GB2108787A - Electrical circuit for braking an AC electric motor - Google Patents

Abstract

A capacitor 1 is arranged for connection across a first winding of the motor 10 and first and second thyristor switches 2a and 2b are connected across the second and third windings. A control circuit 4 controls the first and second switches to short- circuit the respective second and third windings. A circuit 3a detects when the voltages across both the second and third windings are negative and enables circuit 3b to provide a first control signal to the control circuit 4 when the voltage across the second winding becomes a small positive voltage and provides a second control signal to the control circuit 4 when the voltage across the third winding becomes a small positive voltage. After connection of the capacitor across the first winding the second and third windings are short-circuited at the appropriate times in the order to provide optimum braking. <IMAGE>

Description

SPECIFICATION Electrical circuit for braking an electric motor This invention relates to an electrical circuit for braking an electric motor.

It is known to brake a three-phase electric motor by connecting a capacitor across one phase winding and subsequently short-circuiting the other two phase windings. It has been found that for each of the two phase windings to be shortcircuited there are ranges of phase angles during each cycle of the voltage across the respective winding during which the commencement of short-circuiting should preferably take place.

Referring to Figure 1 of the drawings, which shows an exemplary cycle of the voltage waveforms for the three phase windings A, B and C of a 3-phase electric motor as curves VA, VB and Vc, it will be seen that the three phase voltages are in normal phase relationship, each being 1200 displaced from each of the other phase voltages.

Figure 1 also shows the self excitation current 1A produced when a capacitor is connected across the winding A of the motor. It will be seen that the current 1A leads the corresponding phase voltage VA by 900. Once the capacitor has been connected across the winding A for a sufficient time to allow for the build up of the current IAT thus gaining the full capacitive effect on the braking of the motor, it is desirable to short-circuit the other two windings, i.e.B and C, and the preferred phase angles for carrying this out with respect to each winding are known and determined with reference to the self-excitation current 1A In particular, it is known that it is preferable to short-circuit the winding B in a quadrant of a current cycle after a zero crossing of the current 1A It is further preferabie that the short-circuiting takes place soon after a zero crossing of the voltage VB. Thus, the range of phase angles during which the winding B is to be preferably short-circuited is further limited and two empirically determined preferred periods are shown as RB1 and RB2. It will be noted that these ranges of phase angle follow a positive and negative crossing of VB respectively and both ranges extend for 30 after the zero crossing.

Contrastingly, the preferred quadrants fo short-circuiting the winding C are those which occur immediately prior to a zero crossing of the current 1A Again, as for short-circuiting the winding B, the short-circuiting of the winding C preferably takes place immediately following a zero crossing of the voltage Vc.However, in the case of the winding C it has been determined that the phase range within which the short-circuiting preferably commences should be limited to about 150 following the zero crossing of the voltage Vc, i.e. periods Rci and RC2 It will be appreciated that the difference in preferred ranges for the shortcircuiting following the zero crossing of the voltages VB and Vc is a consequence of their respective phase relationships with the selfexcitation current 1A Figure 1 thus shows the importance of knowing which of the phase voltages VB and Vc is being short-circuited.

Circuitry has been proposed for effecting the short-circuiting of the windings B and C within the preferred ranges of phase angle determined.

However, such circuitry will only operate fully when connected to the windings B and C in the proper order. Clearly if circuitry is provided to short-circuit the winding B and is constructed so that the short-circuit will be effected during the phase range known to be allowable and the circuitry is inadvertently connected to the winding C, the short-circuit may be effected outside the allowable period for short-circuiting the winding C. This is of course highly undesirable as ineffective braking of the motor may result. The problem is often compounded in practical situations when it is not known which of the three windings of the motor is being dealt with. That is to say a motor may mereiy have the three phases of the supply arbitrarily connected to the windings and the phase inter-relationships of the three windings may not be fully known.It is also possible for the three phases of the supply to be incorrectly identified.

