WO1979000619A1

WO1979000619A1 - Speed control device for asynchronous motors

Info

Publication number: WO1979000619A1
Application number: PCT/SE1979/000030
Authority: WO
Inventors: B Wihk
Original assignee: B Wihk
Priority date: 1978-02-13
Filing date: 1979-02-13
Publication date: 1979-09-06
Also published as: SE422862B; SE7801640L

Abstract

Current control for asynchronous motors of short-circuit type, for multiphase, usually threephase, operation. In series with the stator windings (La, Lb, Lc) separate switches (10) are coupled, preferably triacs or pairs of thyristors, Each switch is controlled by its own control circuit (A, B, C) which ignites the switches at points in time determined by the zero crossings for reference voltages which are obtained from individual voltage dividers (R1, R2). According to a preferred embodiment, using triacs, their ignition is effected via reversed diodes, which should not be too fast. A suitable circuit for achieving braking with such a controlled motor is also shown. The principle for this is essentially that two of the motor phase windings each have a pair of different control circuits, whose reference phases are determined, for the first circuit in each pair, by voltage division between a first one of the phases connected to the winding in question and one of the two other phases, while the other control circuit in the pair receives its reference phase from said first phase and the remaining phase of said two other phases. A pulse determining circuit is coupled to one of these control circuits in the pair and detects the absence of control pulses, meaning that the motor is running too fast, and then emits a relay control signal, whereby, firstly, two phases are interchanged, and secondly, the second control circuit in each pair is coupled in, resulting in a braking current feed counteracting the direction of movement of the motor, the recouplings reverting to their original arrangements when control pulses are again received.

Description

Speed control device for asynchronous motors

The present invention relates to control of power input to a three-phase motor, the supply of current to the motor being regulated with thyristors or triacs, so that current is supplied in each phase only during a portion of the respective alternating current half cycle. The power supply can be varied by varying the time intervals during which the thyristors or triac units, functioning as valves, are conducting.

Such a construction is previously known by Swedish Lay-open Print 365 673, where the supply of current during each alternating current cycle is regulated in the above manner. In each of the phase windings, a controlled electronic switch (thyristor or triac)^" is coupled in in such a manner that it only allows current to pass to the respective winding during a certain portion of the cycle, beginning at a regulatable point in time within a half cycle up to the zero value crossing of the current. The position of the point in time is determined by a control value which is related to the zero value crossing of the current.

In this known construction, the voltage over one of the phase windings is used via a zero value crossing detector to derive its zero crossings, which are the starting instants for ramp voltages. These ramp voltages are compared with a control voltage, which in the example shown in the lay-open print is derived as the difference between a set ideal value and the voltage from a proportionally acting tachometer, coupled to the motor shaft. At the moment whe the ramp voltage reaches the value of the control voltage an ignition pulse is formed which activates the respectiv switch. When the current drops to zero, the switch is shu off in accordance with the well-known thyristor mechanism

According to said lay-open print, a corresponding ignitio point is obtained for the two additional phases in the A.C. system by allowing pulses corresponding to the ignition in the observed phase to pass through two differ delay circuits. After the first delay circuit there is a delay of one-sixth of a cycle, and after the second there is an additional delay of one-sixth of a period. This results in pulses delayed by one-sixth and one-third of a cycle respectively, thereby igniting the switches of the additional phases. In principle, symmetrical ignition is obtained thereby, which is an advantage because it result in, inter alia, symmetrical loading of the utility net.

However, in this previously known invention there is the disadvantage that one must use delay circuits which must of relatively high precision and are therefore not inexpe ive. Especially at low r.p. . high precision is necessary in any case if the load is low and the ignition must take place rather close to the zero crossing of the current. I the ignition occurs occasionally on the wrong side of the zero crossing, disturbances can be the result, such as uneven operation of the motor.

It is a purpose of the present invention to achieve a sim and not too expensive control device for speed control according to the control principle described in the intro duction in "phase synchronism" .

