SYNCHRONOUS MOTOR CONTROL

Abstract: Apparatus and method for controlling a synchronous motor driving a square-law torque load at variable speeds by employing a frequency changer. The armature voltage Va may be controlled so that the ratio Va/N at motor speeds above a predetermined value is greater than Va(100)/N(100), where N(100) is the rated speed of the motor and Va(100) is the armature voltage at that speed. Below a predetermined speed, the armature voltage may be con­ trolled so that the ratio Va/N is less than (Va(100)/N(100).

Description: This invention relates to the control of a synchronous motor which is driving a square-law torque load, particularly when operated at variable speeds by a frequency changer. Technological progress in recent years has led to the use of many methods of variable speed operation of synchronous motors, including those methods which make use of a forced commutation frequency changer and those which employ a natural commutation (load commutation) frequency changer. The range of application for such methods has also expanded over the years from small and medium motors to large motors. Although the types of load driven by these synchronous motors are also numerous, they can be divided into two general types; constant torque loads where a roughly constant torque is required irrespective of the motor speed N; and square-law torque loads where the torque required varies in proportion to the square of the speed N, such as with fans, blowers and pumps. Figure 1 is a graph of the characteristics of a conventional control system for driving a synchronous motor. The armature voltage Va of the synchronous motor and the torque load T are plotted on vertical axes, while the speed N of the synchronous motor is plotted on the horizontal axis. As the graph shows, it was usual, in conventional synchronous motors, for the Va (armature voltage)/N (rpm) ratio to be controlled to a constant value when the load torque exhibits square-law characteristics. The conventional drive control system for a synchronous motor driving a square-law torque load gives rise to the following problems: (1) In general, with a square-law torque load, the power required is proportional to N³, and the system is, therefore, usually operated with the rpm of the synchronous motor set at less than 100% of the rated figure, with relatively long periods of operation at 50 to 80% of the rating. In this typical operating range, the power factor PFα at the input to the control circuit is low, and a large amount of reactive current is taken into the control circuit from the input AC source. (2) Since the excitation input voltage EF₁ of the magnetic field input source is at its maximum when the speed N of the synchronous motor is zero, the capacity of the field input source is required to be large and the power factor of the current drawn from this supply source is poor. An article entitled "Über neue Betriebsarten der Stromrichtermaschine synchroner Bauart" by Günter Gölz, Peter Grumbrecht und Frank Hentschel, Wiss. Ber. AEG-TELEFUNKEN 48 (1975) 4, pages 170-180, is hereby acknowledged. Figure 16 of this article discloses a control arrangement for the regulation of a synchronous motor energised by a frequency changer which can vary its output voltage and output frequency. The arrangement employs means for detecting the speed of the motor, means for generating a reference voltage related to the speed, means for monitoring the actual armature voltage of the motor, means for comparing the actual armature voltage and the reference voltage to produce an error signal, and means responsive to the error signal for controlling the field current of the motor so that the actual armature voltage approaches the reference voltage. In accordance with a first aspect of the present invention, a method of controlling a synchronous motor driving a square-law torque load by means of a frequency changer which can vary its output voltage and output frequency comprises the steps of: (a) monitoring the speed N of said synchronous motor; (b) controlling the current fed to the motor in accordance with the desired speed; (c) controlling the timing of current commutation in the output inverter of the frequency changer in accordance with signals derived from a sensor coupled to the synchronous motor; (d) generating a reference armature voltage V a that is related to the speed of the motor; (e) regulating the armature voltage in accordance with said reference voltage V a by controlling the field current of the synchronous motor, whereby the ratio of said reference armature voltage V a and the speed N is larger than V a (100)/N(100) for a range of motor speeds above a predetermined value and/or is lower than V a (100)/N(100) for a range of motor speeds below a predetermined value, where N(100) is the rated motor speed and V a (100) is the armature voltage at said rated motor speed N(100). According to a second aspect of the invention, apparatus for controlling a synchronous motor driving a square-law torque load by means of a frequency changer which can vary its output voltage and output frequency comprises: means for detecting the speed N of said motor; means for generating a reference voltage V a related to the output of said detecting means; means for monitoring the actual armature voltage of said motor; means for comparing said actual armature voltage with said reference armature voltage V a and producing an error signal related thereto; means, responsive to said error signal, for controlling the field current of said motor so that said actual armature voltage approaches said reference armature voltage V a ; and wherein the ratio of said reference voltage V a and said speed N is larger than V a (100)/N(100), where N/(100) is the rated motor speed and said V a (100) is the armature voltage of said motor at a speed of said N(100), for motor speeds above a predetermined value. