GB2090027A

GB2090027A - Current limiting circuit

Info

Publication number: GB2090027A
Application number: GB8124777A
Authority: GB
Original assignee: BROWN S G Ltd
Current assignee: BROWN S G Ltd
Priority date: 1980-12-22
Filing date: 1981-08-13
Publication date: 1982-06-30

Abstract

In a ship's bearing repeater system having three-phase motors driven from a common transmitter, the motor windings (9a....9n) are connected together in parallel to be supplied from a transmitter through a current limiting circuit. The circuit has current sensing means (11) including an optocoupler and arranged to cut off the supply if the load current reaches an upper level, and to restore the supply when current through the load due to an inductor (12) reaches a lower level. The supply is switched by an FET (10) which may be controlled by a Schmitt trigger circuit. The FET can be independently controlled by an optical link (31). Each motor winding has a series fuse (25a....25n) to provide short circuit protection. <IMAGE>

Description

SPECIFICATION Current limiting circuit The invention relates to a current limiting circuit and to a bearing repeater system incorporating a current limiting circuit.

A bearing repeater system, as used on a ship, comprises a plurality of repeater stations, distributed around the ship at suitable positions, each station being driven from a control station so as to display the ships bearing as determined at the central station. The display at each repeater station is driven by a three-phase stepping motor, and if a fault should develop in one of the motor coils so that the coil is short circuited, the whole repeater system fails and there is no convenient way of locating the particular station, which may be one of as many as fourteen, at which the fault has developed.

The invention accordingly provides a bearing repeater system comprising a plurality of threephase motors driven from a common transmitter, the plurality of motor windings for each phase being connected together in parallel to be supplied from the transmitter through a current limiting circuit arranged to limit the current supplied to the windings, whereby after removal of a faulty motor winding the other motor or motors can continue to function. Preferably each winding is associated with a fuse arranged to rupture at a current level such that a winding is effectively removed from the circuit immediately its resistance drops to a predetermined value or it becomes short circuited.

The invention also provides a current limiting for limiting current drawn by a load from a supply source, the circuit being arranged to be connected between the load and the supply source and having an inductor and current sensor means responsive to current flowing through the load, the current sensor means being arranged to disconnect the source from the load when the load current has risen to a predetermined upper limit and to re-connect the source and the load when load current due to electrical energy stored in the inductor falls to a predetermined lower limit.

Such a current limiting circuit can be used in the bearing repeater system of the invention but is not limited to such use.

The invention is further described below, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows in schematic block diagram form a current limiting device embodying the invention connected between a supply source and a load; Figure 2 graphically indicates the variation of load current during operation of the device of Figure 1; Figure 3 shows in schematic block diagram form a current limiting device embodying the invention in use with an ac supply source and a load; Figure 4 shows in schematic block diagram form the current limiting device of Figure 1 in use in a bearing repeater system embodying the invention; Figure 5 corresponds to Figure 4 but shows the circuit of the current limiting device in greater detail; and Figure 6 resembles Figure 1 but shows a modified form of the device.

As shown in Figure 1 ,the current limiting device 1 of the invention is connected between terminals 2, 4 of a supply source 5 providing a dc voltage and terminals 6, 8 of an inductive load 9.

The device 1 includes in series, between the negative terminal 4 of the supply source and the load terminal 8, an electronic switch 10, an electronic current sensor 11 and an inductor 1 2.

The sensor 11 controls the switch 10 through a Schmitt trigger circuit 14. The device 1 provides a line 1 5 directly connecting the positive supply source terminal 2 to the load terminal 6, but a line 1 6 connects between this line 1 5 and the junction between the electronic switch 10 and the current sensor 1 This line 16 contains a rectifying diode 18 arranged so that no current flows in the line 1 6 when switch 10 is closed.

Referring now to Figure 2, suppose switch 10 to be closed at time to. A current i3 will flow through the load 9, which is equal to current i through the switch 10 which is sensed by the sensor 11. This current rises on the exponential curve shown because of the inductance in the load 9 and the inductor 14. Current i2 in the line 16 is zero. At time t1 the load current i3 reaches an upper limit current 1,, equal to the current at which the current sensor 11 is arranged to cause the switch 10 to open.

The opening of the switch 10 causes the current1 to drop instantaneously to zero, but because of the load inductance and the inductor 12, a current i2 now flows through the load 9, the inductor, the sensor and the diode 18.

Instantaneously at time t1 this current i2 is the load current i3 and will decline from this value exponentially, as shown in Figure 2.

The current sensor 11 is however arranged to cause the switch 10 to close again when the current i3 (=i2) has reached a predetermined lower limit 12, which occurs at time t2. At this time t2, current i2 becomes 0 and ii becomes equal to i3 which increases again towards i1 between times t2 and t3, at which current limit i1 is again reached and the current sensor 11 opens the switch 10.

