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GB1601463A - Dynamic braking of ac motors - Google Patents

Dynamic braking of ac motors Download PDF

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Publication number
GB1601463A
GB1601463A GB2477677A GB2477677A GB1601463A GB 1601463 A GB1601463 A GB 1601463A GB 2477677 A GB2477677 A GB 2477677A GB 2477677 A GB2477677 A GB 2477677A GB 1601463 A GB1601463 A GB 1601463A
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United Kingdom
Prior art keywords
phase
braking
capacitor
winding
excitation
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GB2477677A
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National Research Development Corp UK
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National Research Development Corp UK
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Priority to GB2477677A priority Critical patent/GB1601463A/en
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02PCONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P3/00Arrangements for stopping or slowing electric motors, generators, or dynamo-electric converters
    • H02P3/06Arrangements for stopping or slowing electric motors, generators, or dynamo-electric converters for stopping or slowing an individual dynamo-electric motor or dynamo-electric converter
    • H02P3/18Arrangements for stopping or slowing electric motors, generators, or dynamo-electric converters for stopping or slowing an individual dynamo-electric motor or dynamo-electric converter for stopping or slowing an AC motor
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02PCONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P3/00Arrangements for stopping or slowing electric motors, generators, or dynamo-electric converters
    • H02P3/06Arrangements for stopping or slowing electric motors, generators, or dynamo-electric converters for stopping or slowing an individual dynamo-electric motor or dynamo-electric converter
    • H02P3/18Arrangements for stopping or slowing electric motors, generators, or dynamo-electric converters for stopping or slowing an individual dynamo-electric motor or dynamo-electric converter for stopping or slowing an AC motor
    • H02P3/22Arrangements for stopping or slowing electric motors, generators, or dynamo-electric converters for stopping or slowing an individual dynamo-electric motor or dynamo-electric converter for stopping or slowing an AC motor by short-circuit or resistive braking

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Stopping Of Electric Motors (AREA)

Description

(54) DYNAMIC BRAKING OF A.C. MOTORS (71) We, NATIONAL RESEARCH AND DEVELOPMENT CORPORA TION, a British Corporation established by statute of Kingsgate House, 66-74 Victoria Street, London, SW1, do declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to control apparatus for the dynamic braking of a threephase of single-phase induction motor particularly in an arrangement in which on disconnection of the motor from the power supply a capacitor is connected to a pair of terminals of the primary windings such that self-excitation is produced.
It should be noted that the terms 'threephase' and 'single-phase' are used to define only the type of supply for which a particular motor is intended without reference to the specific construction of the motor. Thus a single-phase motor will have primary windings or polyphase form and some means of phase correction, such as a capacitor, to enable operation from a single-phase supply. The polyphase windings may be arranged as for a three-phase supply, or may have their axes in quadrature.To avoid possible ambiguity, particularly as between star-connected and delta-connected motors, the terms "a pair of terminals of the primary windings" is used in this specification to refer in the context of a three-phase motor to a pair of the three terminals to which the supply is connected and in the context of a single-phase capacitor-start motor to a pair of the three terminals normally provided.
The effect of self-excitation is to produce a strong dynamic braking torque which is of short duration but sufficient to cause a significant reduction in speed. Thereafter the motor will normally coast to a standstill which is particularly undesirable either for emergency stops or for cyclic production processes in which a motor must be stopped and then reversed or restarted in the shortest time consistent with tolerable mechanical stress. Various secondary braking mechanisms, mechanical or electrical, have been proposed to eliminate coasting and it is usual to arrange that such a mechanism should take effect before the end of selfexcitation.The precise operating conditions appear to be more critical for some arrangements than for others and it has been observed that a particular mode of braking is very effective on some occasions but ineffective on other occasions when the identical procedure appears to have been followed.
It is an object of the present invention to provide control apparatus for more consistently effective dynamic braking.
According to the invention there is provided control apparatus for the dynamic braking of an induction motor having primary windings for connection to a three-phase or single-phase power supply comprising a capacitor, switching means operative following disconnection of such a motor from a power supply to connect the capacitor across one pair of terminals of the primary windings of the motor to establish selfexcitation therein and control means effective after a predetermined delay from the connection of the capacitor and within a predetermined range of values of phase angle in a waveform of such self-excitation to cause a respective low-impedance path to be completed across each of at least one other pair of terminals of the primary windings, the arrangement being such that the motor undergoes initial braking during self-excitation and further braking on completion of the low-impedance path.
The apparatus may include delay means responsive to the operation of the switching means to produce a first enabling signal after the predetermined delay and phase selection means responsive to the occurrence in an excitation waveform of a selected phase angle related to the predetermined range of values to produce a second enabling signal, the control means being operative in response to the first and second enabling signals.
