GB2118790A - Universal motor control - Google Patents

Universal motor control Download PDF

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Publication number
GB2118790A
GB2118790A GB08204521A GB8204521A GB2118790A GB 2118790 A GB2118790 A GB 2118790A GB 08204521 A GB08204521 A GB 08204521A GB 8204521 A GB8204521 A GB 8204521A GB 2118790 A GB2118790 A GB 2118790A
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GB
United Kingdom
Prior art keywords
motor
voltage
switch
circuit
capacitor
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Withdrawn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
GB08204521A
Inventor
Arthur Briscoe Bryan
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BONAR INSTR Ltd
Original Assignee
BONAR INSTR 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 BONAR INSTR Ltd filed Critical BONAR INSTR Ltd
Priority to GB08204521A priority Critical patent/GB2118790A/en
Publication of GB2118790A publication Critical patent/GB2118790A/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H11/00—Emergency protective circuit arrangements for preventing the switching-on in case an undesired electric working condition might result
    • H02H11/006—Emergency protective circuit arrangements for preventing the switching-on in case an undesired electric working condition might result in case of too high or too low voltage
    • H02H11/007—Emergency protective circuit arrangements for preventing the switching-on in case an undesired electric working condition might result in case of too high or too low voltage involving automatic switching for adapting the protected apparatus to the supply voltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/08—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors
    • H02H7/085—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors against excessive load

Landscapes

  • Control Of Ac Motors In General (AREA)
  • Protection Of Generators And Motors (AREA)
  • Stopping Of Electric Motors (AREA)

Abstract

A control system for an electric motor for driving power tools has an electronic overcurrent protection circuit for the motor, wherein the peak voltage developed across a series resistor (R2) is detected and charges a capacitor (C4) accordingly. Should the voltage on the capacitor (C4) rise above a predetermined level corresponding to the motor current rising above an acceptable limit, this is detected by a comparator (IC2) which triggers a bistable circuit (IC1) whereby to render a triac (T1) in series with the motor (10) non-conductive. The system can accept either 110V a.c. or 240V a.c. mains by virtue of the provision of a voltage sensitive relay (12) which, in the event that the system is connected to 240V a.c., introduces extra resistance (R3) into a motor speed control circuit 18. <IMAGE>