According to the invention there is provided an electrical circuit for braking a three-phase electric motor, comprising: a capacitance arranged for connection across a first winding of the motor; first and second electrically controllable switch means arranged for respective connection across second and third windings of the motor; and a detection circuit arranged for connection across the second and third windings of the motor; the detection circuit being adapted to detect the occurrence of the voltages across the second and third windings of the motor being simultaneously of one polarity and subsequently to provide a first control signal to turn on the first switch means when the voltage across the second winding goes through a zero crossing to the other polarity and to provide a second control signal to turn on the second switch means when the voltage across the third winding goes through a zero crossing to the other polarity; so that in use, following the connection of the capacitance across the first winding of the motor, the detection circuit detects the occurrence of both the voltages across the second and third windings being of said one polarity and then the first of the second and third windings to have the other polarity across it will be short-circuited and the remaining winding will be short-circuited when the voltage across it goes to said other polarity, irrespective of the order in which the braking circuit is connected to the phase windings of the motor.

For example, if the first and second switch means are forward biassed when the voltages across the second and third windings are positive; the detection circuit is arranged to detect when the voltage across the second and third windings are both negative and then to switch on the first of the two switches to become forward biassed and then to switch on the other switch when it becomes forward biassed.

It will be seen with reference to Figure 1 that the condition of both the voltage waveforms V8, V0 being negative occurs in the period N. Thus the first condition which must be satisfied for the detection circuit of the invention to operate is satisfied in this period. The first subsequent zero crossing to a positive voltage occurs when V8 goes positive at the beginning of the period R81.

At this time the detection circuit will supply a control signal to the switch means across the winding B and thus cause the winding B to be short-circuited. At the positive-going zero crossing of V0 the winding C will be shortcircuited by a control signal from the detection circuit being supplied to the switch means across that winding. It will be seen that since the control of the switch means is carried out independently of the waveforms VA and 1A it does not matter which way round the windings are connected.

That is to say, the three windings A, B and C can be transposed without the possible loss in braking efficiency of the previously known circuits.

Preferably there is additionally provided a time delay circuit which provides a delay period following the connection of the capacitance across the winding of the motor during which the detection circuit is disabled. Thus following the connection of the capacitance across the first winding of the three-phase motor the delay period will be initiated and during that time the other two windings may not be short-circuited. This allows the self-excitation current of the capacitor and the first winding to stabilise. Thus the full effect of the capacitive braking is achieved before the further braking is carried out by the application of the short-circuits.

Preferably the detection circuit comprises a first sub-circuit adapted to detect both the voltages across the second and third windings being of said one polarity, e.g. negative; and a second sub-circuit arranged to detect a small opposite polarity, e.g. positive voltage across either of the second or third windings; the first sub-circuit being adapted to be disabled by the time delay circuitry and to provide an enable signal to the second sub-circuit after the time delay when both the second and third winding voltages are negative; the second sub-circuit being adapted to provide the first control signal when provided with the enable signal from the first sub-circuit and when the voltage across the second winding is the small opposite polarity voltage, and to provide the second control signal when provided with the enable signal from the first sub-circuit and when the voltage across the third winding is the small opposite polarity voltage. The first sub-circuit may include a latch so that following the end of the time delay period and the occurrence of both the voltage across the second and third windings being of said one polarity the enable signal is subsequently continuously provided to the second sub-circuit.

The provision of the first and second sub-circuits allows for a simplified overall construction of the detection circuitry.

Preferably the switch means are thyristors and are connected across the second and third windings so that the two cathodes of the thyristors are connected to the same terminal and there is provided a control circuit for converting the first or second control signal into a respective first or second firing signal which is applied to the gate of the respective first or second thyristor.

There is preferably also provided reset circuitry arranged to be responsive to the operation of the latch and arranged to ready the circuit for another braking operation, a predetermined time after the operation of the latch.

Preferably the detection circuit, the control circuit, the time delay circuit and the reset circuitry are supplied with electrical power from a power supply circuit adapted to be connected across the first winding of the motor. The power supply circuit preferably comprises a transformer and rectifier to provide an unregulated voltage which is then regulated by means of suitable regulation circuitry.

