various problems. For example, as the variation width is greatly influenced by the length of the instantaneous interruption interval Tc and the load condition of the induction motor 15, it is necessary to restart the motor by making the decrement ΔE of the output voltage frequency reference signal e 2  to have sufficiently large margin, that is to make small the primary frequency at the time of restarting. For this reason, it takes a long time to resume the normal rotation and if the decrement were too small it would be impossible to resume the normal or original speed. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an improved method and apparatus for controlling an AC motor capable of returning to the original speed when a source of supply is interrupted for a short interval. 
     According to one aspect of this invention there is provided a method of controlling a speed of an AC motor energized by a source of alternating current through a frequency converter wherein the speed of the motor is controlled by controlling a primary frequency and a primary voltage of the motor with the frequency converter, the primary voltage is controlled by a closed loop including a minor loop, primary current of the AC motor is controlled by the minor loop and the primary frequency is controlled by an open loop, characterized by the steps of decreasing the primary frequency with the frequency converter at a rate faster than a rate of decrease in a number of revolutions of the motor when the source of alternating current is interrupted and then recovered after a short interval, and accelerating the AC motor to an original speed with the primary current limited to a predetermined value smaller than a value limited by the minor loop while maintaining the primary frequency and the primary current in a predetermined relation. 
     According to another aspect of this invention there is provided apparatus for controlling a speed of an AC motor energized by a source of alternating current through a frequency converter comprising a closed loop including a voltage detector for controlling a primary voltage supplied to the AC motor from the frequency converter; a minor closed loop including a current detector for controlling a primary current supplied to the AC motor from the frequency converter; an open loop including an input limiter for controlling a primary frequency supplied to the AC motor from the frequency converter, and a limiter connected between the current detector and an input voltage detector responsive to source voltage and the input detector; the limiter and the input limiter being connected to decrease the primary frequency at a rate faster than a rate of decrease in a number of revolutions of the AC motor when the source of alternating current is interrupted and then recovered after a short interval and to accelerate the AC motor to an original speed with the primary current limited to a predetermined value smaller than a value limited by the minor loop while maintaining the primary frequency and the primary current in a predetermined relation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a block diagram showing a prior art induction motor control apparatus; 
     FIG. 2 is a diagram showing the operation mode of the prior art apparatus shown in FIG. 1 at the time of momentary interruption of the supply of an AC power; 
     FIG. 3 shows torque-primary current-number of revolutions characteristics of an induction motor; 
     FIG. 4 is a block diagram showing one embodiment of the control apparatus embodying the invention; 
     FIG. 5 is a diagram showing the operation mode of the apparatus embodying the invention at the time of momentary interruption of the power supply; 
     FIG. 6 is a block diagram showing the detail of a portion of the circuit shown in FIG. 4; and 
     FIG. 7 is a connection diagram showing a modification of the circuit shown in FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a preferred embodiment of this invention shown in FIG. 4, circuit elements 11 through 23 have the same functions as those shown in FIG. 1. 
     According to this invention a limiter 24 is connected between the input voltage detector 22, the input current detector 21 and the input limiter 17. The operation of the limiter 24 will be explained with the aid of the operation mode shown in FIG. 5 and one example of the construction of the input limiter 17 and the limiter 24 is shown in FIG. 6. FIG. 5 shows waveforms of the signals at various portions under a condition when the voltage of the AC source 11 decreases below a predetermined value at time t 1  and restores the original or normal value at time t 2 . In the same manner as in FIG. 2, curve (a) represents the source voltage, curve (b) an operation signal, curve (c) the number of revolutions of the induction motor 15, curve (d) the output voltage frequency reference signal e 2  and curve (e) the primary current (input current to the rectifier 12) of the induction motor 15. In the same manner as in FIG. 2, during an interval Tc between times t 1  and t 2  the operations of the rectifier 12 and the inverter 14 are interrupted, and the recovery of the source voltage is detected by the input voltage detector 22 to restart the rectifier and inverter. As above described when the rectifier and inverter are restarted at time t 2 , the variation ΔN 11  in the speed of the induction motor 15 does not correspond to the variation ΔE 11  in the output voltage frequency reference signal e 2 . More particularly, when ΔE 11  is small the induction motor cannot produce a predetermined acceleration torque so that during an interval between times t 2  and t 21  the speed of the motor decreases further by ΔN 12 . During an interval between times t 2  and t 21  the limiter 24 detects the fact that the source voltage has recovered through the input voltage detector, as well as the fact that the primary current of the induction motor has exceeded a predetermined value through the input current detector 21 so as to decrease the output voltage frequency reference signal e 2  by ΔE 12  during an interval between times t 2  and t 21  until the primary current of the induction motor reaches a predetermined value I 1  smaller than the limiting value of the current or minor control loop. As a consequence, the variation (ΔN 11  +ΔN 12 ) in the motor speed will come to have a predetermined relation with respect to the variation (ΔE 11  +ΔE 12 ) in the output voltage frequency reference signal e 2  so that the motor 15 begins to accelerate at time t 21  to reach the normal speed at time t 3 . FIG. 5 shows a case wherein the motor accelerates during an interval betweeen times t 21  and t 3  with its primary current limited to the predetermined value I 1 . 