The consequence is that the load current i3 continues to oscillate between i1 and 12.

The operation of the circuit I depends on electrical energy stored in inductor 1 2. The load 9 can be of resistance, inductance and capacitance together. The value of the inductor 12 affects the time intervals t1-t2 and t2-t3, and the inductor may be variable to facilitate adjustment of these intervals. The inductor 1 2 of course introduces into the circuit an additional voltage drop to its resistance, and it imposes a small time lag.

By suitable selection of design parameters, the difference between i1 and 12 can be chosen to be as large or as small as desired. If chosen to be very small, the load current i3 has in effect a constant value with a small superimposed excursion. It will be noted that the limiting circuit 1 can be employed as an off/on control device for the current load, if means are provided for opening and closing the switch 10 independently of the current sensor 11.

In Figure 3, the current limiting circuit 1 is shown in use with the load 9, which is now supplied with an ac input from a source 20. Correct functioning is achieved by means of diodes 21,22,23,24 connected, as shown, so that in one direction of current flow through the load, a dc current is applied to the circuit 1 through diodes 21 and 22, whilst in the other current flow direction, dc current is still applied to the circuit 1 in the same direction through the diodes 23 and 24.

Figure 4 shows a modified version of the circuit of Figure 1, in which the single load 9 is replaced by a load in the form of a plurality of motor windings 9a, 9b 9n, each in series with an associated fuse 25a, 25b 25n, and connected together in parallel. Each of the loads 9a 9n represents one winding of a respective three-phase stepping motor of a marine bearing repeater system having n stations with a display driven by one of the motors at each. The motor windings for the other two phases are connected together in the same way in a second and a third circuit similar to that of Figure 4, the three sources 5 being constituted by voltage sources under the control of a control circuit not shown in Figure 4 but described below in connection with Figure 5.

The control circuit receives pulses from a transmitter device so that the n stepping motors are driven in a manner to reproduce, at each of the associated repeaters, the main compass bearing to which the transmitting device is responsive.

The three current limiting circuits 1 alleviate the extent of system failure should one of the motor winding become short circuited. Thus, suppose that the normal currents i through the windings 9a 9n are equal, and suppose that each fuse 2a 25n is rated to rupture at twice this current, that is, at 2i. The total current through the electronic switch is consequently ni, and the electronic switch 10 is capable of carrying a current of at least (n+ 1 )i. The sensor 11 is arranged to open the electronic switch 10 when a current of (n+1)i is reached. If now one of the windings 9a 9n, say winding 9a, becomes short circuited, the current through this winding will rise almost to ni, and the associated fuse 25a will promptly rupture. Thereafter, the remaining windings 9b 9n can continue to function rormally.The bearing repeater with which the winding 9a is associated will cease to function properly or may not function at all, and so the position of the fault within the system as a whole can be readily located, while the rest of the system continues to operate.

It will be evident that the fuses 25a 25n are not essential to the protection of the system, because of the upper limit to the current supplied to the windings 9a 9n imposed by the circuit 1.

However, the bearing repeaters still in operating condition cannot resume functioning until the short circuited winding is removed from the circuit as by rupture of the associated fuse. The circuit 1 can be employed with a bearing repeater system incorporating any number of repeaters, and the fuse rupture currents and maximum currents can be appropriately selected as desired.

In Figure 5, the current limiting circuit 1 is shown in greater detail in combination with the windings 9a 9n and fuses 25a 5a......2 25n. The electronic switch 10 is represented by a power field effect transistor, and the current sensor 11 is an opto-coupler. The Schmidt trigger circuit 14 is an open coliector comparator.

A second opto-coupler 31 and resistors 32, 34 are provided in a control circuit for switching the electronic switch 12 independently of the current limiting function. This is done by applying a voltage at terminals 30 to turn on the optocoupler 31 so that a positive potential is applied to the gate of the switch 1 0, unless the circuit 14 sinks this to ground. Components 35 to 41 provide a 1 6 volt supply to the resistor 32 of the control circuit, and also a voltage to the positive terminal of the comparator 14.

Inthe normal operation of the circuit, a 40 volt supply at terminals 33 is applied to the windings 9a 9n, across a capacitor 28, in response to 5 volt pulses from the transmitter received at terminals 30 of the control circuit. With an input signal at the terminals 30, the transistor of the opto-couplerwill conduct. For the electronic switch 10 to be turned on and conductive, a positive potential is required at its gate and this is being provided by the 1 6 volt supply through the resistor 32, and the transistor of the opto-coupler 31. Consequently, the electronic switch 10 passes the current i which is less than but approaches the upper current limit 11. The current i2 is zero.

