EP0145774A1 - Electrical circuit for switching a multi-phase a.c. supply to a load - Google Patents

Electrical circuit for switching a multi-phase a.c. supply to a load

Info

Publication number
EP0145774A1
EP0145774A1 EP84902521A EP84902521A EP0145774A1 EP 0145774 A1 EP0145774 A1 EP 0145774A1 EP 84902521 A EP84902521 A EP 84902521A EP 84902521 A EP84902521 A EP 84902521A EP 0145774 A1 EP0145774 A1 EP 0145774A1
Authority
EP
European Patent Office
Prior art keywords
semiconductor rectifier
input
variable resistance
scrs
switching means
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP84902521A
Other languages
German (de)
French (fr)
Inventor
Gilbert C. Dumont
Pierre-Yves Dumont
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Limitorque Ltd
Original Assignee
Limitorque Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Limitorque Ltd filed Critical Limitorque Ltd
Publication of EP0145774A1 publication Critical patent/EP0145774A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/005—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection avoiding undesired transient conditions
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P23/00—Arrangements or methods for the control of AC motors characterised by a control method other than vector control
    • H02P23/24—Controlling the direction, e.g. clockwise or counterclockwise

Definitions

  • This invention relates to an electrical circuit for switching a multi-phase a.c. supply to a load, and has particular application to reversibly switching a three phase a.c. supply to an electric motor, for running, stopping and reversing the motor.
  • the invention can be used with particular advantage to control an electric motor used to drive a valve actuator.
  • a known valve actuator for example our Limitorque Model SMC 2005* comprises a three phase electric motor which drives a reduction gearing having an output for connection to a valve to drive its valve member.
  • the reduction gearing and the motor are mounted in a heavy explosion proof cast metal housing.
  • Mounted in a compartment within the housing is a set of electrical contactors which switch a three phase electrical supply to the motor so that the motor can be stopped, started and reversed.
  • the contactors are operated by relay coils themselves driven from a single phase of the a.c. supply. RE?
  • a further variable resistance device is connected across the downstream ends of the non-linear inductances of one or more phases.
  • a three phase motor switching circuit will now be described in detail with reference to the embodiment thereof shown in the drawing.
  • the three input phases R.S.T. of an a.c. supply are fed to input terminals 20, 21, 22 and the three phase inputs of a three phase motor are connected to terminals U.V. and W.
  • Semiconductor rectifier switches in the form of triacs SCR1 - 5 control the connection of the input phases R.S.T. to the terminals U.V.W.
  • the phases are connected as follows:-
  • a relay comprising a coil RLl and switching contacts RLl - 1, 2 and 3 *
  • the relay coil RLl is energised in response to a control signal on line l6 from the control logic circuit (not shown) , the control signal being indicative that the motor is to be run forwardly.
  • Each of the relay contacts is connected in series with a dropping resistor Rg so that when the contacts are closed a suitable gate potential is derived from the a.c. supply and applied to the gates of SCRl, and 5 to fire them to a conductive state.
  • the relay RLl is released, thus opening the contacts RLl -1 to RLl - 3, thus causing the SCRs 1, and 5 to switch off and stop the motor.
  • Reverse running of the motor is controlled in a similar way by means of a relay RL - 2 having a coil energised by a reverse running signal on line 17 from the control logic circuit (not shown).
  • the relay RL2 has contacts RL2 - 1, 2, 3 which when closed, fire SCRs 2, 3 and to a conductive state.
  • the electric motor connected to the terminals U V and W essentially comprises an inductive load.
  • the triac SCRl when it has been conductive and is then switched off, transients occur which would produce spurious firing of the triac if the following precautions were not taken.
  • OMPI > - WIPO triac SCRl a substantial rapidly rising transient back emf is developed in the motor coils and the back emf is applied to SCRl.
  • the impedance presented by the switched off triac SCRl is defined predominately by the capacitances of the pn junctions on either side of the SCR's gate and thus, in the presence of the transient the SCR's equivalent circuit is two capacitors connected in series and on opposite sides of the gate electrode.
  • the rapidly rising back emf can cause charge to build up in the capacitors defined by the triacs pn junctions so as to raise the gate potential sufficiently retrigger the triac into conduction.
  • a snubber circuit comprising a snubber resistor Rs and a snubber capacitor Cs, provides a by-pass for the transient back emfs.
  • the time constant of the snubber circuit is selected so that at the a.c. frequency of the supply, the snubber circuit presents a high impedance, but at the high frequency of the transient back emfs the transient currents pass preferentially through the snubber circuit so as to by-pass SCRl, thereby to prevent the spurious firing.
  • Each of the SCRs 1 to 5 is provided with its own snubber circuit Cs Rs.
  • each of the input phases R and S can be selectively connect ⁇ ed to the terminals U V by means of the SCRs 1 to 4 5 so that the motor can be run both forwardly and backwardly.
  • An SCR behaves as a capacitor when in its "off" condition which means that the input circuit is a capacitively terminated line when the motor is de-energised or is running in one direction.
  • SCRl will produce spurious firing of SCR3 which would result in a short circuit being produced across the phases R and S.
  • a similar short circuit could be established if SCRs 2 and 3 became conductive sim ⁇ ultaneously.
  • This step wavefront has a substantially greater rate of rise than the back emfs developed upon switching off of the SCRs and consequently the snubber circuit Cs Rs associated with SCR 3 would be unable to suppress the rapid voltage rise at the cathode 2k.
  • the rapid voltage rise associated with the switching on of SCR 1 would be likely to produce spurious firing of SCR 3 as a result of the transient voltage rise raising the gate potential of SCR 3»
  • firing on SCR 3 could produce spurious firing of SCR 1 and SCRs 2 and k would interact in a similar manner.
  • Such spurious firing and short circuiting of the phases is prevented by providing in series with the SCRs 1 to 5 respective inductor coils ⁇ L1 - each provided with a core of a material exhibiting a non- linear permeability.
  • the permeability of the core varies as a non-linear function of the frequency and magnitude of the applied field, and the core is typically made of ferrite material i.e. sintered carbides or iron powder material.
  • the core may also be made of permalloy, for example, Mulypermalloy manufactured by Magnetics, a division of Spang Industries Inc., P.O. Box 391, Butler Pa, l ⁇ OOl, U.S.A. or powdered soft iron.
  • the inductor coils ⁇ , -* have the effect of reducing the rate of rise of current to which the SCRs are subjected so that no spurious firing occurs.
  • the inductors appear to operate preferentially on the rapid transients which occur upon switch on; if a solid soft iron bar is used, the core inductors are heated by the a.c. supply whilst the motor is running. Conversely if no core is used in the inductors, resonance at radio frequencies may be produced in response to switching of the SCRs 1 to 5.
  • each device 30R ⁇ 30S, 30T is a varistor which operates to clamp the input voltage to a maximum preset value, say Vtrum of its associated rectifier device.
  • Varistors have the advantage of preventing sharp dv/dt changes such as might exist with an air gap arrestor.
  • each varistor has an internal capacitance which is larger than the capacitance of each rectifier device and the presence of this upstream non-negative capacitance could result in a reflected or standing wave signal being generated when varistor 30R, 30S, 30T operates.
  • the signal may also be the result of jnanufacturing tolerances which cause a mismatch.
  • each phase which is capable of being switched to reverse the motor with a further variable resistance device 31?-, 31S which is connected between the downstream ends of the inductor coils of that phase.
  • the effect of the further resistance device 31R and 31S is to cause a circulating current to flow around the loop comprising