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a graph of the V a -H and T-N characteristics in a conventional system; Figure 2 is a schematic diagram showing an embodiment of the invention; Figure 3 is a graph of the V a -N and T-N characteristics of the embodiment of Figure 2; Figures 4a-4d are graphs of the characteristics of the synchronous motor drive control system of the invention; Figures 5 and 6 are graphs of the V a -N and T-N characteristics of other embodiments of the invention; and Figures 7, 8 and 9 show one form of speed controller, α controller and excitation reference generator, respectively. Referring to the schematic block diagram of Figure 2, 11 in this drawing is an input AC source; 12 a rectifier; 13 a DC reactor; 14 an inverter; 15 the synchronous motor; 15₁ the field winding of the synchronous motor 15; 15₂ a rotor rectifier; 15₃ an induction frequency changer (rotating AC transformer); 15₄ a sensor; 16 a speed reference input control; 17 a speed controller; 18 a D/A converter; 19 an α controller; 19₁ a current detector; 20 a β controller; 21 a magnetic field current input source; 22 a thristor switch; 23 a voltage detector; 24 a voltage controller; 24₁ a current detector; and 25 an excitation reference generator. The AC power from input AC source 11 is converted to DC by rectifier 12. The rectified energy is then smoothed by DC reactor 13, inverted to AC of a different frequency by inverter 14, and applied to synchronous motor 15 to drive it at variable speeds. The frequency of the AC power applied to synchronous motor 15 is determined by speed reference input control 16 as follows. The rotation of synchronous motor 15 is detected by sensor 15₄. The detection signal is converted by D/A converter 18 into an analog signal which is compared by the speed controller 17 with the speed reference from speed reference input control 16. The result of the comparison is employed by a controller 19 to control the power output of rectifier 12. α controller 19 also detects the input current to rectifier 12. Thus, the power fed to synchronous motor 15 is variably controlled so that the speed of motor 15 approaches the desired value N. Inverter 14 is controlled via β controller 20 by the signal from the sensor 15₄, which detects the angular position of the rotating magnetic field of synchronous motor 15. The point at which the inverter undergoes load commutation (natural commutation), due to the counter electromotive force of synchronous motor 15, is detected using the signal from sensor 15₄ and taken by β controller 20 as the reference to determine the timing of the inverter turn-on. As for the magnetic flux of synchronous motor 15, the AC power of field input source 21 is voltage controlled by thyristor switch 22, the output of which is applied to induction frequency changer 15₃. The AC power output at the secondary side of induction frequency changer 15₃ is converted to DC by rotor rectifier 15₂, and magnetic flux is produced by applying this to field winding 15₁. Control of the magnetic flux of synchronous motor 15 is achieved as follows. The output signal of the voltage detector 23, which detects the armature voltage of synchronous motor 15, is compared with the output signal of the D/A converter 18 (speed detection signal) as processed by excitation reference generator 25, to establish a reference for voltage controller 24. The input current of thyristor switch 22 is detected by current detector 24₁ and the required voltage control is thus effected. Excitation reference generator 25, receiving the output signal from D/A converter 18, generates an output signal according to the rotation of synchronous motor 15, as shown in Figure 3. Voltage controller 24 compares the output voltage of inverter 14, which is detected by voltage detector 23, with the output of excitation reference generator 25, and the difference is applied to thyristor switch 22 to control the field current I f so that the armature voltage value approaches the desired value for a particular speed as determined by generator 25. The characteristics of the control system are as follows, with reference to Figures 3 and 4a-4d. Figure 3 is a graph of the V a -N and T-N characteristics, corresponding to the depiction in Figure 1 of the characteristics of a conventional system. In Figure 3, the load torque T changes as function of the rotation N of the synchronous motor 15 in the same manner as the T-N characteristic in Figure 1. The armature voltage V a of the synchronous motor 15 is, however, controlled by field excitation reference generator 25 which adjusts the field current so that Va/N is not constant over the whole