The major purpose is to achieve improved control of an A. motor, preferably of the short-circuit type; and even if_ control of speed with recoupling via a tachometer is the most obvious arrangement, the person skilled in the art will see upon reflection that the inventive idea enables one to control a motor with reference to another measur- able quantity, e.g. developed torque or the like if a suitable sensor is arranged.

The invention has a number of interesting applications. One need only think of the problems arising in trying to achieve satisfactory A.C. operation of passenger elevators (lifts)^'. In many applications requiring exact speed control, one must use D.C. current motors, with the obvious problems involved. In other cases where the problem is to achieve a high starting torque, double wound motors are used, Leonard systems and the like. _

The advantages described above and other advantages and characteristics which will be revealed in the description below are obtained according to the invention through a control device which has the features disclosed in Claim 1.

In the construction according to the invention, an indivi¬ dual control circuit is used for each of the different phases. Due to the development of electronic microcircuits, this is not a disadvantage, either economically or space- wise, or as regards the risk of electronic failure in complicated systems. Basically, the invention reveals an advantageous manner of obtaining the synchronic pulses which make possible the synchronic ingnition of the switches. This is achieved through the arrangement of special auxili¬ ary phases which constitute weighted vector sums of two different phases for each winding. To carry this out, the use of ordinary resistors in voltage divider coupling is preferred, but arbitrary types of impedances could be used equivalently.

According to a preferred embodiment of the invention, the motor is triangle-connected, with the switches arranged not outside the "triangle" in respective phases, but in series with each phase winding and thus on the. "sides of the triangle". It has been found that in this way an especially even running can be obtained, if a motor designed for star connection with the normal circuit volt¬ age, is coupled in triangle together with a device accord¬ ing to the invention. The motor will thus have a certain over -capacity and would be overloaded if the switches wer closed all the time. Thus there is a suitable control leeway for the power, and there is the possibility of extracting power for short periods, in excess of the rated power of the motor. It is possible thereby to improve the starting torque and the torque at low speeds.

The invention will now be described with reference to the figures. Fig. 1 shows a circuit diagram partially in block diagram form, for the three phase windings in a three-phas motor. Fig. 2 is a vector diagram which explains how a reference phase is obtained. Fig. 3 is a block diagram o a control circuit for an electronic switch coupled in series to a phase winding. Fig. 4 shows an oscilloscope curve of a voltage. Fig. 5 is a schematic block diagram o a. device for braking a motor.

Fig. 1 shows the general circuit scheme in an embodiment o the invention. The coupling of the three phases R, S and T to the three windings of the motor La, Lb and Lc via individual switches 10 is clearly evident from the figure. The switches are in this case triacs, but it is obvious th they can completely analogously be replaced with pairs of thyristors in the manner which will be explained in more detail in connection with Fig. 3. The switches are control ed with individual control circuits A, B and C. Individual reference phases are. coupled to each of them. The referenc are obtained from voltage dividers R -R , which are couple in the manner shown in the figure.

Fig. 2. shows how the respective reference phase lies in

A wiio relation to the voltages between the phases. For the sake of simplicity, only the reference phase used for control circuit A in Fig. 1 is treated here. The voltage over the winding La and the associated switch 10 follows the vector S-R according to the vector diagram in Fig. 2. The reference voltage is obtained by voltage division between the phases S and T. The resistances R.. and R^ are in the example 1.8 ohm and 0.8 ohm, making a ratio between the resistances of 2.25. Elementary geometrical and mathematical considerations will show that the vector REF in Fig. 2 is almost exactly parallel to vector S-R. The resistors should have at least one percent nominal precision.

Fig. 3 shows a suitable circuit for the control circuits A, B and C in block diagram form. The circuit drawn corre¬ sponds to circuit A in Fig. 1, with regard to the controlled phases. The circuit shown is accomplished with the use of a trigger module which is sold under the designation Philips TCA 280 A by the firm Elcoma. For detailed information, reference is made to the data sheet from the manufacturer.