Preferably there is provided circuitry for connecting the first winding of the motor to the power supply circuit when the motor is disconnected from an external power source and which disconnects the power supply circuit from the first winding when the electric motor is supplied with external power.

A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, wherein: Figure 1 shows the known voltage waveforms for a three-phase motor and also shows the selfexcitation current produced when a capacitance is connected across one winding of the motor; Figure 2 shows a block diagram of an electrical circuit according to the invention for braking a three-phase electric motor; and Figure 3 shows a schematic diagram of the circuit of Figure 2.

Referring to Figures 2 and 3 of the drawings, an electrical circuit for braking a 3-phase electric motor 10 generally comprises a capacitor 1 arranged for connection across a first winding of the motor, first and second switch means 2a and 2b arranged for connection across second and third windings of the motor, a detection circuit 3 arranged for connection across the second and third windings of the motor and adapted to detect the voltages across the second and third windings and to produce first and second control signals upon certain conditions being satisfied (as hereinafter described) and supply these to a control circuit 4 which control circuit is adapted to control the first and second switch means 2 to short-circuit each of the second and third windings.

The detection circuit 3 includes a first subcircuit 3a for detecting the occurrence of the voltages across both the second and third windings being negative and subsequently provides an enable signal to a second sub-circuit 3b which, when provided with the enable signal, provides a first control signal to the control circuit 4 when the voltage across the second winding becomes a small positive voltage and provides a second control signal to the control circuit 4 when the voltage across the third winding becomes a small positive voltage. The sub-circuit 3a includes a latch 3c which provides the enable signal continuously to sub-circuit 3b once both the voltages across the second and third windings are negative, when the latch 3c is not itself disabled by a time delay circuit 5.The time delay circuit 5 provides a disable signal to latch 3c for a delay period following the connection of the electrical braking circuit to the motor 1 0.

A reset circuit 7 is provided to reset the circuit a predetermined time after the operation of the latch 3c.

The motor 10 is connected to a 3-phase power supply 8 by means of a contactor 11. Part of the contactor 11 is arranged to connect the first winding of the motor 10 to an operating circuit 9 when the contactor 11 disconnects the motor 10 from the 3-phase supply 8. The operating circuit 9 is adapted to then operate a contactor 12 to connect the first winding of the motor 10 to the electrical braking circuit. The common lead of the second and third windings is permanently connected to the electrical braking circuit.

Referring now particularly to Figure 3, the detection circuit 3 comprises sub-circuits 3a and 3b. The sub-circuit 3a which detects the occurrence of both the voltages across the second and third windings of the motor being negative includes the diodes D17 and D18, resistors R29 and R30, resistor R4, diode D2, inverter IC1/3, resistors R6 and R7, diode D3 and capacitor C2.

The latch circuit 3c comprises NAND gate IC3/2, Schmitt NAND gate IC2/4, inverter IC1/6, diode D5, resistor R9, and capacitor C4. The reset circuit 7 comprises resistor R10, diode D6, capacitor C5, inverters IC1/2 and IC1/1, resistors R1 1 and R12 and transistor Q2. The time delay circuitry 5 comprises variable resistor VR1, resistor R8, diode D4, Schmitt NAND gate IC2/3, and capacitor C3. The sub-circuit 3b includes diodes D16 and D15, resistors R28 and R27, capacitors C6 and C7, resistors R13 and R14, diodes D7 and D8, Schmitt NAND gates IC2/1 and IC2/2 and inverters IC1/5 and IC1/4.

The control circuit 4 includes resistors R1 5 and R20, transistors 03 and Q4 and Q5 and Q6 together with associated resistors R 16, R1 7, R1 8 and R19, and resistors R21, R22, R23 and R24 respectively and also diodes D9, D10 and D11, and diodes D12, D13 and D14 respectively.

The switch means 2a and 2b comprise thyristor TH 1, having gate G1, and resistor R25, and thyristorTH2, having gate G2, and resistor R26.

The power supply circuit 6 includes transformer TR1, full wave bridge rectifier BR1, capacitor C1, zener diode Z1, resistors R1 and R2, regulation transistor 01, zener diode Z2, diode D1 and resistor R3.