range of speeds N. In the example depicted in Figure 3, the pattern is such that, when the speed N is in the range of 0 to 25%, the armature voltage Va is controlled in the low range; when N is in the range 25% to 75%, Va/N is constant and Va is controlled in the high range; and when N is in the range 75% to 100%, Va is controlled to a virtually constant value. Figures 4a-4d show the characteristics of a drive control system for a synchronous motor controlled in this manner. Figure 4a depicts the armature current Ia of synchronous motor 15; Figure 4b the field current I f flowing in field winding 15₁; Figure 4c the excitation input power Pf₁ and the excitation input voltage Ef₁ of the induction frequency changer 15₃; and Figure 4d the input power factor PFα of the rectifier 12. In Figures 4a-4d, each solid line depicts the above-mentioned value when the armature voltage Va varies, according to Figure 3, and each dotted line depicts the above-mentioned value when the armature voltage Va varies according to Figure 1 (hereinafter called "in the conventional controller"). In Figure 4a, in this invention, the armature current Ia: 1) is proportional to the speed N cubed when N is in the range of 75% to 100%; 2) is proportional to the load torque T or square of the speed N when N is in the range of 25% to 75%; and 3) increases by more than the load torque T when N is in the range 0% to 25%. In the conventional controller, the armature Ia is proportional to the load torque T or the square of the speed N. In Figure 4b, the field current I f , in this invention, is smaller than that of the conventional controller when the speed N is in the range of 0% to 25%. The field current in the former case is, in turn, more than the field current in the latter case when N is in the range of 25% to 75%. In this invention, the field current I f does not vary very much when N is in the range of 75% to 100%. The field current I f in the conventional controller is a function of the load torque T and the minimum field current I f is one-half of that when N is 100%. In Figure 4c, the excitation input power Pf₁ is a function of the load torque T, as is the field current I f , and the minimum excitation input power Pf₁ is one-quarter of that when N is 100%. The excitation input voltage Ef₁ in the conventional controller is inversely related to the speed N. The excitation input voltage Ef₁ in this invention is smaller than that in the conventional controller when N is in the range of 0% to 25%, and the excitation input voltage in this invention is, in turn, greater than that in the conventional controller when N is in the range of 25% to 100%. In Figure 4d, the input power factor PFα of rectifier 12 in both this invention and in the conventional controller is the same as the corresponding armature voltage Va. The input power factor PFα in this invention and that in the conventional controller are roughly the same when N is in the range of 0% to 25%, and the power factor in this invention is better than in the conventional controller when N is in the range of 25% to 75%. With this invention, as explained above, the input power factor PFα of rectifier 12 is increased by 10% to 15% in the typical range of operation (where N is 50% to 80%), while the reactive current taken from input AC source 11 in the typical range of operation is considerably reduced. Moreover, with regard to the capacity of the excitation power source of induction frequency changer 15₃, since the maximum value of the excitation input voltage Ef₁ is lower than in that of conventional control method when N is 0, the capacity of field input source 21 can be reduced, and its power factor increased. The reason why this advantageous effect is obtained is that the invention is applied only to a square-law torque load. Further, even though the armature voltage Va for the N range of 75% to 100% is controlled to a constant value, the load torque T reaches its maximum when N is 100% and, for this reason, the armature current Ia and field current I f do not increase even when N decreases, and there is no need for an increase in the capacity of synchronous motor 15. In the case of a constant torque load, since the magnetic flux of synchronous motor 15 is weakened when N is 75% to 100%, the armature current Ia increases in proportion to N. In the case of constant output power control, when the output power of the synchronous motor 15 is controlled to a constant value, the armature current Ia is at its maximum when N is in the region of 75%. Both the case of constant torque load mentioned above and the case of constant output power control involve an increase in capacity, weight and dimensions of synchronous motor 15, with a consequent increase in cost. The reason the armature voltage Va in this invention (Figure 3) can be set lower than in Figure 2 when N is 0 to 25% is that the load torque is smaller in this range, and the torque generated by synchronous motor 15 can also be reduced. These advantages can only be obtained in the case of square-law torque. As the above explanation will have made clear, the invention was proposed after a thorough study of the characteristics of synchronous motor 15 and of the characteristics of square-law torque loads; and only when it is applied to square-law