Abstract:
A speed control system for an induction motor controlled by a converting arrangement coupled with a commutatorless motor which operates without commutation failure notwithstanding wide variations in the speed of the commutatorless motor. The speed control system includes an arrangement for maintaining the value of a direct current signal of the converting arrangement less than the maximum current value for ensuring proper operation of the converting arrangement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a speed control system for an induction motor wherein control is effected in accordance with the Kramer system utilizing a commutatorless motor. 
     2. Description of the Prior Art 
     The speed control system of an induction motor controlled by the so-called &#34;Kramer&#34; system has an inverter connected with a commutatorless motor. That is, the speed control of the induction motor is usually effected in that a secondary winding output of the induction motor is rectified by a rectifier, converted to alternating current by a thyristor frequency converter, and the alternating current is supplied a synchronous motor. This synchronous motor is directly connected with the induction motor by a mechanical axis. In such a system, the speed of the induction motor is controlled by varying the advance angle to turn on the thyristors of said frequency converter with this speed control system being generally called the Kramer system. 
     In commutatorless Kramer speed control systems, the most important factor to ensure proper operation is to turn off the thyristors of the frequency converter without failure of commutation. In order to ensure turn off of the thyristors of the converter, the input direct current Ic of the thyristor converter must be less than a maximum current Icmax which is the maximum current permissible to ensure proper turn off of the thyristors. Another factor is that the input current Ic must be larger than load current I L  with the currents Icmax and I L  being varied according to the speed N of the Kramer system. 
     When the speed of the Kramer system changes suddenly, that is, at large acceleration or deceleration, the input current Ic becomes greater than Icmax; Ic &gt; Icmax &gt; I L  such that the above-noted requirements are not met proper turn off of the thyristors of the converter is not ensured. Accordingly, the speed of the Kramer system must, therefore, be changed slowly or maintained within a small speed range. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a speed control system for an induction motor which overcomes the above-noted disadvantages of the general Kramer system. 
     Another object of the present invention is to provide a speed control system for an induction motor which permits sudden variation of speed or over a wide range. 
     A further object of the present invention is to provide a speed control system which ensures that the input current Ic of the frequency converter is larger than the load current I L  but less than the current Icmax. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, several embodiments in accordance with the present invention, and wherein: 
     FIG. 1 is a schematic diagram of a speed control system according to an embodiment of the present invention; 
     FIG. 2 shows a characteristic curve of the Kramer system in accordance with the invention; 
     FIG. 3 illustrates a speed reference device for use with the circuit in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein like reference numerals are utilized to designate like parts throughout the several views, there is shown in FIG. 1 a speed control system for an induction motor wherein an alternating power source 1 supplies power to an induction motor 3. An output of a secondary winding of the induction motor 3 is supplied to a diode rectifier 7 with the direct current from the rectifier 7 being supplied to a thyristor frequency converter 9 through a direct current reactor 8. The thyristor frequency converter 9 is controlled by an automatic pulse phase shifter 14 and the output of the converter 9 is supplied to synchronous motor 4. A position detector 5 detects a magnetic pole position of the synchronous motor 4 and supplies a pulse to the automatic pulse phase shifter 14. A pilot generator 6 detects a rotating speed of the synchronous motor 4. A load 2 is coupled to the induction motor with the induction motor 3, the synchronous motor 4, the position detector 5 and the pilot generator 6 being mechanically connected to each other. 
     A speed reference device 11 sets a desired speed for the induction motor 3 and a signal indicative of the deviation between the speed reference signal from the device 11 and an actual speed signal from the pilot generator 6 is supplied to an operational amplifier 12, reference numeral 13 also being an operational amplifier. 
     The above-mentioned system is a conventional Kramer system and in accordance with the present invention, the Kramer system is modified by utilizing a function generator 15. 
     The Kramer system usually operates when the output voltage SE 2 , of the diode rectifier 7 and the input voltage Et cos γ of the thyristor frequency converter 9 is well-balanced. From this relation, the following equation can be obtained: 
     
         SE.sub.2 = Et cos γ                                  (1) 
    
     S = slip of the induction motor 3 
     E 2  = secondary winding output voltage of the induction motor 3 when S = 1 
     Et = counter electromotive force 
     γ = advance angle of the converter 9 
     Then, generally a relation among the advance angle u and a turn-off reactance X of the motor 4 becomes as follows: ##EQU1## 
     Ic = current flowing into the converter 9. The counter electromotive force Et of the motor 4 is in proportion to a revolutional speed of the motor 4 and the following equation is obtained: 
     
         Et = KN                                                    (3) 
    
     k = a proportional constant 
     A revolutional speed N of the induction motor 3 which is equal to the speed of the synchronous motor 4 is as follows: 
     