The control means may include normally non-conducting switch means operative to complete each said low-impedance path respectively and trigger means operative to transmit a trigger signal to render the switch means conductive.
The trigger means may include means responsive to the first enabling signal to produce a repetitive trigger signal and means responsive to the second enabling signal to transmit the trigger signal to each normally non-conducting switch means.
The phase-selection means may include means responsive to a parameter of the capacitor current waveform or of a voltage waveform and may include a phase shift network.
The normally non-conducting switch means may be unidirectional (typically a thyristor) or bi-directional (typically a contactor which may be controlled by electronic means) and may be connected across one or each of the pairs of terminals of the primary windings which is not connected to the capacitor.
The predetermined ranges of values of phase angle are specific to the switch configuration and manner of operation and preferred values may be determined, for example with respect to zero-crossings in the capacitor current waveform, for each case.
Preferably when two switches across respective windings are caused to conduct simultaneously and the arrangement is such that a low-impedance path is thereby completed across the capacitor, conduction should be initiated close to a zero-crossing in the waveform of capacitor voltage to avoid damage to the capacitor and to the switches.
The control means may be energised from the main power supply or from an auxiliary power supply derived from a self-excitation voltage.
The present invention is based on an appreciation that it is not sufficient that a short-circuit or similar low-impedance path should be initiated randomly during a stage of the self-excitation sequence determined by a delay device. In general it is considered that the conditions for braking which is reliably and repeatedly effective can be defined in terms of the relationship between the time at which the short-circuit is initiated and the value of a parameter of the self-excitation function at that time. It is convenient to refer timings to the zero crossings of the waveform of excitation current flowing in the capacitor, but it will be understood that the process by which a switch is operated to provide a short-circuit may be initiated by reference to other self-excitation parameters.Both the process of initiation and the phase angle at which the short-circuit should occur must be determined in dependence on the configuration of short-circuit switching to be used. In particular these requirements apply to the use of unidirectional switches such as thyristors which are the most probable choice.
It is found that for each thyristor configuration the most favourable initiation condition exists in only one quadrant of the self-excitation waveform and is then limited to a small sector of that quadrant. This segment always follows a sign change in the related voltage waveform such that a trigger signal can be applied in advance of the sign-change to await the favourable condition. The process will be referred to in this specification as 'lie-in-wait' triggering. A further constraint must be applied in the special case where two windings are to be individually short-circuited in such a way that the capacitor is also short-circuited. In oder to avoid damage to the capacitor and to the switches this condition should only occur when the capacitor voltage is low and preferably close to zero.
The importance of the correct choice of the range of phase angle lies in the fact that, in general, once a short-circuit is applied during a part of the cycle in which braking is ineffective the dynamic braking capability of the machine has been dissipated and the continued presence of the short-circuit means later in the cycle will have no effect.
Since for a configuration employing a single thyristor the preferred range of phase-angle within a cycle ma be as little as 15 the probability of effective braking by switching without regard to phase is proportionately small. In order to provide the necessary phase discrimination in the control system, the use of electronic switching is preferred.
Electromechanical means may be satisfactory if the response is sufficiently rapid and consistent and particularly if the electromechanical switch is controlled by an electronic switch.
The length of the initial delay which is desirable is related to the self-excitation behaviour of the machine. It is known that during self-excitation the reactance of the external capacitor must be substantially equal to the total inductive reactance of the machine as seen from the terminals to which the capacitor is connected and the total apparent resistance of the machine as seen from the same terminals must be substantially zero. These conditions are continuously accommodated by changes in the magne tic state of the machine as the speed falls until self-excitation can no longer be sustained. The relationships involved are not readily calculable and the value of capacitor to be most effective for a particular machine is best determined by experiment.Typically, for a suitable value of capacitor, excitation builds up (as indicated by the selfexcitation voltage) within two or three cycles to a level which would be maintained if the machine were driven by a load and then declines over a few tens of cycles.
Assuming an initial frequency somewhat lower than the normal supply frequency experience indicates that the minimum initial delay should be made at least equal to the build-up time. For a 50 Hz supply, a minimum delay in the range 75 to 100 ms would be appropriate with a maximum in the region of 400 ms. In general, braking occurs less abruptly as the delay period is extended.
The control of a dynamic braking process therefore requires provision for a delay period after connection of the capacitor to enable self-excitation to be properly established followed by the selection of a favourable instant in a self-excitation cycle at which to apply a short-circuit. It will be apparent that on disconnection of the motor from the supply no appreciable reduction in speed will occur immediately so that if desired a delay may be allowed between disconnection of the supply and connection of the capacitor.