     One example of the circuit that performs this function is illustrated in FIG. 6 in which the input limiter 17 is constituted by operators OA1 and OA2 in the form of operational amplifiers. When there is a difference between the set reference signal e 1  and the output voltage frequency reference signal e 2 ,the operator OA1 produces a signal e 11  with its amplitude limited by an amplitude limiter L 1 , the signal e 11  normally controlling the rate of variation of the output signal of the operator OA2, i.e., the output voltage frequency reference signal e 2 . The rate of variation is determined by a ratio e 11  /(R 13 .C 11 ) where R 13  represents the resistance value of a resistor R 13  connected between operators OA1 and OA2, and C 11  represents the capacitance value of a capacitor C 11  connected across the operator OA2. When an equation e 1  /R 11  =e 2  /R 12  holds (where R 11   represents the resistance value of a resistor R 11  connected on the input side of operator OA1, and R 12  the resistance value of a variable resistor R 12  connected between the input of the operator OA1 and the output of the operator OA2), the output signal e 11  of the operator OA1 becomes substantially zero, whereby the output voltage frequency reference signal e 2  does not vary and is held at a constant value. As shown in FIG. 6, a relay 27 is connected to input voltage detector 22 to be actuated by a source voltage decrease detection signal V so as to be closed under a normal condition but opened under an abnormal condition. In the comparator 25, the output signal I of the input current detector 21 is compared with a primary current limiting value CL 1  set by a rheostat 26, and the difference thus detected is inputted to a current limiter CL 2  to produce signals e 12  and e 13 . During the interval Tc between times t 1  and t 2  shown in FIG. 5, the variation ΔE 11  in the output voltage frequency reference signal e 2  varies the set reference signal e 1  and decreases the resistance value of the resistor R 12 . During the interval between times t 2  and t 21 , as the primary current I 1  of the motor exceeds a predetermined value I 1  as shown in FIG. 5e, the current limiter CL 2  would produce signal e 13  to decrease the resistance values of the variable resistors R 12  and R 13  to quickly decrease the output voltage frequency reference signal e 2  by ΔE 12 . When the primary current reduces to the predetermined value I 1  at time t 21 , the current limiter CL 2  stops producing signal e 13  and output signal e 12  for controlling the rate of variation de 2  /dt=e 11  /R 13 . C 11 ) of the output voltage frequency reference value e 2  by controlling the amplitude of the output signal e 11  of the operator OA1 so as to accelerate the motor 15 until time t 3  with the primary current limited to the predetermined value I 1 . Thus, at time t 3  the speed of the motor 15 returns to the original value before time t 1 . Thus, signal e 12  outputted from the current limiter CL 2  is stopped and signal e 11  becomes substantially to zero because an equation e 1  /R 11  =e 2  /R 12  holds. 
     As above described according to this invention, when power supply of an AC source is momentarily interrupted the primary frequency of the motor is decreased at a rate faster than the rate of decrease in the motor speed so as to accelerate the motor to the original speed with the primary current limited to a predetermined value while maintaining the relation between the primary frequency and the primary current or voltage such that the motor would produce a predetermined torque. For this reason, it is possible to continue stable operation of the motor under such transient condition as momentary interruption of the AC source. 
     It should be understood that the constructions of the input limiter 17 and the limiter 24 are not limited to those shown in FIG. 6 and that any combinations of the circuit elements may be used so long as they can decrease the primary frequency (the output voltage frequency reference signal e 2 ) at a rate faster than the rate of decrease in the motor speed. 
     For example, the circuit shown in FIG. 6 can be modified as shown in FIG. 7 in which the set reference signal e 1  is applied to operator OA1 via resistor R 11  and the output i 1  thereof is applied to operator OA2 via resistor R 13  to produce the output voltage frequency signal e 2  and the operator OA2 is shunted by a capacitor C 11 . Resistor R 12  is connected in the same manner as that shown in FIG.6. In this case, however, resistors R 12  and R 13  are fixed resistors. The connection of the current limiter CL 2  and the comparator 25 is similar to that shown in FIG. 6. The output of the current limiter CL 2  is supplied to the input of the operator OA2 via a polarity discriminator 30 which outputs -i 2  in accordance with the polarity of the output of the comparator 25. Thus, when i&lt;&lt;-i 2  the capacitor C 11  discharges to generate a predetermined output voltage frequency signal e 2 . Provision of the polarity discriminator 30 makes it possible to use fixed resistors R 12  and R 13  so that the current limit CL 2  is not required to produce control signals e 12  and e 13 . 
     Further, it should be understood that it is not always necessary to decrease the output voltage frequency signal e 2  by a predetermined value ΔE 11  during momentary interruption of the source of power and that the value ΔE 11  may be zero in which case the interval between times t 2  and t 21  becomes slightly longer. 
     Further, it should be understood that the invention is applicable to any type of the frequency converter. 
     As above described this invention makes it possible to rapidly resume the original speed of an AC motor and continue its stable running at the time of interruption of the power supply for a short interval.
In a method and apparatus for controlling a speed of an AC motor energized by a source of alternating current through a frequency converter, a limiter responsive to input voltage and input current is provided for decreasing a primary frequency at a rate faster than a rate of decrease in a number of revolutions of the motor when the source is interrupted for a short interval and for accelerating the motor with a primary current thereof limited to a predetermined value while maintaining the primary frequency and the primary current in a predetermined relation.