GB2044023A - Semiconductor switching circuit - Google Patents

Abstract

A semiconductor switching circuit comprises three triacs 1, 2 and 3 having terminals A, B and C for connection to an alternating current supply, terminals X, Y and Z for connection to a load, and gate electrodes connected to a triggering arrangement. The triggering arrangement comprises three transformers having primary and secondary windings, the primary windings applying triggering voltages to the gate electrodes of the triacs 1, 2 and 3 when the secondary windings are effectively short circuited through a circuit including rectifiers 10, 11 and 12, and SCR 23, when a switch 21 is closed. In an alternative arrangement, the triggering arrangement includes optical isolators. <IMAGE>

Description

SPECIFICATION Improvements in and relating to control systems The present invention relates to A.C. control systems and particularly, but not exclusively to an alternating current electric motor control circuit.

One of the problems to be overcome in the control an alternating current electric motor is that the normal contactor contacts connecting the motor to the A.C. supply tend to wear out rapidly due to electrical arcing. It will be appreciated in this regard that with an electric motor being an inductive ioad, quite considerable voltages in switching can be developed across the supply contacts.

It is thus an object of the present invention to provide a semiconductor switching circuit suitable for use as an alternating current motor starter circuit.

The present invention according to one embodiment thereof thus provides a semiconductor switching circuit for switching an alternating current supply to a load, comprising a triac having a first terminal for connection to the load, a second terminal for connection with the alternating current supply and a gate electrode, said gate electrode being connected with a triggering arrangement electrically isolated from the first and second terminals and an output circuit which develops a triggering voltage to be applied to said gate electrode.

One embodiment of the present invention will now be described by way of example with reference to the circuit diagram shown in the accompanying drawing.

Referring now to the accompanying drawing a three phase input voltage supply, for example at 400 V 50 HZ, is connected with terminals A, B, C.

Output terminals X, Y, Z, can then be connected to the A.C. load, for example in one particular use of the present invention, a three phase A.C.

electric motor.

Whilst the present invention will now be described in respect of a three phase A.C. supply it is to be appreciated that this is by way of example only. Connected between each of the respective input and output terminals of each phase is a respective triac 1, 2 or 3. A respective transformer 7, 8, or 9, able to provide isolation on each phase between the input and the control circuitry has its primary winding connected between a respective phase input terminal and the gate electrode of the triac of that same phase.

The secondary winding of a respective transformer 7, 8, or 9 is connected across a respective full wave rectifier 10, 11 or 12 with each secondary voltage being independently full wave rectified and all three fed into a smoothing capacitor 1 9 via a resistor 1 8. A suitable turns ratio for each of the transformers 7, 8 or 9 may be ofthe order of 20:1.

The resultant positive voltage is then fed through cut-out 1 6 and 1 7 to a stop button 20.

With the start button 21 in its open position an SCR 20 remains turned off. When the start button 21 is moved to its closed position a positive voltage is then applied to the gate of the SCR 23 via resistor 24. This turns SCR 23 on and into a latched condition. A current of higher magnitude can flow via SCR 23 and resistor 22 effectively short-circuiting most of the voltage originally appearing across the secondaries of the transformers 7, 8 and 9. Accordingly, the back EMF of the transformers 7, 8 and 9 is so reduced that the original high impedance of the primary windings of transformers 7, 8 and 9 is reduced substantially to zero with substantially only the D.C. resistance of the primary windings remaining, this possibly being of the order of 3000 ohms.

With this occurrence respective triggering voltages are developed across the primary windings of respective transformers 7, 8 and 9 causing a current of a few milliamps to flow between the gates of triacs 1,2 and 3 and their respective terminals A.C. and C with triacs being turned on into full conduction of load current.

The power supply on terminals A, B and C is now applied across the respective output terminals X, Y and Z via respective resistors 13, 1 4 and 1 5. In the absence of overload conditions the heat generated from resistors 13, 14 and 1 5 is insufficient to operate thermal cut-outs 1 6 and 17, which having normally closed contacts will in overload current conditions then go open-circuit.

Therefore, under normal load conditions all three triacs 1, 2 and 3 are, via the other control circuitry, permitted to remain in full load conduction.

However, in the event of an overload occuring the increase in heat generated by the resistors 13, 14 and/or 1 5 will cause the cut-out(s) 1 6 and/or 1 7 to open-circuit. If either cut-out 1 6 or 1 7 opencircuits then the current through SCR 23 is interrupted thus allo.wing the inductance and consequent impedance to build again in all three transformers 7, 8 and 9 and therefore to again present in effect an open circuit as seen by the gates of the three triacs 1, 2 and 3. Accordingly, the triacs 1, 2 and 3 are then turned off and A.C.

load connected to the terminal X, Y and Z is thus disconnected from the A.C. supply across input terminals A, B, C.

After an appropriate time interval, generally of the order of several minutes, when resistors 13, 14 and 1 5 have cooled down sufficiently, the cutouts 1 6 and/or 1 7 will re-close. However, in order to switch on all three triacs 1,2 and 3 again it is necessary to one more press the start button 21 when the previously above mentioned start sequence will commence.