An embodiment of the invention and its mode of operation will be described with reference to the accompanying drawings in which: Figure 1 represents diagrammatically a three-phase motor with a control circuit in accordance with the invention; Figure 2 illustrates the operation of the control circuit of Figure 1 by means of representative waveforms; Figure 3 represents a three-phase motor having an alternative form of control circuit in accordance with the invention; Figure 4 represents a single-phase motor having a control circuit in accordance with the invention; Figure 5 represents a switch configuration for use in the circuits of Figures 1 or 3; Figure 6 illustrates the operation of various switch configurations in relation to representative waveforms; and Figure 7 (a to d) represent alternative switch configurations for use in the circuits of Figures 1 and 3.
With reference to Figure 1 a stator 10 is shown for a three-phase induction motor having delta-connected primary windings A, B and C connected between pairs of terminals (12, 14), (14, 16) and (16, 12) respectively. The following description would apply equally to a star-connected winding. The windings A, B, C, are connected for energisation via a contactor 18 in the phase sequence 12, 14, 16. The contactor 18 includes a pair of normally-open contacts 20 in each phase of the supply and two pairs of normally-closed contacts 22 and 24. one contact of the pair 22 is connected direct to the terminal 16 and the other via a capacitor 26 to the terminal 12. When the motor is running contacts 22 and 24 are held open but the contactor 18 is so arranged that, immediately after the supply is disconnected, contacts 22 and 24 close.Via contact pair 22, capacitor 26 is thus connected across the winding C. The value of the capacitor 26 as has been explained can best be determined by experiment but a value in IsF equal to 15 x 103 X Rated Current/ Rated Voltage has been found to be satisfactory for a three-phase motor.
The closing of contact pair 24 causes an auxiliary power unit 28 to be connected across winding B so that the control circuit becomes independent of mains power. The second stage of the mode of braking which is to be demonstrated now requires a switch to be connected across one of the windings A, B. The closing of contact pair 24 is therefore also arranged to connect a thyristor 30 across winding B, the direction of connection shown being that for which thyristor 30 conducts only during positive half-cycles in winding B, i.e. when terminal 14 is positive relative to terminal 16.
Referring now to Figure 2 idealised waveforms of arbitrary amplitude are illustrated for the self-excitation current I, (curve 50) in the path which includes capacitor 26 and the voltages VA, VB and Vc (curves 52, 54, 56 respectively) on the corresponding windings A, B, C. The scale represents electrical degrees and a zero value of Is, preceding a negative half-cycle, is taken as the origin. It has been established experimentally by the inventors that for a bi-directional switch across winding B the most effective braking results from closing the switch within the quarter-cycle or quadrant preceding those points were Is=o at 1800 and 360". Braking is not uniformly effective within these ranges and for the lower range the preferred region is that between 60 and 15 before current zero.In the second quadrant this region is from 1200 to 165". When, however, thyristor 30 is employed as the switch, conduction only begins at 1500 where VB goes positive. In this instance therefore the preferred region is limited to a period of 15 from 1500 to 1650 and thyristor 30 must receive a trigger signal during this time.
Conveniently, since thyristor 30 cannot conduct during the preceding half-cycle in VB a trigger signal can be applied at any time after 300 to 'lie in wait' for the positive going zero-crossing of VB at 1500.
Referring again to Figure 1 the trigger signal is derived from a pulse generator 32 controlled by a timer 34, the generator 32 and the timer 34 being supplied by the auxiliary power unit 28. On the initiation of braking, unit 28 is energised by winding B and causes timer 34 to start a predetermined running period equal to the required initial delay. At the end of this period the pulse generator 32 is switched on by timer 34 to produce a train of pulses at a frequency very much higher than the frequency of selfexcitation, 15 KHz being suitable. The pulse train is fed to the trigger electrode of thyristor 30 through a pulse transformer 36 and a thyristor 38 which is in turn triggered by the secondary voltage of transformer 40.
The primary winding of transformer 40 in series with a capacitor 42 is connected across thyristor 30 and consequently, when contact pair 24 is closed, across winding B. Transformer 40 and capacitor 42 thus form a phase shift network which functions as a phase-selector with respect to the phase of waveform Vu. The value of capacitor 42 will therefore govern the period during which thyristor 38 conducts to enable trigger pulses to reach thyristor 30. The phase-shift network is adjusted so that trigger pulses are applied from a point subsequent to --300 to a point within the desired range of 1500 to 165 where thyristor 30 becomes conducting.