torque loads can the advantages mentioned above be obtained without an increase in the capacity, weight and cost of synchronous motor 15. Figure 5 shows the characteristics of another embodiment of the invention. There is no need to restrict the ranges of speed N where the ratio Va/N is controlled to a lower value than the constant slope straight line and where Va is controlled to a constant value which are shown in Figure 3. A pattern such as shown in Figure 5 can also be set, with any N range being selected at will to match the normal range of synchronous motor 15. Figure 6 shows the characteristics of yet another embodiment of the invention. This graph shows a pattern which provides only for a range where the armature voltage Va is greater than the previously-mentioned straight line of constant slope Va/N. It is clear that the effects obtained in the range where the armature voltage Va coincides with the broken line will be unaltered. In the case of Figure 6, the capacity of field input source 21 is not reduced, but the reactive current taken from input AC source 11 is considerably reduced. As Figures 3, 5 and 6 demonstrate, in this invention the ratio of the armature voltage Va to the rotation N is not restricted to one particular value. If the armature voltage Va is set, in the normal working range, to giva a Va/N ratio larger than the ratio Va(100)/N(100) of the rated armature voltage Va(100) and the rated rotation N(100), (both values being the 100 percentage points), of synchronous motor 15, or if, in addition to this, the armature voltage is set, in the low speed range, to give a Va/N ratio smaller than Va(100)/ N(100), it is clear that the results described above can be obtained. One possible embodiment of speed controller 17 will now be explained by referring to Figure 7. In Figure 7, elements 171 to 173 are resistors; 174 a condenser; and 175 an operational amplifier. The output signal from speed reference input control 16 and the output of signal from D/A converter 18 are applied respectively to resistors 171 and 172. The difference between the former and the latter is detected by operational amplifier 175. Thus, the standard output signal e₂ of speed controller 17 is supplied to α controller 19. One possible embodiment of α controller 19 will now be explained by referring to Figure 8. In Figure 8, elements 191 to 193 are resistors; 194 a condenser; 195 an operational amplifier; 196 a phase controller; and 197 a rectifier. α controller 19 controls the output of rectifier 12, so that the value of the output current corresponds with that of the standard output signal e₂ in Figure 7. The detected signal e₃ of the current detector 19₁ is applied to rectifier 197 in order to convert it into DC. On the other hand, the standard output signal e₂ of speed controller 17 is supplied to resistor 191. The phase signal from operational amplifier 195 is supplied to phase controller 196 after the comparison between e₂ and e₃. The phase signal of the input AC source 11 is supplied to phase controller 196 through e₄. The output signal e₅ of the phase controller 196 controls the ignition timing of rectifier 12 after the comparison by phase controller 196. β controller 20 in Figure 2 and phase controller 196 in Figure 8 are well known, commonly used, circuits. Examples of perfectly acceptable circuits may be found in The Theory and Operation of Thyristor Motors by Kamiyama, published by Denki Shoin in Japan in 1974. Voltage detector 23 in Figure 2 simply monitors the AC armature voltage applied to motor 15 and employs a rectifier to produce a DC voltage related thereto. Voltage controller 24 in Figure 2 performs the same function for thyristor switch 22 as α controller 19 performs for rectifier 12. Therefore, voltage controller 24 may be similar to the example of α controller 19 illustrated in Figure 8. One possible embodiment of excitation reference generator 25 will now be explained by referring to Figure 9. In Figure 9, elements 251 to 257 are resistors; 258 a variable resistor; 259 to 261 operational amplifiers; 262 and 263 diodes; 264 a relay; and 256 a switch depending upon relay 264. R₁ represents a resistive value of resistors 251, 252; R₂ a resistive value of resistors 254, 255; R₃ a resistive value of resistors 256, 257; and R₄ a resistive value of a resistor 253. Variable resistor 258 is adjustable so that a voltage e₆ on a point 266 is shifted from a 0 to a negative voltage. When input signal e₇ from D/A converter 18 varies from 0V to +10V and voltage e₆ at point 266 is 2.5V, the relationship between speed N of synchronous motor 15 and the armature voltage Va is as follows. When speed N is from 0% to 25% of the rated value, the value of input voltage e₇ is smaller than that of voltage e₆ at point 266 and, consequently, an output signal is produced by operational amplifier 261. This causes relay 264 to be on and switch 265 to be closed. Consequently, resistor 253 is short-circuited. As a result, since resistors 251 and 252 have the same value, the value of the output voltage e₈ is equal to that of the input voltage e₇. Furthermore, when speed N is over 25% of the rated rotation, the value of input voltage e₇ is bigger than that of the voltage e₆ at point 266 and, consequently, an output signal is not produced