Landscapes

  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Ac Motors In General (AREA)

Abstract

Un circuit électrique de commutation d'une alimentation à courant alternatif multiphasé utilise des commutateurs redresseurs à semi-conducteur SCR 1 à 5 pour commuter de manière réversible l'alimentation à un moteur électrique multiphasé de manière à pouvoir faire tourner le moteur en marche avant ou arrière. Les SCRs sont arrangés en deux groupes, l'un pour commuter l'alimentation nécessaire à la marche avant du moteur, et l'autre pour l'alimentation du moteur en marche arrière. Des éléments inducteurs DELTAL sont connectés en série avec les SCRs. Dans certaines situations, des surtensions parasites importantes peuvent se produire aux phases d'entrée, par exemple en cas de foudre, ce qui peut donner lieu à un déclenchement des SCRs. L'installation de varistors entre les phases d'entrée et la terre permet de limiter la tension d'entrée à une valeur maximum prédéterminée, empêchant ainsi le déclenchement parasite des SCRs. On décrit également l'utilisation de varistors supplémentaires permettant de réduire les effets d'ondes stationnaires de tension pouvant se produire à cause des capacitances internes du premier groupe de varistors.An electrical circuit for switching a multi-phase AC power supply uses SCR semiconductor rectifier switches 1 to 5 to reversibly switch the power supply to a multi-phase electric motor so that the motor can be run in forward or back. The SCRs are arranged in two groups, one for switching the power supply necessary for the forward operation of the engine, and the other for supplying the engine with reverse gear. DELTAL inductors are connected in series with the SCRs. In certain situations, significant parasitic overvoltages can occur at the input phases, for example in the event of lightning, which can give rise to a triggering of the SCRs. The installation of varistors between the input phases and earth makes it possible to limit the input voltage to a predetermined maximum value, thus preventing parasitic tripping of the SCRs. The use of additional varistors is also described, making it possible to reduce the effects of standing voltage waves which may occur due to the internal capacitances of the first group of varistors.