The circuit in the example can be understood schematically as containing a direct current source 30, a zero crossing detector 31, a differential amplifier 32, a pulse generator 33 (corresponding to the "ramp generator" in the data sheet for the circuit), and a power amplifier 34. This is coupled to a transformer with a ratio of transformation of 1:1 and two secondary windings, which are coupled to the control electrodes on individual thyristors. The thyristors are coupled in parallel and function as switches for the motor winding La.

The zero crossing detector, in combination with the resist¬ ance Rll and the capacitor C2 generates saw-tooth ramp voltages, following one another and having a length corre¬ sponding to 180 current degrees. The starting points coin¬ cide with the zero crossings for the reference phase, which is coupled to the zero crossing detector. The ramp voltages are compared to a control voltage, and as soon as the ram voltages exceed the control voltage, the pulse generator 33 starts and emits, as long as this condition_^ is main¬ tained, a shorter or longer series of pulses with a time spacing of approximately 0.6 ms. These are amplified by the final amplifier 34 and the one of the thyristors 36,37 which is ignitable, will ignite.

When using a TCA 280 A according to the above, the values of the components used are as follows:

R10 = 4.7KΩ10W Cl = lOOOyF

Rll = 10-15 KΩ C2 = 1

R12 = 1MΩ C3 = 2.2 " R13 = 150 KΩ Dl = IN 4006

R14 = 680 KΩ D2 = IN 14148

R15 = 100 KΩ D3 = D4 = IN 4006

R16 = 82 KΩ

R17 = 47 Ω

A surprising detail for the person skilled in the art is the coupling of the diodes D3 and D4. In relation to the ignition pulses, which are to activate the thyristors 36 and 37, they are actually placed backwards. Chance and a reversed polarity marking on a diode, which was thus coupl "incorrectly", led to the realization of this advantage ous effect. It has been demonstrated that this coupling produces an exceptionally elegant and trouble-free ingniti of thyristors. To obtain the best effect, it has been foun that one should no.t have fine, expensive and fast diodes. Rather, one should have cheap and slow ones. Examples of diodes which have proved suitable are IN 4006 and BA 148. Without wishing to commit myself to a specific explanation which could very well be erroneous, the best explanation for now is that the rise time of the reverse voltage is crucial.

According to my practical experience, it is important to

-^{ R select diodes D3 and D4 in Fig. 3 so that they are not too fast, since then a clean ignition of the thyristors is obtained. The power generation in the thyristors can be schematically computed as the integral of the voltage thereover times the current. When a thyristor conducts, the voltage drop over the same is negligible. When it blocks, the current is negligible. The power which can be controlled is thus very dependent on the characteristics of the control pulses which control the thyristors. It might be mentioned also that a resolute pulse control of the thyristors tends to eliminate radio frequency disturbances. In view of the fact that the thyristors are choked by the motor winding, it is also clear that a rush of current at thyristor ignition heavy enough to cause high frequency disturbances, is hardly conceivable. Practical tests to detect radio disturbance support such a hypothesis.

Experience has shown that the circuit shown here makes it possible to control thyristors at effective currents almost double the ratings given on the data sheet and still get •them to work with such low heat generation that- the bare hand in any case cannot detect any rise in temperature.

For larger thyristors, experience has shown that this construction with the diodes D3 and D4 is less suitable. In this case it is more suitable to replace these diodes^"shown in Fig. 3 with resistors in series with diodes directed oppositely to those shown. For thyristors which, according to the data sheet, are to have a control pulse of about 2.5 - 3,5 volts, a suitable size for each resistor is 40 ohms for controlling 900A thyristors and 80 ohms for controlling of 75A thyristors.