by operational amplifier 261. This causes relay 264 to be off and switch 265 to be opened. Consequently, resistors 252 and 253 are in series connection between the output and input of operational amplifier 259. Thus, the value of the output voltage e₈ is greater than that of the input voltage e₇. If the value of (R₁+R₄) is equal to 4/3, as it is in the embodiment in Figure 9, when speed N is 75% of the rated rotation, the value of the output voltage e₈ is equal to 10V. When speed N increases over 75% of the rated rotation, the value of the output voltage e₈ remains equal to 10V as a result of diode 262. Therefore, the output voltage e₈ from excitation reference generator 25 in Figure 9 is as shown in Figure 3. A DC type thyristor motor system is shown in Figure 2 as the frequency changer for variable operation of synchronous motor 15. However, clearly a cycloconverter or forced commutation type inverter system can also be used, and there is no particular restriction as to the implementation of the frequency changer. With this invention, as explained above, the power factor of the input AC source of a synchronous motor drive control system can be improved, and the reactive current input can be reduced. This makes possible a reduction in the capacity of the power factor correction apparatus which is needed on the input AC source side of the system, and a reduction is also achieved in the apparent power drawn from the input AC source. The result is an increase in the operating efficiency of the system, and a reduction in the size and cost of the equipment.

Claims: A method of controlling a synchronous motor (15) driving a square-law torque load by means of a frequency changer (12, 14) which can vary its output voltage and output frequency, said method comprising the steps of: (a) monitoring the speed N of said synchronous motor; (b) controlling the current fed to the motor in accordance with the desired speed; (c) controlling the timing of current commutation in the output inverter (14) of the frequency changer in accordance with signals derived from a sensor (15₄) coupled to the synchronous motor; (d) generating a reference armature voltage V a that is related to the speed of the motor; (e) regulating the armature voltage in accordance with said reference voltage V a by controlling the field current of the synchronous motor, whereby the ratio of said reference armature voltage V a and the speed N is larger than V a (100)/N(100) for a range of motor speeds above a predetermined value and/or is lower than V a (100)/N(100) for a range of motor speeds below a predetermined value, where N(100) is the rated motor speed and V a (100) is the armature voltage at said rated motor speed N(100). A method in accordance with claim 1, wherein said predetermined value is approximately 25% of said N(100). A method in accordance with claim 1 or 2, wherein, in a range of motor speeds above a second predetermined value which is larger than said predetermined value, the slope of the relationship of V a to N is smaller than said slope for motor speeds between said predetermined value and said second predetermined value. A method in accordance with claim 3, wherein said slope is zero for motor speeds higher than said second predetermined value. A method in accordance with claim 3 or 4, wherein said predetermined value is approximately 25% of said N(100) and said second predetermined value is approximately 75% of said N(100). A method in accordance with claim 3, wherein in a range of motor speeds above a third predetermined value, which is larger than said second predetermined value, the slope of the relationship of V a to N is smaller than said slope between said second predetermined value and said third predetermined value. A method in accordance with claim 6, wherein said slope is zero for motor speeds beyond said third predetermined value. Apparatus for controlling a synchronous motor driving a square-law torque load by means of a frequency changer which can vary its output voltage and output frequency, said apparatus comprising: means for detecting the speed N of said motor; means for generating a reference voltage V a related to the output of said detecting means; means for monitoring the actual armature voltage of said motor; means for comparing said actual armature voltage with said reference armature voltage V a and producing an error signal related thereto; means, responsive to said error signal, for controlling the field current of said motor so that said actual armature voltage approaches said reference armature voltage V a ; and wherein the ratio of said reference voltage V a and said speed N is larger than V a (100)/N(100), where N/(100) is the rated motor speed and said V a (100) is the armature voltage of said motor at a speed of said N(100), for motor speeds above a predetermined value. Apparatus in accordance with claim 8, wherein said field current controlling means comprises a current source; thyristor switching means for controlling the current flowing from said current source to said motor; and means for controlling said thyristor switch means in response to said error signal.

Inventors: HIRATA, AKIO C/O, PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1983
Main classification: H02P5
IPC codes: H02P25, H02P27