The electric motor 10 is connected to the three phase power supply 8 by means of contactor 11 which includes contacts 1/1 to 1/6. The contactor 11 is operated by a control circuit including relay coil RL1, switches AUX PB3, PB2 and PB1 and fuse F1. The three phase electric motor 10 is connected directly to the electrical braking circuit by one phase lead, the lead common to the second and third windings, and the remaining two leads are connectable to the circuit by means of a contactor 12.The contactor 1 2 is operated by operating circuit 9 including contacts 1/5 and 1/6 of contactor 1 , full wave bridge rectifier BR2, resistors R3 1, R32, capacitor C8, diode Do 7, resistor R33, full wave bridge rectifier BR3, resistor R34 and capacitor C9 which components are together arranged to supply power to relay coil RL2 which operates contacts 2/1 and 2/2 of contactor 12.

In operation, the switch PB1 is closed to start the motor: relay RL1 operates to close contacts 1/1 to 1/4 and relay RL1 is held on via contacts 1/4.

When it is desired to brake the motor 10 one of the switches PB2 and AUX PB3 is operated to break the holding circuit and contacts 1/1 to 1/4 of contactor 11 then open. Contacts 1/5 and 1/6 of contactor 11 close and the operating circuit 9 is supplied with power and contacts 2/1 and 2/2 of contactor 1 2 close. The operating circuit 9, and in particular capacitor C8 thereof, supplies the power required to hold contactor 12 closed during the subsequent braking operation.

The capacitor 1 is thus connected across the first winding of the motor and braking commences. The power supply circuit 6, Q1 etc, now provides a voltage of approximately 40V to the control circuit 4 and 5V to the time delay circuit 5 and detection circuit 3. The capacitors are all initially discharged and capacitor C4 supplies a "0" signal to one input of gate IC2/4 to ensure that the latch 3e is initially reset with the output of inverter IC 1/6 at "O". After a short time defined by the charging of capacitor C4 through resistor R9, gate IC2/4 is enabled by the higher voltage on capacitor C4.

Capacitor C3 of time delay circuit 5 is also initially discharged and holds latch 3c reset. After a time determined by resistor R8 and variable resistor VR 1, e.g. 0.08s up to a few seconds, the voltage on capacitor C3 rises to a logical "1" level to enable the latch 3c to be set. While capacitor C3 is charging the output of gate IC2/3 will be high irrespective of the output of inverter IC1/3.

When the voltage on either of the second or third windings is more than about 1 0V positive a positive signal is supplied to the input of inverter IC1/3 via diode D18 and resistor R29 or diode D17 and resistor R30 to hold the lower input of gate IC2/3 down. When the voltages across the second and third windings are both negative (in fact less than approximately 1 0V positive) the input to It 1/3 will be pulled down by-resistor R4 and the output will go high and consequently the output of gate IC2/3 will go low, thus setting the latch 3c, (IC3/2 and IC2/4).

The setting of latch 3c, a high output from inverter It 1/6, enables gates IC2/1 and IC2/2 to pass the positive voltages detected by diode Di 6 and resistor R28 for the second winding and diode D15 and resistor R27 for the third winding.

Thus when either of the voltages across the second and third winding goes positive (approx.

1 0V) the control circuit 4 will receive an enable signal through either gate IC2/1 and inverter IC1/5 or gate IC2/2 and inverter IC1/4. The control circuit 4 will then fire the appropriate one of the thyristors TH 1 and TH2. The capacitor/resistor networks C6/R 13 and C7/R14 ensure that the positive level to gate IC2/1 or IC2/2 is maintained for a short time after the corresponding thyristor is fired to enable it to latch on. Resistors R15 and R16 and associated components, and resistors R20 and R21 and associated components prevent the firing of the thyristors when the output voltage from transistor Q1 is less than about 3V and the operation of the logic circuitry correspondingly unreliable.