Referring to Figure 3 an alternative form of control apparatus is shown in which phase information is derived directly from the self-excited current. A three-phase motor 10 is shown with supply and switch arrangements identical to those of Figure 1 as indicated by common reference numbering.
On disconnection of the main supply via contactor 18 a capacitor 26 is connected across terminals 12, 16 of winding C and an auxiliary power unit 28 for the control circuit is energised from winding B. A thyristor 30 is also connected across winding B. As before the control circuit includes a timer 34 which introduces a delay sufficient to allow self-excitation to be fully established and thereafter produces an output signal for switching on a pulse generator 70.
The signal is transmitted via and AND gate 72. A second input is required to open gate 72 and is derived from a phase-angle selector 74. When gate 72 is open pulses from generator 70 are applied via a pulse transformer 76 to the trigger electrode of 30. In the manner of operation explained for the circuit of Figure 1 with reference to Figure 2, thyristor 30 must be triggered within the range of phase angle from 1500 to 165 ; in the circuit of Figure 3 the precise point of triggering within this range is determined by the phase-angle selector 74. The input to selector 74 is derived via an amplifier 80 from a current transformer 78 in the selfexcitation circuit and therefore provides information on the phase of self-excitation current at any instant.Selector 74 comprises a peak detector arranged to produce a pulse when the current waveform passes through a negative peak, followed by delay means such that the pulse is passed to gate 72 at the desired point in the excitation current cycle.
Alternatively, other points of the cycle could be detected, in response for example to a zero-crossing or to a specific gradient, and an appropriate delay provided.
Referring again to Figure 2, it was stated earlier that for a bidirectional switch across winding B 2 further region of effective braking was found for a short-circuit occurring before the current zero point at 360" but this region proves not to be reliably effective for a unidirectional switch conducting in the direction shown for thyristor 30.
A complementary mode of operation is however available if the polarity of thyristor 30 in Figure 1 or Figure 3 is reversed when the region 330" to 345" with VB negativegoing permits 'lie-in-wait' triggering.
In this case there is no reliably effective braking region in the second quadrant, each thyristor configuration having a preferred range of operation in a single quadrant.
On replacing thyristor 30 by a bidirectional switch the second and fourth quadrants and patt---ularly the ranges 1200 to 1650 and 3000 to 345" become available wit-h delayed tfiggering. In this case a hea-ny-duty electromechanical switch may be preferred; precision of timing is then maintained by the use of electronic means to control the actuating current of the switch with an appropriate allowance for the delay in response of the switch.
As an alternative to the use of thyristor 30 connected across winding B, a short-circuit can be introduced across winding,4 by means of a unidirectional electronic switch such as a thyristor or by means of a bi-directional switch. A similar analysis to that discussed in relation to winding B can be applied with respect to the voltage variation Vg on winding A. The experimental basis in this case is that in general the most effective braking occurs, for a bidirectional switch, when a short-circuit is applied in the quadrant following each zero-crossing in Is (curve 50 of Figure 2).
(To recall the effect of short-circuiting winding B, the most effective regions then occurred in the quadrant preceding each zero-crossing in the Is curve). More specifically it is considered that, for a bi-directional switch, the desirable operation regions lie between 15 to 600, and between 195 and 240". For uni-directional switches parts of these regions are preferred according to the relative polarities of the switch and the winding. Thus for a thyristor having its positive terminal connected to terminal 12 of winding A conduction can be initiated only during the positive half-cycle of curve VA and reliably effective braking is limited to the range 30 to 600; in the reverse sense the complementary range 210 to 240 applies.As with winding B, each region allows the use of 'line-in-wait' triggering. It will be seen that the preferred regions for winding A lie in the quadrant which were not reliably useful for winding B.
It is also of practical importance that a control system in any of the forms so far described can be applied to the dynamic braking of machines having primary windings arranged for operation with quadrature (or lesser) phase displacement from a singlephase supply. The only limitation over the three-phase winding is that the selfexcitation capacitor must be connected between a predetermined pair of the primary winding terminals. With reference to Figure 4 a quadrature wound motor 84 is shown together with the immediately associated switching arrangement for connection to the control circuits of Figure 1 or Figure 3. Two primary windings have independent terminals 86 and 88 and a common terminal 90. A single-phase main power supply is connected to terminals 86, 90 via contactor contacts 92.When the supply is disconnected, contacts 92 open and contact 94 closes to connect a capacitor 96 across terminals 86, 88. An auxiliary power supply may be energised by the self-excitation voltage between terminals 88 (via contact 95) and 90. A dotted connection is also shown between terminals 86, 88 to indicate the position of a starting capacitor 98. After self-excitation is established the braking short-circuit may be applied to either pair of terminals 86, 90 or 88, 90.