Description

SPECIFICATION Universal motor control The present invention is concerned with the control of universal electric motors of the type used to drive power tools, such as hammer drills.
One problem with power tools such as hammer drills is that of the potential damage to the motor and/or, injury to the user in the event of excessiveiy high motor currents resulting from jamming or partial jamming of the motor. It is known to include over-current detection circuits in this connection, which acts so as to reduce the power to the motor in the event of excessively high currents occurring. Such circuits are known as Current Limiting or Anti-Kick Back circuits.
It is an object of the present invention to provide an over-current protection circuit which has an improved operating characteristic in relation to such known devices.
In accordance with the present invention, there is provided an overcurrent protection current which, in the event of the motor current exceeding a predetermined level, is arranged to cut-off the current supply to the motor altogether.
Preferably, the current supply to the motor is only returned, following the occurrence of an overload condition, by the operation of a manual device on the tool, i.e. by a positive act on the part of the operator.
Advantageously, the overload protection circuit includes a semi-conductor switch, such as a triac, disposed in series with the motor field winding or windings, the semi-conductor switch being arranged to be operated to break the supply to the motor in the event of an overload condition occurring.
Preferably, the motor is also provided with a circuit which reacts automatically to the magnitude of the mains supply voltage to enable the motor to be supplied with the correct operating voltage irrespective of whether the tool is connected to 11 0V ac or 24V ac mains.
Advantageously, the latter circuit includes a voltage sensitive relay which, in the event that the mains voltage exceeds a predetermined level, is arranged to introduce extra resistance into a regulated speed-control circuit.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which is a circuit diagram of a hammer drill control system embodying the present invention.
The drill motor which is of the universal type, is shown only diagrammatically at 10 and has brushes 12, 14 and an armature 16. The brush 12 is connected via one field winding F1 and a resistor R1 and capacitor C, to a terminal L for connection to the live side of an a.c. mains supply.
The other brush 14 is connected via a second field winding F2, a triac T, and a resistor R2 to a terminal N for connection to the neutral side of the mains supply.
Speed control of the motor is provided by a conventional regulator device indicated diagrammatically at 1 8. One possible form of regulator is indicated within the box 1 8 but any alternative suitable speed regulating arrangement could be used here. Preferably, however, the speed is controlled in dependence upon a feedback signal from the brush-gear picked up on a line 20 from the brush 14. This signal is fed, via an ON/OFF switch 22, a variable resistance Vr, and a resistor R3to pin 1 of the regulator. The components Vr, and R3 combine with the internal capacitance and resistance (C8, Cg, R,2, R4) within the regulator to provide a phase-shifted voltage to control the gate of an internal triac T2 and hence to control the motor speed to a value set by the variable resistance Vr,.
In order to enable the mains supply to the terminals L, N to be either 240 volt or 110 volt a.c. the resistor R3 can be selectively shortcircuited by means of the switching contacts S of a relay R whose solenoid is connected on one side to the neutral terminal N and on the other side to the live terminal L via a diode D1, and resistor R6.
The relay solenoid has a capacitor C2 in parallel with it. The latter arrangement operates as follows.
The motor used is wound for 11 0V a.c.
operation. The contacts S are normally closed so as to remove R3 from the circuit. If the motor is connected to 1 1 0V a.c. then Vr, alone provides the external resistance in the motor speed control circuit. Relay R is designed to operate only when the input voltage to the hammer is above say 1 80 V a.c. Components R5 and D, are used to provide the correct voltage for the relay from the mains supply. Capacitor C2 provides some smoothing. If the supply voltage is above 1 80 V a.c. (e.g. 240V) then the relay operates and the contacts S open.
This causes extra resistance (resistor R3) to appear in series with Vr, which is dimensioned to limit the voltage to the motor to 11 OV. When the supply voltage is only 11 0V, on the other hand, the relay does not operate and its contacts S short-circuit the resistance R3. This thereby allows the full range of the speed control to provide the full supply voltage of 100 to 1 20V to the motor.
The purpose of the resistance R, and capacitor C, connected across the triac connections of the regulator 1 8 is to provide the necessary voltage spike suppression for the triac T2.
In order to minimize the possibility of damage to the motor, and/or operator injury in the event of jamming of the tool, the system includes an overload trip which is constructed and operates as follows.
The neutral return for the motor 10 is connected in series with the resistance R2, which is of low ohmic value (e.g. 0.1 Q) and is used as a current shunt, and the triac T,. The gate control for this triac T, is supplied from a transistor Tr, controlled by a bistable circuit IC1 described in detail hereinafter.
The voltage across the current resistor R2 is fed by line 24 and a capacitor C3 to the base of a further transistor Tr2. The emitter of this transistor Tr2 is connected to the neutral line 26 by the parallel combination of a resistor Re and capacitor C4. The base of Tr2 is coupled to the neutral line 26 by a diode D2. The latter components act as a peak detecting circuit which monitors the voltage developed across the shunt resistance R2,the latter voltage being dependent upon the load current of the motor When Tr2 conducts, its emitter capacitor C4 is charged to a d.c. voltage.