         N = (1 - S)N.sub.C                                         (4) 
    
     n c  = synchronous revolutional speed of the induction motor 3 
     If the margin angle (γ - u) is equal to the minimum margin angle δ, the current I c  flowing into the converter 9 becomes its maximum value Icmax, so that the equation (2) is represented as follows: ##EQU2## 
     A curve of Icmax with respect to the speed N as shown in FIG. 2 is obtained from the above equations (1), (3), (4) and (5). 
     Icmax is the maximum direct current permissible to ensure proper turn off of the thyristor of the frequency converter 9 which flows through the direct current reactor 8. When the current Ic exceeds Icmax, the converter 9 fails to turn off. Therefore, the current Ic flowing into the converter 9 through the reactor 8 must be less than the current Icmax. The current Icmax varies according to the revolutional speed of the motor 4. On the other hand, the direct current I L  of the motor 3 which is required by the load 2, such as a water pump or an air blower, varies in response to the speed of the motor 4, as shown in FIG. 2. 
     The current Ic must be larger the current I L . Thus, 
     
         Icmax &gt; Ic &gt; I.sub.L 
    
     from this condition, the speed control range becomes generally narrower than the range N 1  to N 3  of FIG. 2. Now in FIG. 2, when the motor 4 is driven at the speed N 1 , and the speed reference of the speed reference device 11 varies suddenly and widely from N 1  to N 3 , the deviation between the speed reference and the actual speed of the pilot generator 6 becomes large and is supplied to the operational amplifier 12. The operational amplifier 12 amplifies the deviation up to the limit value Ia&#39;. 
     Although the actual speed of the motor is at N 1 , the value of the direct current of the converter 9 rises to the value Ia which is larger than Ic 1  which is the maximum current at the speed N 1 . Accordingly, the thyristor frequency converter fails to turn off. For this reason, the Kramer system generally has a narrow speed control range in order to avoid commutation failure. In FIG. 2, the actual current Ic must be maintained below the maximum current Icmax at any speed variation and the speed control was generally limited to the range from N 2  to N 3 . 
     In accordance with the present invention, a function generator 15 is utilized together with the operational amplifier 12 which serves to generate a constant signal Ia&#34; as shown in FIG. 2. The function generator 15 generates an output signal Ip in proportion to the actual signal from the pilot generator 6. The current (Ip + Ia&#34;) which is larger than I L  and less than Icmax is compared with the direct current detected by a current detector 10, and is supplied to the automatic pulse phase shifter 14 through the operational amplifier 13 in order to control the thyristor frequency converter 9. 
     At the speed N 1 , the load requires the current I L1  and the induction motor 3 begins to accelerate according to an accelerating current (Ia 1  - I L1 ). Since the current Ic 1  is larger than Ia 1 , the thyristor frequency converter does not fail to turn off. And then, according to the acceleration of the motor, the output signal Ip of the generator 15 rises to the limit value Ip 3 . 
     The signal Ip from the function generator 15 and the signal Ia&#34; from the operational amplifier 12, that is to say, the current (Ip + Ia&#34; = Ia) causes the induction motor 3 to accelerate. When the speed of the motor 3 goes up to N 3 , the generator 15 generates the current Ip 3  and the output signal of the operational amplifier 12 is kept at the value -(Ip 3  - I L3 ), and the induction motor is kept rotating at the speed of N 3 . Thus, proper operation of the thyristor, frequency converter without commutation failure over a wide range of speed values at sudden accelerations of the induction motor 3 is ensured. 
     In the above-mentioned system, the output of the function generator 15 can be linearly approximated such as the line Ip&#39; in FIG. 2 whereby the output of the operational amplifier is always negative. Therefore, maximum acceleration can be possible without commutation failure. Additionally, a cycloconverter may be utilized in the system replacing the diode rectifier 7, the reactor 8 and the thyristor converter 9. 
     Another embodiment of the present invention is shown in FIG. 3 which illustrates a portion of the circuit of FIG. 1. The operational amplifier 12 amplifies the output of the speed reference device 11. When the output of the pilot generator 6 is larger than the output of the speed reference device 11 (which is selected to be negative in FIG. 3), the output of the operational amplifier 12 is maintained at the value Ia limited by the circuit including a resistor 18 and a diode 17. As shown in FIG. 3, the circuit also includes resistors 21, 22 and 23, and capacitor 20 with capacitor 20 and resistor 21 forming unit 19 in the feedback path of the operational amplifier 12. 
     The value Ia varies corresponding to the output of a function generator 16 with the output of the generator 16 being varied in the manner of Ia in FIG. 2 according to the speed of pilot generator 6. 
     The induction motor in accordance with the present invention is utilizable as a variable speed motor over a wide speed range in applications such as a water pump or air blower drive. 
     While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but also contemplates numerous changes and modifications as would be known to those skilled in the art given the present disclosure of the invention, and we therefore do not wish to be limited to the details shown and described herein only schematically, but intend to cover all such changes and modifications.