A three-phase machine will now be considered in which switches are arranged to apply a capacitance across one pair of terminals of the primary windings and then to short-circuit both of the other pairs of terminals. It is found that the braking effect is enhanced over the effect of a single short-circuited winding provided that the short-circuits are correctly timed in relation to the phase of the excitation waveform.
With reference to Figure 5 a three-phase motor 10 is represented by windings A, B and C as in Figures 1 and 3 and capacitor 26 is shown connected across the terminals 12, 16, but the contactor 18 is not shown.
Similar control arrangements to those shown in either of Figures 1 and 3 would be employed. A bi-directional switch 100 is connected across winding A and a similar switch 102 across winding B. It is supposed that the connection of capacitor 26 has produced self-excitation so that the waveforms in the motor windings are those shown in Figure 6, substantially reproducing the curves 50, 52, 54, 56 of Figure 2. One mode of braking would be to operate the switches 100 and 102 separately within the favourable regions of phase-angle identified for the individual windings in the discussion referring to Figure 2. An alternative mode is to operate switches 100 and 102 simultaneously when effective braking is found to occur although clearly the desirable phase criteria cannot be satisfied for both windings.A criterion which can be observed, since capacitor 26 is now immediately shortcircuited, is that the value of Vc (curve 56 of Figure 6) can be chosen to be very close to zero to avoid the risk of damage both to the capacitor and to the switches. Phase angles of 90" and 270 are therefore preferred for the switch combination 100, 102.
In the arrangements of Figures 7 (a to d) again showing details for incorporation in Figure 1 or Figure 3, switches 100 and 102 of Figure 5 are replaced by unidirectional switches. The individual and mutual polarities then become significant in analysing the operation of the system. Thus in Figure 7(a) thyristors 104 across winding A and 106 across winding B are similarly directed with positive terminals connected to terminals 12 and 14 respectively; in Figure 7(b) thyristors 108 and 110 are similarly directed in the opposite sense to those of Figure 7 (a). In Figure 7(c) thyristors 112 and 114 are opposed with a common positive connection to terminal 14 and in Figure 7 (d) thyristors 116 and 118 are opposed with a common negative connection to terminal 14.Figures 7 (a to d) will be discussed with reference to Figure 6, the thyristors being referred to by the symbols TA, TB for simplicity, the suffixes A and B referring to the corresponding windings, with the reference number added to identify the configuration.
It will be shown that conditions can be selected for the switch configurations of Figures 7 (a to d) which will lead to simultaneous firing of the two thyristors or which produce a distinctly sequential action.
To be reliably and repeatably effective, braking with sequential action requires that at least for the first of the switches to be fired, conduction should start within the phase range which was found to be preferred for a switch in that position and of that polarity when used alone. In the preferred order of firing the thyristor having a preferred range in the earlier quadrant is the first to be triggered. From the results of the preceding discussion the scale of phase angle of Figure 6 is marked with the preferred ranges for the relevant single thyristors.Thus the reference TA (104, 116) indicates the preferred range 30 to 60 for either of those thyristors, TB (106, 114) the preferred range 1500 to 165 , TA (108, 112) the preferred range 210 to 2400 and TB (110, 118) the preferred range 330" to 3450.
Referring to Figure 7(a) and Figure 6, the combination TA (104), TB (106) is required. TA (104) can be switched on in the first quadrant as soon as VA becomes positive and VB being negative TB (106) will be unaffected. Consider 'lie-in-wait' triggering to cause conduction at a point 120 on curve VA. (In order to preserve clarity in the diagram, the point 120 is shown displaced slightly from its actual position close to the zero-crossing). The value of VA then falls to a low positive level which for convenience may be considered to be zero.In order that the machine voltages should remain balanced when VS, becomes zero, Va and Vc must subsequently equalise with opposite sign and since Vc cannot change rapidly the balance must be provided by a step change in VB. Thus a step in Va to a level 122 is required. Vc then continues to decay with a possible departure from the original curve 56 suggested by the curve 124, while VB decays in mirror fashion from level 122 to a zero crossing 126. As Vc continues negatively, VB becomes positive so that TB (106) also conducts. Both Vg and Vc then return to zero, the braking sequence then being completed.
Referring to Figure 7b and Figure 6 it will be seen that the desirable order of firing is TA (108) followed by TB (110). TA (108) is caused to conduct in the third quadrant as soon as VA goes negative and a braking sequence closely by similar to that desired for the configuration of Figure 7a is then followed.