As long as the switch 10 is conducting, a portion of the load current flowing through it passes through a resistor 42 and the current sensor 11.

The resultant current through a resistor 41 determines the voltage applied to the negative terminal of the circuit 14 which is a measure of the current through the switch. During normal operations this voltage is such as to maintain the comparator output in a non-conductive state.

If now the resistance of one of the resistors 9a 9n drops to a very low value, or the resistor even becomes short circuited, then the current ii increases, so that the voltage drop across the resistor 44 increases until the comparator 14 switches. The switching of the comparator 14 sinks the gate of the electronic switch 10 approximately to ground. The switch 10 is thus turned off.

The current i2 now flows and soon decays to a level (12) at which the voltage developed across the resistor 44 causes the circuit 14 to switch back to the non-conductive state, so that the switch 10 can be turned on again. It can thus be seen that the circuit of Figure 5 fulfills the functions of the circuit of Figure 1.

Exemplary values and identities for components of the circuit of Figure 5 are as follows: 10 IRF 230 11 6N136 12 500,us 14 LM 2401 18 60V.7A.

28 100,uF 31 4N35 32 1KQ 34 300Q 35 60V.1A.

36 330Q 37 4KS2 38 1us? 39 20,uF 40 20yF 41 1 6V.Zener diode 42 100Q 44 1KQ 45 470Q 46 0.6Q 47 370Q 48 1 OKQ 49 1OAKS1 50 220,u,uF 52 60V.7A.

In the device 1 of Figure 1, the single current sensor 11 senses both the upper and the lower current levels i1 and 12. In the modified device 100 of Figure 6 in which parts similar to those of Figure 1 are given the reference numerals of that figure, separate current sensors 110 and 111 are providedof which sensor 110 located between the source terminal 4 and the switch 12 is arranged to cause the switch to open when the current ii (13) reaches the limit 11, whilst sensor 111 is arranged to cause the switch to close again when the current i3 (i2) reaches the predetermined lower limit 12. The sensor 110 can be constituted by a comparator and the sensor 111 by an optocoupler in an approximately modified form of the circuit of Figure 5.

It will thus be evident that the invention can be modified in a variety of different ways within its scope as indicated in the following claims.

Claims

1. A current limiting circuit for limiting current drawn by a load from a supply source, the circuit being arranged to be connected between the load and the supply source and having an inductor and current sensor means responsive to current flowing through the load, the current sensor means being arranged to disconnect the source from the load when the load current has risen to a predetermined upper limit and to re-connect the source and the load when load current due to electrical energy stored in the inductor falls to a predetermined lower limit.

2. A circuit as claimed in claim 1 having a switch controlled by the current sensor means connected in series therewith and with the inductor in a line connecting one terminal of the source to one terminal of the load, a second line connecting together the other terminals of the sources and the load, and a third line between the second line and the junctions between the switch and the current sensing means in the first line, the third line containing means preventing current flow therein when the switch is closed.

3. A circuit as claimed in claim 2 wherein the switch is an electronic switch controlled through a comparator circuit by an opto-coupler constituting the current sensing means.

4. A circuit as claimed in claim 1 wherein the current sensing means comprises a first current sensor arranged to open the switch when the upper current limit is reached, and a second current sensor arranged to close the switch when the lower current limit is reached, the first sensor being connected in series with the switch and the inductor in a first line connecting one terminal of the source with one terminal of the load, a second line connects together the second terminals of the source and the load, and a third line extending between the second line and the junction of the switch and the inductor contains means preventing current flow therein when the switch is closed in series with the second sensor.

5. A circuit as claimed in claim 1, 2, 3 or 4 having means for opening and closing the switch independently of the current sensing means.

6. A current limiting circuit substantially as herein described with reference to Figures 1,2 and 5 or Figure 6 of the accompanying drawings.

7. A bearing repeater system comprising a plurality of three-phase motors driven from a common transmitter, the plurality of motor windings for each phase being connected together in parallel to be supplied from the transmitter through a current limiting circuit arranged to limit the current supplied to the windings, whereby after removal of a faulty motor winding the other motor or motors can continue to function.

8. A bearing repeater system as claimed in claim 7 wherein the current limit circuit comprises a current limiting circuit as claimed in any one of claim 1 to 6.

9. A bearing repeater system as claimed in claim 8 having a fuse connected in series with each motor winding, each fuse being arranged to rupture of a current level such that the associated winding is affectively removed from the circuit when its resistance drops below a predetermined value.

10. A bearing repeater system substantially as herein described with reference to Figures 4 and 5 of the accompanying drawings.