The latter voltage is provided as one input of a comparator device IC2 which is arranged to detect when this voltage exceeds a predetermined level.
Thus, the emitter of Tr2 is connected to one input of the comparator IC2 via a resistor R7. A second input to the comparator 28 is provided from a potentiometer Vr2 connected to a d.c.
supply (+VE). Vr2 is set to determine the level (i.e.
the motor current magnitude) at which the comparator trips.
On reaching the preset trip level, the output of comparator 28 changes state and resistor R8 and capacitor C8 provide a pulse which is used to trigger a pulse shaping circuit IC3. The output of the pulse-shaping circuit IC3 is used to switch the bistable switch IC1 which in turn is arranged to switch off the gate supply to the triac T1 by means of the transistor Tr, as now described.
Bistable switch IC1 comprises two AND gates 1C1 a and Cl b interconnected as shown. The inputs 1 and 6 of these AND D gates are normally held high (logic 1) by way of respective resistors R9,R10. Input 6 can be supplied with the shaped output of the pulse shaper IC3 by way of a capacitor Ce. The output of bistable switch IC1 appears on a line 30 and is transferred to the base of PNP transistorTr1 by way of an inverter 32.
During normal operation, therefore, the inputs 1 and 6 are high (1), the output 4 is low (0), input 2 is low (0) and the output 3 on line 30 is high (1).
The latter signal is inverted in inverter 32 so as to hold the base of Tr1 low. Since the transistor is of the PNP type it is held permanently ON whereby triacT1 is permanently conducting. However, should an overload condition arise so that the comparator IC2 is tripped and a pulse appears at the capacitor Ce, the input 6 is momentarily dragged low (0). This has the result that the output 4 goes high (1) and output 3 goes low.
Input 5 therefore also goes low and maintains output 4 high (1) even when the pulse disappears and input 6 returns to high (1). The output of the bistable on line 30 thus becomes permanently low (until reset) and the transistor Tr1 is switched OFF via the inverter 32. With the transistor Tr1 OFF, the triac T1 can no longer conduct and power is totally removed from the motor.
In order to prevent the overload circuit from being tripped by the relatively high starting currents during motor start-up, this particular embodiment employs a manually operable switch 34 on the tool handle. This has a pair of normally closed contacts (N/C) and a pair of normally open contacts (N/O). When the switch is moved to its N/O position, a d.c. voltage is applied to a capacitor C7 so as to charge this capacitor to a positive voltage. Upon subsequent return (release) of the button to the N/C position, the charge on the capacitor acts via an inverter 36 to pull input 1 low (0) and thereby prevent output 3 from going low for several cycles (dependent upon the value of a resistor R11) even though input 1 may have gone temporarily low as a result of the high starting current tripping the comparator IC2.
Thus, the motor is permitted to start without the over-current protection operating.
Operation of the switch 34 also provides a reset facility following operation of the overload protection.
The internal low-voltage d.c. supplies are obtained by the use of a conventional rectifier arrangement comprising diodes, D3, D4 Zener diodes 2 2 and capacitors C8, C9.
The above described circuit thus has the advantages that: (1) it cuts off the electrical power to the motor entirely in the event of the motor current exceeding a predetermined level, e.g. as a result of jamming; (2) it senses automatically the supply voltage to the hammer and enables the motor to run off either 240V or 11 0V a.c. supplies without any manual adjustment; and (3) the selected motor speed is held constant independent of varying load conditions.
Claims (Filed on 1 5/2/83) 1. A control system for a universal electric motor of the type used to drive power tools, the control system including an electronic overcurrent protection circuit which, in the event that the motor current exceeds a predetermined level, is arranged to cut off the current supply to the motor.
2. A control system as claimed in claim 1, wherein the current supply to the motor is only returned, following the occurrence of an overload condition, by the operation of a manual device on the tool, i.e. by a positive act on the part of the operator.
3. A control system as claimed in claim 2, wherein the overload protection circuit includes a semi-conductor switch, such as a triac, disposed in series with the motor field winding or windings, the semi-conductor switch being arranged to be operated to break the supply to the motor in the event of an overload condition occurring.
4. A control system as claimed in claim 3, wherein the overload protection circuit further includes a peak detecting device which monitors the current through the motor and provides a d.c.
signal representative of the level of the peak motor current, the d.c. signal being compared with a preset reference in a comparator which, in the event that the d.c. signal exceeds the reference, is arranged to switch over a bistable circuit to change the conducting state of said semi-conductor switch.
5. A control system as claimed in claim 4, wherein the bistable circuit, once tripped, maintains the semi-conductor switch in its non