Description

ELECTRICAL CIRCUIT FOR SWITCHING A MULTI-PHASE A.C. SUPPLY TO A LOAD
This invention relates to an electrical circuit for switching a multi-phase a.c. supply to a load, and has particular application to reversibly switching a three phase a.c. supply to an electric motor, for running, stopping and reversing the motor. The invention can be used with particular advantage to control an electric motor used to drive a valve actuator.
Electrically driven valve actuators are well known and have been used for many years to drive valves for controlling hydraulic and pneumatic flows in industrial process plants, for example in the operation of butterfly valves used in oil refineries. A known valve actuator, for example our Limitorque Model SMC 2005* comprises a three phase electric motor which drives a reduction gearing having an output for connection to a valve to drive its valve member. The reduction gearing and the motor are mounted in a heavy explosion proof cast metal housing. Mounted in a compartment within the housing is a set of electrical contactors which switch a three phase electrical supply to the motor so that the motor can be stopped, started and reversed. The contactors are operated by relay coils themselves driven from a single phase of the a.c. supply. RE?
OMPI An improved circuit for controlling such an arrangement is disclosed in our European Patent Application 82303368.3 which circuit utilises a non-linear inductance in series with a controlled rectifier device for each phase in order to prevent spurious operation of the controlled rectifier devices.
While this circuit is effective in normal operation, there are some exceptional circumstances when even the improved circuit will not prevent spurious operation e.g. a lightning strike.
It is an object of the present invention to improve upon the circuit disclosed in our European Patent Application 82303368.3 and to that end a variable resistance device is connected between one end of each non-linear inductance and ground.
Preferably, a further variable resistance device is connected across the downstream ends of the non-linear inductances of one or more phases. Features and advantages of the present invention will become apparent from the following description of an embodiment thereof given by way of example with reference to the accompanying drawing which shows a circuit diagram of a switching circuit according to the present invention.
A three phase motor switching circuit will now be described in detail with reference to the embodiment thereof shown in the drawing. The three input phases R.S.T. of an a.c. supply are fed to input terminals 20, 21, 22 and the three phase inputs of a three phase motor are connected to terminals U.V. and W. Semiconductor rectifier switches in the form of triacs SCR1 - 5 control the connection of the input phases R.S.T. to the terminals U.V.W. To run the motor in a forward direction the phases are connected as follows:-
R to U, S to V and T to W.
To reverse the motor the connection of the R and S phases is reversed, and the connections are as follows:-
R to V, S to U, and T to W.
Thus, to run the motor forwardly, triacs SCR1, and 5 need to be fired to a conductive state, which is achieved by means of a relay comprising a coil RLl and switching contacts RLl - 1, 2 and 3* The relay coil RLl is energised in response to a control signal on line l6 from the control logic circuit (not shown) , the control signal being indicative that the motor is to be run forwardly. Each of the relay contacts is connected in series with a dropping resistor Rg so that when the contacts are closed a suitable gate potential is derived from the a.c. supply and applied to the gates of SCRl, and 5 to fire them to a conductive state. When the control signal on line 16 ceases, the relay RLl is released, thus opening the contacts RLl -1 to RLl - 3, thus causing the SCRs 1, and 5 to switch off and stop the motor.
Reverse running of the motor is controlled in a similar way by means of a relay RL - 2 having a coil energised by a reverse running signal on line 17 from the control logic circuit (not shown). The relay RL2 has contacts RL2 - 1, 2, 3 which when closed, fire SCRs 2, 3 and to a conductive state.
It will be appreciated that the electric motor connected to the terminals U V and W essentially comprises an inductive load. Considering for the moment the triac SCRl, when it has been conductive and is then switched off, transients occur which would produce spurious firing of the triac if the following precautions were not taken. Upon switching off of the
OMPI > - WIPO triac SCRl a substantial rapidly rising transient back emf is developed in the motor coils and the back emf is applied to SCRl. In response to such rapid transients, the impedance presented by the switched off triac SCRl, is defined predominately by the capacitances of the pn junctions on either side of the SCR's gate and thus, in the presence of the transient the SCR's equivalent circuit is two capacitors connected in series and on opposite sides of the gate electrode. The rapidly rising back emf can cause charge to build up in the capacitors defined by the triacs pn junctions so as to raise the gate potential sufficiently retrigger the triac into conduction. To prevent such spurious retriggering of the triacs, a snubber circuit comprising a snubber resistor Rs and a snubber capacitor Cs, provides a by-pass for the transient back emfs. The time constant of the snubber circuit is selected so that at the a.c. frequency of the supply, the snubber circuit presents a high impedance, but at the high frequency of the transient back emfs the transient currents pass preferentially through the snubber circuit so as to by-pass SCRl, thereby to prevent the spurious firing. Each of the SCRs 1 to 5 is provided with its own snubber circuit Cs Rs.