Since in Figure 7a TA (104) and TB (106) conduct in the same direction, (as do TA (108) and TB (110) in Figurue 7b in the opposite sense), the capacitor 26 is short circuited as soon as both thyristors in either configuration become conducting. As described for the switches of Figure 5, simultaneous triggering can produce effective braking but in practice such action must be limited to the condition when Vc is close to zero.
Referring to Figure 7c, the desirable order of firing is indicated by the order of the preferred quadrants in Figure 6 as being first TB (114) and then TA (112). Thus TB (114) should be fired at a point on curve VB within the range 1500 to 1650 which will correspond for 'lie-in-wait' triggering to the point 130 just after the zero-crossing. As Vg falls to zero, VA rises to a level 132 and then follows a curve such as the curve 134 which tends to zero at a point 136. As soon as VA goes negative TA (112) will fire to complete the braking sequence. In practice Vc, and therefore also VA, will approach zero at a rate dependent on the precise conditions in the machine but TA (112) is always found to fire at an instant such that effective braking results.It will be apparent that since the waveforms of Figure 6 represent the initial uniform excitation frequency the waveform during braking is not truly indicated, there being a progressive reduction in frequency as the speed of the motor is reduced. Any estimate of the decay time of Vc is therefore subject to uncertainty for this reason.
Referring to Figure 7d, the desirable order of firing is indicated by the order of the preferred quadrants in Figure 6 as being first TB (118) and then TA (116). In a similar manner to that described with reference to Figure 7c, firing TB (118) within the range 330" to 345 , VA mirrors the decay of Vc towards a zero crossing at a point in the first quadrant of the following cycle.
Preferred ranges of phase angle have been defined for the initiation of the shortcircuit in the configurations of Figure 7 in order to provide convenient and reliable braking. Points outside the defined ranges can be found at which braking may be produced but with varying degrees of uncertainty and no compensating advantage and in particular such points are not amenable to -lie-in-wait' triggering. At some of these points the risk of damage to circuit compo aents is substantial.
In this connection an aspect of the opposed polarity configuration of Figures 7c and 7d to be noted in comparison with the series configuration of Figures 7a and 7b is that the two thyristors when conducting do not represent a unidirectional current path across the capacitor 26 and so do not introduce the risk of damage if Vc should be non-zero.
In using the auxiliary power supply (28) connected as shown in Figures l and 3, with any configuration of Figures 7(a to d) which leads to sequential conduction in TA and TB, the power input will be cut off or rapidly reduced on the occurrence of the first short-circuit. The auxiliary power supply should therefore be designed to provide a regulated output which remains adequate with reduced input. The smoothing time constant can readily be made sufficient to cover the normal braking period after the input has fallen to zero. The auxiliary power supply may of course be connected to derive its input from windings other than the winding B to provide fail-safe braking in the event that the machine becomes completely isolated from the main power supply. In a situation where such an eventuality need not be taken into account the braking controls may be energised directly from the main supply.
Tests carried out on single-phase and three-phase induction motors with a short circuit applied to a single winding show that dependable braking is obtained when the short-circuit is initiated within the preferred ranges of phase angle. For a 3KW threephase motor a 200 ijF capacitor was used and with normal inertia the motor was stopped in 0.ls after little more than two revolutions. A high value of peak torque is experienced, in comparison for example with braking by normal d.c. injection when the stopping time is nine times longer. The peak torque can be reduced and the stopping time extended by lengthening the period of the initial delay but the braking effect will fail if the delay is made too long.
Control systems for 3-phase motors in which short circuits are applied to two windings, in accordance with the invention, in general provide more effective braking than when only one winding is shortcircuited. An advantage may be derived in alternative ways. For example: a higher value of inertia can be accommodated; the value of capacitance can be reduced in the self-excitation stage; the period of initial excitation can be reduced and a very short overall stopping time achieved; or, the initial excitation period can be increased and the overall stopping time extended so that the peak torque is reduced.
Whereas the embodiments described in this specification have included provision for the application of a 'short-circuit' across a machine winding the general requirement will be satisfied by a path having a low but non-zero value of impedance. Selection of this value provides a further means of variation in the braking performance and therefore of the peak torque. It will be appreciated that the means by which the braking capacitor is switched across the machine winding is described in terms of a conventional type of contactor and that other means of producing such a switching action could be employed within the ambit of the invention. Similarly it is intended that means other than those described may be employed to determine the value of any parameter of the self-excitation function to which the instant of application of the winding short-circuit may advantageously be related.