**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. parallel combination of a resistor Re and capacitor C4. The base of Tr2 is coupled to the neutral line 26 by a diode D2. The latter components act as a peak detecting circuit which monitors the voltage developed across the shunt resistance R2,the latter voltage being dependent upon the load current of the motor When Tr2 conducts, its emitter capacitor C4 is charged to a d.c. voltage. The latter voltage is provided as one input of a comparator device IC2 which is arranged to detect when this voltage exceeds a predetermined level. Thus, the emitter of Tr2 is connected to one input of the comparator IC2 via a resistor R7. A second input to the comparator 28 is provided from a potentiometer Vr2 connected to a d.c. supply (+VE). Vr2 is set to determine the level (i.e. the motor current magnitude) at which the comparator trips. On reaching the preset trip level, the output of comparator 28 changes state and resistor R8 and capacitor C8 provide a pulse which is used to trigger a pulse shaping circuit IC3. The output of the pulse-shaping circuit IC3 is used to switch the bistable switch IC1 which in turn is arranged to switch off the gate supply to the triac T1 by means of the transistor Tr, as now described. Bistable switch IC1 comprises two AND gates 1C1 a and Cl b interconnected as shown. The inputs 1 and 6 of these AND D gates are normally held high (logic 1) by way of respective resistors R9,R10. Input 6 can be supplied with the shaped output of the pulse shaper IC3 by way of a capacitor Ce. The output of bistable switch IC1 appears on a line 30 and is transferred to the base of PNP transistorTr1 by way of an inverter 32. During normal operation, therefore, the inputs 1 and 6 are high (1), the output 4 is low (0), input 2 is low (0) and the output 3 on line 30 is high (1). The latter signal is inverted in inverter 32 so as to hold the base of Tr1 low. Since the transistor is of the PNP type it is held permanently ON whereby triacT1 is permanently conducting. However, should an overload condition arise so that the comparator IC2 is tripped and a pulse appears at the capacitor Ce, the input 6 is momentarily dragged low (0). This has the result that the output 4 goes high (1) and output 3 goes low. Input 5 therefore also goes low and maintains output 4 high (1) even when the pulse disappears and input 6 returns to high (1). The output of the bistable on line 30 thus becomes permanently low (until reset) and the transistor Tr1 is switched OFF via the inverter 32. With the transistor Tr1 OFF, the triac T1 can no longer conduct and power is totally removed from the motor. In order to prevent the overload circuit from being tripped by the relatively high starting currents during motor start-up, this particular embodiment employs a manually operable switch 34 on the tool handle. This has a pair of normally closed contacts (N/C) and a pair of normally open contacts (N/O). When the switch is moved to its N/O position, a d.c. voltage is applied to a capacitor C7 so as to charge this capacitor to a positive voltage. Upon subsequent return (release) of the button to the N/C position, the charge on the capacitor acts via an inverter 36 to pull input 1 low (0) and thereby prevent output 3 from going low for several cycles (dependent upon the value of a resistor R11) even though input 1 may have gone temporarily low as a result of the high starting current tripping the comparator IC2. Thus, the motor is permitted to start without the over-current protection operating. Operation of the switch 34 also provides a reset facility following operation of the overload protection. The internal low-voltage d.c. supplies are obtained by the use of a conventional rectifier arrangement comprising diodes, D3, D4 Zener diodes 2 2 and capacitors C8, C9. The above described circuit thus has the advantages that: (1) it cuts off the electrical power to the motor entirely in the event of the motor current exceeding a predetermined level, e.g. as a result of jamming; (2) it senses automatically the supply voltage to the hammer and enables the motor to run off either 240V or 11 0V a.c. supplies without any manual adjustment; and (3) the selected motor speed is held constant independent of varying load conditions. Claims (Filed on 1 5/2/83)
1. A control system for a universal electric motor of the type used to drive power tools, the control system including an electronic overcurrent protection circuit which, in the event that the motor current exceeds a predetermined level, is arranged to cut off the current supply to the motor.
2. A control system as claimed in claim 1, wherein the current supply to the motor is only returned, following the occurrence of an overload condition, by the operation of a manual device on the tool, i.e. by a positive act on the part of the operator.
3. A control system as claimed in claim 2, wherein the overload protection circuit includes a semi-conductor switch, such as a triac, disposed in series with the motor field winding or windings, the semi-conductor switch being arranged to be operated to break the supply to the motor in the event of an overload condition occurring.
4. A control system as claimed in claim 3, wherein the overload protection circuit further includes a peak detecting device which monitors the current through the motor and provides a d.c.
signal representative of the level of the peak motor current, the d.c. signal being compared with a preset reference in a comparator which, in the event that the d.c. signal exceeds the reference, is arranged to switch over a bistable circuit to change the conducting state of said semi-conductor switch.
5. A control system as claimed in claim 4, wherein the bistable circuit, once tripped, maintains the semi-conductor switch in its non
conducting state until a switch in the bistable circuit is manually operated, whereupon the bistable circuit is re-set.
6. A control system as claimed in any of claims 1 to 5, including a circuit which reacts automatically to the magnitude of the mains supply voltage to the control system to enable the motor to be supplied with the correct operating voltage irrespective of whether the system is connected to 11 0V a.c. or 240V a.c. mains.
7. A control system as claimed in claim 6, wherein the mains voltage responsive circuit includes a voltage sensitive relay which, in the event that the mains voltage exceeds a predetermined level, is arranged to introduce extra resistance into a regulated speed control circuit.
8. A control system as claimed in claim 7, wherein the motor speed is controlled in dependence upon a feedback signal taken from the motor brush-gear.
9. A cdntrol system for a universal motor, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
GB08204521A 1982-02-16 1982-02-16 Universal motor control Withdrawn GB2118790A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB08204521A GB2118790A (en) 1982-02-16 1982-02-16 Universal motor control