To disconnect the load from the input supply at any time this can be achieved by pressing the stop button 20. Similarly, if at any time there is a slight pause in the supply voltage to the terminals A, B and C then the SCR 23 will turn of and subsequently turn off all three triacs 1, 2 and 3, giving the standard facility of "no-volt-release".

Connected across the gate circuit of the SCR 23 are resistors 24 and 25, connected in series with start button 21, with resistor 24 providing the appropriate voltage bias on the gate when the start button 21 is closed and resistor 25 providing a negative bias to the gate of SCR 23 to avoid spurious triggering.

Resistors 4, 5 and 6 shown connected across load terminals of the triac 1, 2 and 3 are suitably voltage dependent and by-pass voltage spikes from inductive loads connected to the terminals X, Y and Z when the triacs 1,2 and 3 are being turned off.

Resistors 13, 14 and 1 5 would each have a value dependent on the overload conditions required to be detected in operating the cut-outs 16 and 17 and thus may be of lower values than the 2.2 ohms indicated in the schedules hereinafter where heavier cut-out current overload sensing is required.

The control circuitry including the start button 21 and the stop button 20 is electrically isolated from the voltage input terminals A, B and C and the output voltage terminals X, Y and Z, the offstate voltage with a 400V supply would be nominally of the order of 60V and the on-state voltage would be of the order of 5V.

The electronic switching circuit of the present invention may be operated as frequently as may be required without adversely effecting its efficiency of operation such as is the experience with standard electro-mechanical starters having physical make/break type contacts, as there are no physical contacts in the main current carrying circuits between respective terminals A-X, B-Y and C-Z.

If desired, the switching circuit of the present invention may be used purely as a semiconductor 3 phase contactor in which case the cut-outs 1 6 and 17 and the resistors 13, 14 and 1 5 may be omitted, all other circuitry remaining as shown.

SCHEDULE OF SUITABLE COMPONENT VALUES FOR A 3 PHASE 400 VOLT 50 HZ SUPPLY ITEM VALUE 7,8,9(N:1) 20:1 13,14and15 2.2 Ohms,5 W 18 820 Ohms 19 50uf 22 10 Ohm 23 BTX 181-00 24 56K Ohm 25 3.3K Ohm In an alternative embodiment of the present invention as shown in the drawings accompanying the provisional patent specification, the triggering arrangement for the respective gate electrodes of the triac connected between the input voltage supply terminals and the output load-connected terminals includes an output circuit of an optical isolator arrangement with at least one optical isolator providing the necessary isolation between the load current path through the semiconductor switching circuit of that embodiment of the invention and the control current circuitry of that embodiment. The provisional patent specification drawing and the description relative thereto included in the provisional specification is incorporated herein in its entirety by way of reference.

Where in the aforegoing description reference has been made to specific components or integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described by way of example and with reference to possible embodiments thereof it is to be understood that modifications or improvements may be made thereto without departing from the scope of the invention as defined in the appended claims.

Claims (10)

1. A semiconductor switching circuit for switching an alternating current supply to a load, comprising a triac having a first terminal for connection to the load, a second terminal for connection with the alternating current supply and a gate electrode, said gate electrode being connected to a triggering arrangement electrically isolated from the first and second terminals and an output circuit which develops a triggering voltage to be applied to said gate electrode.

2. A semiconductor switching as claimed in claim 1 wherein said triggering arrangement comprises a transformer including a primary transformer winding mutually coupled with a secondary transformer winding and a triggering means adapted to effectively short circuit said secondary transformer winding to cause said triggering voltage to be developed across said primary winding.

3. A semiconductor switching circuit as claimed in claim 2 wherein said secondary winding is connected across a rectifying means the rectified voltage output from which provides an input to said triggering means.

4. A semiconductor switching circuit as claimed in claim 3 wherein said triggering means comprises a thyristor connected with said rectified output via a normally-closed switching means, a normally open switching means connected with a biasing arrangement across said thyristor whereby operation of said normally open switching means causes said thyristor to conduct and effectively short circuit said transformer secondary winding.

5. A semiconductor switching circuit as claimed in claim 4 and including a cut-out means connected between said rectified output and said normally-closed switching means, said cut-out means being adapted to detect an overload current condition and to disconnect said thyristor from said rectified output resulting in the effective short circuiting of the secondary transformer winding being removed and the triac to cease to conduct.

6. A semiconductor switching circuit as claimed in claim 5 wherein said cut-out means is thermally responsive and is in thermal association with a resistor connected between said triac and said second terminal.

7. A semiconductor switching circuit as claimed in any one of the preceding claims wherein a voltage dependent resistor is connected in parallel across said triac to protect same whilst it is being turned off from voltage spikes from an inductive load connected to said second terminal.

8. A semiconductor switching circuit as claimed in claim 4 wherein said biasing circuit includes a resistor providing a negative bias to the gate of said thyristor to protect same against spurious triggering.

9. A semiconductor switching circuit as claimed in claim 1 wherein said triggering arrangement comprises optical isolating means having an output circuit in which the triggering voltage is developed.

10. A semiconductor switching circuit substantially as herein described with reference to and as illustrated by the accompanying drawing or the drawing accompanying the provisional specification.