It will be seen that whichever of the first and second windings goes positive after the setting of latch 3c will be short circuited by its associated thyristor. The voltage across the winding will then collapse but the thyristor firing pulse is extended by capacitor C6 or C7. Because the voltage has collapsed no further firing current is supplied which reduces the drain on power supply circuit 6 enabling a relatively small capacitor C1 to be used. Should the thyristor come out of conduction, e.g. because of inadequate holding current the voltage is restored and another firing pulse is immediately provided. The other thyristor fires as soon as the voltage across its associated winding goes positive and the motor rapidly stops.

Following the setting of the latch 3c, a time delay determined by resistor R10 and capacitor C5 is initiated after which transistor Q2 conducts to discharge capacitor C1. This takes place after the thyristors have been fired and enables the circuit to be ready for another braking operation very quickly. Diodes D4, D5, D6, D7 and D8 discharge their associated capacitors very quickly when transistor Q2 conducts, to ensure that the capacitors are discharged ready for the next braking operation.

Claims (Filed on 26.10.82.) 1. An electrical circuit for braking a threephase electric motor, comprising: a capacitance arranged for connection across a first winding of the motor; first and second electrically controllable switch means arranged for respective connection across second and third windings of the motor; and a detection circuit arranged for connection across the second and third windings of the motor; the detection circuit being adapted to detect the occurrence of the voltages across the second and third windings of the motor being simultaneously of one polarity and subsequently to provide a first control signal to turn on the first switch means when the voltage across the second winding goes through a zero crossing to the other polarity and to provide a second control signal to turn on the second switch means when the voltage across the third winding goes through a zero crossing to the other polarity: so that in use, following the connection of the capacitance across the first winding of the motor, the detection circuit detects the occurrence of both the voltages across the second and third windings being of said one polarity and then the first of the second and third windings to have the other polarity across it will be short-circuited and the remaining winding will be short-circuited when the voltage across it goes to said other polarity, irrespective of the order in which the braking circuit is connected to the phase windings of the motor.

2. A circuit as claimed in claim 1 wherein there is additionally provided a time delay circuit which provides a delay period following the connection of the capacitance across the winding of the motor during which the detection circuit is disabled.

3. A circuit as claimed in claim 2 wherein the detection circuit comprises a first sub-circuit adapted to detect both the voltages across the second and third windings being of said one polarity, and a second sub-circuit arranged to detect a small opposite polarity voltage across either of the second or third windings; the first sub-circuit being adapted to be disabled by the time delay circuitry and to provide an enable signal to the second sub-circuit after the time delay when both the second and third winding voltages are of said one polarity; the second subcircuit being adapted to provide the first control signal when provided with the enable signal from the first sub-circuit and when the voltage across the second winding is the small opposite polarity voltage, and to provide the second control signal when provided with the enable signal from the first sub-circuit and when the voltage across the third winding is the small opposite polarity voltage.

4. A circuit as claimed in claim 3 wherein the first sub-circuit includes a latch so that following the end of the time delay period and the occurrence of both the voltage across the second and third windings being of said one polarity the enable signal is subsequently continuously provided to the second sub-circuit.

5. A circuit as claimed in claim 4 further comprising reset circuitry arranged to be responsive to the operation of the latch and arranged to ready the circuit for another braking operation, a predetermined time after the operation of the latch.

6. A circuit as claimed in claim 5 wherein the control circuit, the time delay circuit and the reset circuitry are supplied with electrical power from a power supply circuit adapted to be connected across the first winding of the motor.

7. A circuit as claimed in claim 6 including circuitry for connecting the first winding of the motor to the power supply circuit when the motor is disconnected from an external power source