However, the idea with reversed diodes is especially advantageous in igniting power triacs. Very good results have been obtained, for example, with triacs with a rated current of 90A, which have been able to be used up to 150 A without becoming warm. In the embodiments which have been tried up to now, the control voltage has been obtained by mounting a tachomete generator on the motor axle. The tachometer is coupled to a voltage divider network which is rather obvious to the person skilled in the. art, with the possibility of controlling the weighting of the tachometer voltage for t size of the control voltage. By controlling Rll in Fig. 3 one can then set the entire arrangement so that a zero- speed position is obtained where the output signal of the zero crossing detector has a top value agreeing with the control voltage intended for zero r.p. .

The principle of the control process is that a reduced control voltage causes the ramp to the differential amplifier for each ramp to exceed the control voltage for longer period and thus the pulse generator 33 gives off more pulses. Only the first pulse coming in such a series fires the one of the two thyristors which is ignitable. T result is obviously earlier ignition of the thyristor. Th thyristor will then conduct for a longer period, the moto is supplied with more power, increases -its torque and therefore runs faster, which leads to increased compensat control voltage. This results in r.p.m.. control.

By virtue of the fact that the reference phase is arrange according to the above, it is clear that if only one sing pulse is emitted per 180 period for the current, this pu will be placed approximately at the zero crossing for the voltage R-S (see Fig. 4). If there is more than one pulse then it must necessarily come earlier.

Under the conditions present here it is rather difficult make a calculation of what function the current in the win ings of the motor will follow and what phase displacement it has in relation to the voltage. One would have to take into account r.p.m.-dependent counter-electromotive force etc. and in fact expand the theory for the A.C. motor quite a bit. It would be too involved to go into it here ,

OM but it should suffice to say that the current curves in practice hardly look like those in Fig. 2 in the cited Swedish Patent Specification 365 673, since th y must pass through inductances. An appreciation for the difficulty of the problems can be gotten from reading Shepard: Thyristor Control of AC circuits (London 1977) .

Fig. 4 shows the result of a measurement made with an oscilloscope at point P in Fig. 3. The solid curve shows the voltage cycle at point P, when the thyristors are turned on for a relatively short portion of the time. The major portion of the time, the curve follows a sine wave corresponding to phase S, because the thyristors are then non-conducting and the motor in this trial is going rather slowly, so that hardly any MF is developed over La. Just before the voltage between S and R crosses zero one of the thyristors will become conducting and the point P will assume essentially the voltage R. The curve resumes follow¬ ing S when the current has crossed zero. This occurs after the zero crossing of the voltage because the load is essentially reactive. It can be seen that when the experi- ent is .carried out so that the ignition takes place earlier (not shown in the figure) , the shutting-off occurs later and later, until, when the power of the motor is completely engaged, the point P follows the voltage R for the major part of the period and the^" zero crossing of the current is phase-shifted approximately in the same way as in a normally connected three-phase motor.

The above description has been directed to a single phase. Since the rest of the phases are symetrically identical, it should suffice for understanding the invention. It should be pointed out that although the control system has been described based on a certain coupling whose specifications apply to a readily available integrated circuit on the market, this is not intended to limit the invention, which can be embodied in many different ways. For example, the control voltage is stated as being determined by t from a linear tachogenerator, which is suitable for simpl r.p.m. control. This can, of course, be done in a number obvious ways, for example by starting from magnetic sensi of a magnet rotating with the motor axle. Said magnet is 5 . sensed with a magnetometer sensor, thus obtaining a speed dependent signal. One might also wish to control the moto to ahcieve another effect than constant r.p.m. For exampl the control variable can be the mechanical torque generat by the motor, measured with some suitable sensor. One 10 might also wish to have a gradual start, which could be achieved by using a ramp voltage of suitable shape as the control voltage.

In a further development of an additional aspect of the 15 invention, a brake function has also been added. It is we known per se to make a three-phase motor run in the opposite direction, by shifting two phases. The purpose o the further development in question is instead to achieve improved control of the motor so that soft braking can b 20 achieved.