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB08204521A GB2118790A (en) 1982-02-16 1982-02-16 Universal motor control

Publications (1)

Publication Number Publication Date
GB2118790A true GB2118790A (en) 1983-11-02

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Family Applications (1)

Application Number Title Priority Date Filing Date
GB08204521A Withdrawn GB2118790A (en) 1982-02-16 1982-02-16 Universal motor control

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Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1070411A (en) * 1962-09-28 1967-06-01 Energy Conversion Devices Inc Improvements in or relating to symmetrical current controlling devices
GB1108747A (en) * 1964-05-02 1968-04-03 Danfoss As Switching circuits for electric motors
GB1248776A (en) * 1967-10-07 1971-10-06 Danfoss As A circuit arrangement for protecting the winding of an a.c. motor against overheating
GB1298684A (en) * 1968-12-30 1972-12-06 Texas Instruments Inc Electric motor control
GB1312122A (en) * 1969-07-15 1973-04-04 Rca Corp Protection circuit
GB1445711A (en) * 1973-03-22 1976-08-11 Kenwood Mfg Co Ltd Electric motor thermal overload protection circuit
GB1467331A (en) * 1975-01-28 1977-03-16 Secr Defence Electrical power supply controllers
GB1511190A (en) * 1975-10-28 1978-05-17 Singer Co Fast operating over load switching circuit for universal motors

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1070411A (en) * 1962-09-28 1967-06-01 Energy Conversion Devices Inc Improvements in or relating to symmetrical current controlling devices
GB1108747A (en) * 1964-05-02 1968-04-03 Danfoss As Switching circuits for electric motors
GB1248776A (en) * 1967-10-07 1971-10-06 Danfoss As A circuit arrangement for protecting the winding of an a.c. motor against overheating
GB1298684A (en) * 1968-12-30 1972-12-06 Texas Instruments Inc Electric motor control
GB1312122A (en) * 1969-07-15 1973-04-04 Rca Corp Protection circuit
GB1445711A (en) * 1973-03-22 1976-08-11 Kenwood Mfg Co Ltd Electric motor thermal overload protection circuit
GB1467331A (en) * 1975-01-28 1977-03-16 Secr Defence Electrical power supply controllers
GB1511190A (en) * 1975-10-28 1978-05-17 Singer Co Fast operating over load switching circuit for universal motors

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WAP Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1)