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. inverter It 1/6, enables gates IC2/1 and IC2/2 to pass the positive voltages detected by diode Di 6 and resistor R28 for the second winding and diode D15 and resistor R27 for the third winding. Thus when either of the voltages across the second and third winding goes positive (approx. 1 0V) the control circuit 4 will receive an enable signal through either gate IC2/1 and inverter IC1/5 or gate IC2/2 and inverter IC1/4. The control circuit 4 will then fire the appropriate one of the thyristors TH 1 and TH2. The capacitor/resistor networks C6/R 13 and C7/R14 ensure that the positive level to gate IC2/1 or IC2/2 is maintained for a short time after the corresponding thyristor is fired to enable it to latch on. Resistors R15 and R16 and associated components, and resistors R20 and R21 and associated components prevent the firing of the thyristors when the output voltage from transistor Q1 is less than about 3V and the operation of the logic circuitry correspondingly unreliable. It will be seen that whichever of the first and second windings goes positive after the setting of latch 3c will be short circuited by its associated thyristor. The voltage across the winding will then collapse but the thyristor firing pulse is extended by capacitor C6 or C7. Because the voltage has collapsed no further firing current is supplied which reduces the drain on power supply circuit 6 enabling a relatively small capacitor C1 to be used. Should the thyristor come out of conduction, e.g. because of inadequate holding current the voltage is restored and another firing pulse is immediately provided. The other thyristor fires as soon as the voltage across its associated winding goes positive and the motor rapidly stops. Following the setting of the latch 3c, a time delay determined by resistor R10 and capacitor C5 is initiated after which transistor Q2 conducts to discharge capacitor C1. This takes place after the thyristors have been fired and enables the circuit to be ready for another braking operation very quickly. Diodes D4, D5, D6, D7 and D8 discharge their associated capacitors very quickly when transistor Q2 conducts, to ensure that the capacitors are discharged ready for the next braking operation. Claims (Filed on 26.10.82.)

1. An electrical circuit for braking a threephase electric motor, comprising: a capacitance arranged for connection across a first winding of the motor; first and second electrically controllable switch means arranged for respective connection across second and third windings of the motor; and a detection circuit arranged for connection across the second and third windings of the motor; the detection circuit being adapted to detect the occurrence of the voltages across the second and third windings of the motor being simultaneously of one polarity and subsequently to provide a first control signal to turn on the first switch means when the voltage across the second winding goes through a zero crossing to the other polarity and to provide a second control signal to turn on the second switch means when the voltage across the third winding goes through a zero crossing to the other polarity: so that in use, following the connection of the capacitance across the first winding of the motor, the detection circuit detects the occurrence of both the voltages across the second and third windings being of said one polarity and then the first of the second and third windings to have the other polarity across it will be short-circuited and the remaining winding will be short-circuited when the voltage across it goes to said other polarity, irrespective of the order in which the braking circuit is connected to the phase windings of the motor.

2. A circuit as claimed in claim 1 wherein there is additionally provided a time delay circuit which provides a delay period following the connection of the capacitance across the winding of the motor during which the detection circuit is disabled.

3. A circuit as claimed in claim 2 wherein the detection circuit comprises a first sub-circuit adapted to detect both the voltages across the second and third windings being of said one polarity, and a second sub-circuit arranged to detect a small opposite polarity voltage across either of the second or third windings; the first sub-circuit being adapted to be disabled by the time delay circuitry and to provide an enable signal to the second sub-circuit after the time delay when both the second and third winding voltages are of said one polarity; the second subcircuit being adapted to provide the first control signal when provided with the enable signal from the first sub-circuit and when the voltage across the second winding is the small opposite polarity voltage, and to provide the second control signal when provided with the enable signal from the first sub-circuit and when the voltage across the third winding is the small opposite polarity voltage.

4. A circuit as claimed in claim 3 wherein the first sub-circuit includes a latch so that following the end of the time delay period and the occurrence of both the voltage across the second and third windings being of said one polarity the enable signal is subsequently continuously provided to the second sub-circuit.

5. A circuit as claimed in claim 4 further comprising reset circuitry arranged to be responsive to the operation of the latch and arranged to ready the circuit for another braking operation, a predetermined time after the operation of the latch.

6. A circuit as claimed in claim 5 wherein the control circuit, the time delay circuit and the reset circuitry are supplied with electrical power from a power supply circuit adapted to be connected across the first winding of the motor.

7. A circuit as claimed in claim 6 including circuitry for connecting the first winding of the motor to the power supply circuit when the motor is disconnected from an external power source