. ^' An example of this is shown in Fig. 5, which is to be vie in conjunction with Figs. 1 and 3. The figure shows the control for a single phase winding of a motor, but the

25 person skilled in the art will see how the other phases which are interchanged when braking are to be coupled, an that the third phase winding need not be recoupled. C and are control circuits made as in Fig. 3. In addition to th control circuits shown in Fig. 1, it is necessary to arra

30 two additional control circuits, one of which, designated is shown in the figure. This figure also shows how the mo can be controlled via a tachometer 40 coupled thereto. Depending on the r.p.m., the tachometer gives off a curre of 0-140 A which can suitably be smoothed by means not

35 shown. For forward running, the desired r.p.m. is set on potentiometer 44 (which can be replaced with a ramp funct if a special starting pattern is desired, e.g. even accel ration of an elevator) . As long as the value now set is j held, the control circuit C will give off control pulses. If the potentiometer is turned down, the control voltage to the control circuit C will drop and seek to drive up the speed of the motor, so that the tachometer generates more current. If the potentiometer.is turned up, the control voltage will increase, so that the control circuit C ceases completely to give off any output pulses, so that the switch to the winding Lc no longer closes. If we now wish to have a braking effect, it is apparent that it is in such situations it should function. A pulse setting circuit 41, for example consisting of a gate circuit controlled by a pulse extending circuit with appropriate time constant, will then give off an activating signal as soon as the pulses are absent for a significant period of time. This controls two relays 42 and 43, the former of which switches out the control circuit C and switches in the control circuit D to the switch for Lc. The latter relay 43 changes phase from R to S for coupling-in to the switch. It should be pointed out that while the control circuit C receives a reference phase from S and T, the control circuit D recieves its reference phase from R and T. Thus one has a complete interchange between R and S to the motor, which clearly causes a change of direction of the driving of the motor.

When the control circuit C now ceases to give off pulses, the pulse setter 41 will drive the relays 42 and 43 so that they switch over. The control circuit D is in operation the whole time and has a control voltage approximately corre- spσnding to that which the control circuit C had upon reversal. Thus a driving force will be given to the motor in the opposite direction to that previously prevalent. Through the effect of the resistance 46 and the capacitor 47 this voltage will now drop and the braking force will gradually increase, thus providing the advantageous, gradually acting braking effect.

During this whole time, however, the control rcuit C is coupled to the input side. This means that when the motor comes down to the desired r.p.m. set through adjustment of the potentiometer 44, the control circuit C will once again give off pulses. This will mean that the pulse setting circuit 41 will react to the fact that there are pulses at its input, thus causing the relays 42 and 43 to fall. The motor returns to forward drive again. In a norm braking sequence the control circuits C and D will thus work alternately, until the braking is complete.

It is clear that both the starting effect and the braking effect can be controlled in other ways as well, with the aid of specially arranged control voltages. In an elevato for example, it is best to be able to provide uniform acceleration to normal speed with the aid of a constant torque for the motor and maintain normal speed until the elevator is to be braked, and it is then desirable to have a constant braking torque for uniform deceleration. Such performance in elevator machinery is the most comfortable and places the least strain on the mechanical parts. Using the comments here with regard to Fig. 5, the person skille in the art will be able without great difficulty to achie this goal through suitable design choices depending on the characteristics of the machinery in question.

The invention makes it possible to use common A.C. motors in speed control, and in many cases it can be suitable to build in a device according to the invention in existing devices. One need merely think of the importance of maintaining carefully determined cutting speeds when it comes to cutting with modern materials and tools to see th value of being able to modernize otherwise obsolete machines without excesively high costs.