WHAT WE CLAIM IS; 1. Control apparatus for the dynamic braking of an induction motor having primary windings for connection to a three-phase or single-phase power supply comprising a capacitor, switching means operative subsequent to disconnection of such a motor from a power supply to connect the capacitor across one pair of terminals of the primary windings to establish self-excitation therein and control means effective after a predetermined delay from the connection of the capacitor and within a predetermined range of values of phase angle in a waveform of such self excitation to cause a respective low-impedance path to be completed across the or each of at least one of the other pairs of terminals of the primary 'windings, the arrangement being such that the motor undergoes initial braking during selfexcitation and further braking on completion of the or each low-impedance path.
2. Apparatus according to Claim 1 including delay means responsive to the operation of the switching means to produce a first enabling signal after the predetermined delay and phase selection means responsive to the occurrence in an excitation waveform of a selected phase angle related to the predetermined range of values to produce a second enabling signal, the control means being operative in response to the first and second enabling signals.
3. Apparatus according to Claim 1 or Claim 2 in which the control means includes normally non-conducting switch means operative to complete each said lowimpedance path respectively and trigger means operative to transmit a trigger signal to render the switch means conductive.
4. Apparatus according to Claim 3 in which the trigger means includes means responsive to the first enabling signal to produce a repetitive trigger signal and means responsive to the second enabling signal to transmit the trigger signal to each normally non-conducting switch means.
5. Apparatus according to any of Claims 2 to 4 in which the phase-selection means comprises a phase-shift network having a voltage input from a primary winding.
6. Apparatus according to any of Claims 2 to 4 in which the phase-selection means includes means responsive to a parameter of the capacitor current waveform.
7. Apparatus according to any of Claims 2 to 6 in which the phase-selection means includes means for causing a predetermined delay between the production of the second enabling signal and the receipt of said signal by the control means.
8. Apparatus according to any of Claims 3 to 7 in which the or each normally non-conducting switch means is adapted for bi-directional conduction.
9. Apparatus according to Claim 8 in which a low-impedance path is completed across only one of the other pairs of terminals.
10. Apparatus according to Claim 9 in which the predetermined range of values of phase angle includes at least a sector of each of the quadrants preceding a zero-crossing in the waveform of capacitor current, or of each alternate quadrant in dependence on the phase of the winding which is associated with the low-impedance path.
11. Apparatus according to Claim 10 in
**WARNING** end of DESC field may overlap start of CLMS **.

Claims (24)

**WARNING** start of CLMS field may overlap end of DESC **. circuit applied to a single winding show that dependable braking is obtained when the short-circuit is initiated within the preferred ranges of phase angle. For a 3KW threephase motor a 200 ijF capacitor was used and with normal inertia the motor was stopped in 0.ls after little more than two revolutions. A high value of peak torque is experienced, in comparison for example with braking by normal d.c. injection when the stopping time is nine times longer. The peak torque can be reduced and the stopping time extended by lengthening the period of the initial delay but the braking effect will fail if the delay is made too long. Control systems for 3-phase motors in which short circuits are applied to two windings, in accordance with the invention, in general provide more effective braking than when only one winding is shortcircuited. An advantage may be derived in alternative ways. For example: a higher value of inertia can be accommodated; the value of capacitance can be reduced in the self-excitation stage; the period of initial excitation can be reduced and a very short overall stopping time achieved; or, the initial excitation period can be increased and the overall stopping time extended so that the peak torque is reduced. Whereas the embodiments described in this specification have included provision for the application of a 'short-circuit' across a machine winding the general requirement will be satisfied by a path having a low but non-zero value of impedance. Selection of this value provides a further means of variation in the braking performance and therefore of the peak torque. It will be appreciated that the means by which the braking capacitor is switched across the machine winding is described in terms of a conventional type of contactor and that other means of producing such a switching action could be employed within the ambit of the invention. Similarly it is intended that means other than those described may be employed to determine the value of any parameter of the self-excitation function to which the instant of application of the winding short-circuit may advantageously be related. WHAT WE CLAIM IS;
1. Control apparatus for the dynamic braking of an induction motor having primary windings for connection to a three-phase or single-phase power supply comprising a capacitor, switching means operative subsequent to disconnection of such a motor from a power supply to connect the capacitor across one pair of terminals of the primary windings to establish self-excitation therein and control means effective after a predetermined delay from the connection of the capacitor and within a predetermined range of values of phase angle in a waveform of such self excitation to cause a respective low-impedance path to be completed across the or each of at least one of the other pairs of terminals of the primary 'windings, the arrangement being such that the motor undergoes initial braking during selfexcitation and further braking on completion of the or each low-impedance path.
2. Apparatus according to Claim 1 including delay means responsive to the operation of the switching means to produce a first enabling signal after the predetermined delay and phase selection means responsive to the occurrence in an excitation waveform of a selected phase angle related to the predetermined range of values to produce a second enabling signal, the control means being operative in response to the first and second enabling signals.