Another problem of spurious firing of the triacs occurs at switch on of the motor. It is to be noted that each of the input phases R and S can be selectively connect¬ ed to the terminals U V by means of the SCRs 1 to 45 so that the motor can be run both forwardly and backwardly. An SCR behaves as a capacitor when in its "off" condition which means that the input circuit is a capacitively terminated line when the motor is de-energised or is running in one direction. There is a danger that the transients produced on switching on of say SCRl will produce spurious firing of SCR3, which would result in a short circuit being produced across the phases R and S. A similar short circuit could be established if SCRs 2 and 3 became conductive sim¬ ultaneously.
Considering now the switching on of SCRl, when the relay RL 1 is operated, a gate potential is applied to SCR 1 and it switches on. Consequently the potential at its cathode 23 rises to the potential of the input phase R. Thus, a very rapid rise of voltage could occur which if the precautions discussed herein¬ after were not taken, would produce spurious firing of SCR 3* It will be appreciated that a substantially rectangular step wavefront could be established at the cathode 23 of SCR 1, which would be applied to cathode 2k of SCR 3. This step wavefront has a substantially greater rate of rise than the back emfs developed upon switching off of the SCRs and consequently the snubber circuit Cs Rs associated with SCR 3 would be unable to suppress the rapid voltage rise at the cathode 2k. As a result, the rapid voltage rise associated with the switching on of SCR 1 would be likely to produce spurious firing of SCR 3 as a result of the transient voltage rise raising the gate potential of SCR 3» Similarly, firing on SCR 3 could produce spurious firing of SCR 1 and SCRs 2 and k would interact in a similar manner. A similar problem arises when the phases R S and T are initially connected to the terminals 21, 22 and 23* Again, a step wavefront would be applied to the SCRs 1 to 5 which could produce simultaneous spurious firing of the SCRs, thereby producing a short circuit across the phases.
Such spurious firing and short circuiting of the phases is prevented by providing in series with the SCRs 1 to 5 respective inductor coils ΔL1 - each provided with a core of a material exhibiting a non- linear permeability. The permeability of the core varies as a non-linear function of the frequency and magnitude of the applied field, and the core is typically made of ferrite material i.e. sintered carbides or iron powder material. The core may also be made of permalloy, for example, Mulypermalloy manufactured by Magnetics, a division of Spang Industries Inc., P.O. Box 391, Butler Pa, lόOOl, U.S.A. or powdered soft iron.
The inductor coils Δ, -* have the effect of reducing the rate of rise of current to which the SCRs are subjected so that no spurious firing occurs. By using a non-linear permeability core, the inductors appear to operate preferentially on the rapid transients which occur upon switch on; if a solid soft iron bar is used, the core inductors are heated by the a.c. supply whilst the motor is running. Conversely if no core is used in the inductors, resonance at radio frequencies may be produced in response to switching of the SCRs 1 to 5.
The provision of the inductors Δ-.L1 to 5 has been found to provide reliable switching of the SCRs 1 to 5-
Thus far the circuit is as described in our European Patent Application S2303368_3« We have found, however, that in certain situations gross spurious over-voltages may occur at the input phases R,S,T, e.g. • when subjected to lightning strikes or other fault conditions. These over-voltages may also cause spurious firing of the semiconductor rectifier switches which cannot be cured by the inductor coils & 1-5- We have found that if a variable resistance device 30R, 30S, 30T is connected between the upstream end of the inductor coils in each phase respectively and ground then the effects of this particular problem can be ameliorated. Preferably, each device 30R} 30S, 30T is a varistor which operates to clamp the input voltage to a maximum preset value, say Vtrum of its associated rectifier device. Varistors have the advantage of preventing sharp dv/dt changes such as might exist with an air gap arrestor. For many instances, this addition to the circuit is sufficient but with presently available varistors, each varistor has an internal capacitance which is larger than the capacitance of each rectifier device and the presence of this upstream non-negative capacitance could result in a reflected or standing wave signal being generated when varistor 30R, 30S, 30T operates. The signal may also be the result of jnanufacturing tolerances which cause a mismatch. The magnitude of this signal could be such as to cause spurious firing of the rectifier devices. To overcome this difficult we provide each phase which is capable of being switched to reverse the motor with a further variable resistance device 31?-, 31S which is connected between the downstream ends of the inductor coils of that phase. The effect of the further resistance device 31R and 31S is to cause a circulating current to flow around the loop comprising
LI, 31R and L2 for the R phase if the magnitude of the reflected or standing wave signal is greater than a preset value which for convenience is the same as that for the varistors 30, in this case Vtrm of the rectifier device.