Tests have been carried out at the Royal Institute of

Technology in Stockholm, in which an 8 kW motor was brake tested to determine the mechanical effectiveness. The moto had a rated r.p.m. of 1500 r.p.m. At maximum r.p.m. mechanical effect of 82% was obtained. Upon control to 1000 r.p.m. it was 76% and at 750 r.p.m. it was 50%. The reduced mechanical effect is a result of the lag and is hardly surprising but is what should be expected. However, this characteristic of the invention is of minor importance in such applications, in which the mechanical power consumption drops drastically with reduced r.p.m., e.g. in fans, where the power requirements often drop by the fourth power of the r.p.m. Therefore, even if the mechanical effectiveness is relatively poor, the absolute power losses will be insignificant. Thus the invention is still advantageous because of the minimal cost in comparison to competing solutions, which are not based on lag. The previously known systems based on lag .have often proved to be unsuitable for fans, since even driving force from the motors often results in infra-sound from the blades of the fan.

For the most advantageous effect, when using the invention, one should place higher demands on the quality of the motors in view of the fact that the losses in mechanical effect at low r.p.m.s are essentially iron losses. The motor used in the test at the-Royal Institute of Technology was an old motor with cos φ = 0.62. Rated values are now often as high as 0.9, primarily because of better rotor design. Thus through proper selection of a three-phase motor, the losses can be reduced.

It can be noted that the principle for obtaining the reference phase can be generalized, and the person skilled in the art will see that the invention is not limited to triangle-coupled motors. It appears theoretically possible that reference phases for control can be obtained according to the same principle in star-coupled motors, for example, even if practical experiences lead me to prefer triangle- coupled motors at the present time.

OMPI y IPO

Claims

1. A current-time-controlled control device for a short circuit, multi-phase asynchronic motor, in each of the phase windings- (La,Lb,Lc) to the motor there being couple a controlled electronic switch (10,36,37) in such a manne that it oniy allows current to pass through the respectiv windings (La, Lb, Lc) during a certain portion of each cycle beginning at a controllable point in time within a half cycle and ending at the zero crossing of the current and the point in time for the switching on of the respective switch being determined by a control quantity (STYR) and being related to the zero value crossing of th current, characterized in that for each phase winding in the motor, a reference•phase (REF) created for the purpos is used, which phase is coupled to a zero crossing detect (31), each reference phase being taken from a voltage divider (R1-R2) , the ends of which are coupled to individ phase voltages.

2. A control device according to Claim 1, characterized in that the zero crossings of the reference phase coincid with the zero crossings for the voltage coupled over the associated ^"winding and switch.

3. A control device according to Claim 1,^" characterized in that the three mains phase wires are triangle-coupled (Fig. 1) to the motor, that the switches are coupled in series with individual phase windings (La, Lb, Lc), and that for each phase winding there is arranged as said reference phase a voltage divider consisting of two resis ances (Rl, R2) , a first one of the end points of said voltage divider being coupled to one of the mains phases connected to the phase winding in question and the other end point being coupled to the one of the three mains phases which is not connected to the phase winding in question, and the ratio of the two resistance values of t resistances being approximately 2.25.

4. A control device according to Claim 2, characterized in that each switch .consists of a triac, whose ignition electrode is coupled to a pulse generator via a pulse transformer through a diode which is coupled so that its reverse direction is the operative direction for ignition.

5. A control device according to Claim 3, characterized in that for each, of two of the switches of the phase windings a pair of different control circuits are arranged, a first control circuit (C, Fig. 5) with a reference phase determined by one of the mains phases (T) connected to the winding in question, and by one of the other two phases, and the other control circuit (D) in each pair having a reference phase determined by the same one mains phase (T) and the other (R) one of the remaining two phases, control pulses coming from a first control circuit being coupled to a pulse determining circuit (41) which gives off.a relay control signal as soon as the control pulses are absent for more than a few current cycles, signifying that the motor is running too fast, and whereupon the relay control signal interchanges the coupling of the above-mentioned other two phases to the phase windings of the motor and recouples the control input of the respective switch from the output of the first control circuit to that of the second, so that the motor will be imparted a direction of motion counter¬ acting the current feed and upon re-establishment of control pulses to the input of the pulse determining circuit (41) , the relay signal will cease, whereupon the recouplings caused thereby will revert to their original arrangements.