3. Apparatus according to Claim 1 or Claim 2 in which the control means includes normally non-conducting switch means operative to complete each said lowimpedance path respectively and trigger means operative to transmit a trigger signal to render the switch means conductive.
4. Apparatus according to Claim 3 in which the trigger means includes means responsive to the first enabling signal to produce a repetitive trigger signal and means responsive to the second enabling signal to transmit the trigger signal to each normally non-conducting switch means.
5. Apparatus according to any of Claims 2 to 4 in which the phase-selection means comprises a phase-shift network having a voltage input from a primary winding.
6. Apparatus according to any of Claims 2 to 4 in which the phase-selection means includes means responsive to a parameter of the capacitor current waveform.
7. Apparatus according to any of Claims 2 to 6 in which the phase-selection means includes means for causing a predetermined delay between the production of the second enabling signal and the receipt of said signal by the control means.
8. Apparatus according to any of Claims 3 to 7 in which the or each normally non-conducting switch means is adapted for bi-directional conduction.
9. Apparatus according to Claim 8 in which a low-impedance path is completed across only one of the other pairs of terminals.
10. Apparatus according to Claim 9 in which the predetermined range of values of phase angle includes at least a sector of each of the quadrants preceding a zero-crossing in the waveform of capacitor current, or of each alternate quadrant in dependence on the phase of the winding which is associated with the low-impedance path.
11. Apparatus according to Claim 10 in
which the sector lies between 15 and 60 from the related zero-crossing.
12. Apparatus according to Claim 8 in which a respective low-impedance path is completed across each of the other pairs of terminals.
13. Apparatus according to Claim 12 in which the low-impedance paths are caused to be completed simultaneously.
14. Apparatus according to Claim 13 in which the phase angle when the paths are completed is that for which the capacitor voltage is substantially zero.
15. Apparatus according to any of Claims 3 to 7 in which the or each normally non-conducting switch means is adapted for uni-directional conduction
16. Apparatus according to Claim 15 in which first uni-directional switch means is operative such that a low-impedance path is completed across only one of the other pairs of terminals.
17. Apparatus according to Claim 16 in which the predetermined range of values of phase angle includes a sector of that quadrant in which the voltage on the associated winding changes sign to that favourable to conduction through the switch means.
18. Apparatus according to Claim 17 in which the sector extends for a minimum of 15 following the change of sign.
19. Apparatus according to Claim 18 in which the sector extends for a minimum of 15 or a maximum of 60 from the closest adjacent zero-crossing in the waveform of capacitor current.
20. Apparatus according to any of Claims 17 to 19 in which the trigger means is operative to transmit the trigger signal within the half-cycle preceding the change of sign such that 'lie-in-wait' triggering occurs.
21. Apparatus according to any of Claims 16 to 20 in which further unidirectional switch means is operative in response to completion of said path to cause a low-impedance path to be completed subsequently across another one of the pairs of terminals.
22. Apparatus according to Claim 21 in which the first path to be completed is that for which the predetermined range occurs in the earlier quadrant.
23. Apparatus according to any preceding claim in which the control means is energised from an auxiliary power supply derived from one of the primary windings during self-excitation.
24. Apparatus substantially as hereinbefore described wth reference to Figures 1 or 3 alone, or as modified by Figures 4 or 5 or 7a to 7d.
GB2477677A 1978-05-31 1978-05-31 Dynamic braking of ac motors Expired GB1601463A (en)

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Application Number Priority Date Filing Date Title
GB2477677A GB1601463A (en) 1978-05-31 1978-05-31 Dynamic braking of ac motors

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Application Number Priority Date Filing Date Title
GB2477677A GB1601463A (en) 1978-05-31 1978-05-31 Dynamic braking of ac motors

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GB1601463A true GB1601463A (en) 1981-10-28

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN104518709A (en) * 2013-09-28 2015-04-15 安德烈·斯蒂尔股份两合公司 Method for braking an electric drive motor

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN104518709A (en) * 2013-09-28 2015-04-15 安德烈·斯蒂尔股份两合公司 Method for braking an electric drive motor
EP2854283A3 (en) * 2013-09-28 2016-01-27 Andreas Stihl AG & Co. KG Method for braking an electric drive motor
US9306478B2 (en) 2013-09-28 2016-04-05 Andreas Stihl Ag & Co. Kg Method for braking an electric drive motor
CN104518709B (en) * 2013-09-28 2019-01-01 安德烈·斯蒂尔股份两合公司 Method for braking electric drive motor

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