Claims

CLAIMS :
1. An electrical circuit for multi-phase switching of an a.c. electrical supply to a load, comprising input terminals (20,21,22) for receiving respective different phases (R,S,T) of an a.c. supply, output terminals (U,V,W) for supplying the different phases to the load, means defining current paths between said input and said output terminals, said paths being arranged to connect the input terminals to respective ones of the output terminals in a first or a second different predetermined relationship, each of said current paths including a respective semiconductor rectifier switching means (SCRl,SCR2,SCR3. SCR4,SCR5) for rendering the path conductive or non-conductive, said paths further including respective inductors (ΔL1, ΔL2, ΔL3, ΔL4,.ΔL5) each comprising a coil having associated therewith a core, wherein a respective variable resistance device (30R,30S,30T) is connected between each input terminal and ground, each variable resistance device being operable to clampthe input voltage to a maximum preset value.
2. An electrical switching circuit according to claim 1 wherein said core is arranged to have a non¬ linear characteristic.
3• An electrical switching circuit according to claims 1 or 2 wherein an input terminal (20,21) has a first current path to a first output terminal (U) and a second current path to a second output terminal (V) and wherein means are provided for selectively enabling
SJ El
OMPI the respective semiconductor rectifier switching means of each current path whereby to connect the input terminal to the respective first or second output terminal by the enabled semiconductor rectifier switching means.
4- An electrical switching circuit according to claim 3_ wherein a further variable resistance device (31 j31S) is provided between said first and second current paths, said further variable resistance deivce being operable to prevent spurious firing of the respective semiconductor rectifier switching means which may otherwise occur as a result of the effect of the voltage standing wave produced due to the inherent internal capacitance of said first variable resistance device.
5. An electrical switching circuit according to claim 4 wherein each of said current paths said inductor is connected between said input terminal and said semiconductor rectifier switching means and said further variable resistance device has a first terminal connected to said first path between the inductor and the semiconductor rectifier switching means and a second terminal connected to said second path between the inductor and the semiconductor rectifier switching means.
EP84902521A 1983-06-16 1984-06-15 Electrical circuit for switching a multi-phase a.c. supply to a load Withdrawn EP0145774A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8316441 1983-06-16
GB838316441A GB8316441D0 (en) 1983-06-16 1983-06-16 Electrical circuit

Publications (1)

Publication Number Publication Date
EP0145774A1 true EP0145774A1 (en) 1985-06-26

Family

ID=10544329

Family Applications (1)

Application Number Title Priority Date Filing Date
EP84902521A Withdrawn EP0145774A1 (en) 1983-06-16 1984-06-15 Electrical circuit for switching a multi-phase a.c. supply to a load

Country Status (6)

Country Link
EP (1) EP0145774A1 (en)
AU (1) AU3069084A (en)
GB (1) GB8316441D0 (en)
IT (1) IT1177800B (en)
WO (1) WO1985000079A1 (en)
ZA (1) ZA844508B (en)

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6966275B2 (en) * 2001-10-10 2005-11-22 Whitehill David C E Pet bed

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2204087B1 (en) * 1972-10-21 1980-08-08 Licentia Gmbh
US4259705A (en) * 1979-03-27 1981-03-31 Stifter Francis J Combination surge suppressor filter
CA1124819A (en) * 1979-10-26 1982-06-01 Robert F. Rose Power control for vibratory feeder
BE889496A (en) * 1981-07-03 1981-11-03 Dumont Gilbert C POLYPHASE STATIC INVERTER

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8500079A1 *

Also Published As

Publication number Publication date
AU3069084A (en) 1985-01-11
IT8448406A0 (en) 1984-06-15
ZA844508B (en) 1985-01-30
IT1177800B (en) 1987-08-26
GB8316441D0 (en) 1983-07-20
WO1985000079A1 (en) 1985-01-03

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