GB2126437A - Semiconductor switching circuit - Google Patents

Abstract

The circuit is for switching an alternating current supply to a load and comprises a thyristor (1, 2, 3) with a first terminal connected to the load and a second terminal connected with an alternating current supply and a gate electrode which is connected with the output of a gate control circuit, an input of said gate control circuit being connected to said alternating current supply and a switching means (29) included in the gate control circuit operable to enable current to flow through the gate circuit, the gate control circuit providing an output signal which turns on the thyristor to connect the supply to the load upon said alternating current flowing through said gate control circuit. In one example, the load comprises a three phase motor 30 connected to the supply through three triacs 1,2,3. The triacs are fired by a circuit which includes a start button 25, a stop button 24, and thyristor 29 and a transformer 19. The invention thus provides a semi-conductor switching circuit of "solid state" construction which enables stopping and starting of, for example, a three phase motor and which overcomes one of the major problems of hitherto available mechanical motor starters, that is, contact wear due to repeated sparking of the contacts resulting from the inductive load being switched. <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 of 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 as an electric motor is an inductive load, quite considerable switching voltages 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 thyristor having (a) a first terminal which is connected to the load, (b) a second terminal which is connected with an alternating current supply and (c) a gate electrode which is connected with an output of a gate control circuit; an input of said gate control circuit being connected to said supply and a switching means included in said gate control circuit operable to enable current to flow through said gate control circuit, said gate control circuit being adapted to provide an output signal on said output which turns on said thyristor to connect said supply to said load upon said current flowing through said gate control circuit.

In standard mechanical direct-on-line three phase motor starters, operation of appropriate start and stop buttons simultaneously closes or opens three isolated circuits supplying the motor with three phase power. Additionally provision is usually made for the interruption of the three phase supply to the motor in the event of an overload or temporary failure of the mains supply.

However, as mentioned above, with such mechanical motor starters considerable contact wear is experienced due to repeated sparking of the contacts resulting from the inductive load being switched.

The present invention whilst retaining certain features of mechanical starters avoids disadvantages inherent therein inter alia the use of contacts.

The present invention will now be described by way of example and with reference to the accompanying drawings wherein; Figure 1: shows a circuit diagram of a motor starter according to one embodiment of the invention; Figure 2: shows a circuit diagram of a motor starter according to another embodiment of the invention; Figure 3: shows a circuit diagram of a motor starter according to a still further embodiment of the invention.

Referring to Fig. 1 of the accompanying drawings three triacs 1, 2, 3 are connected between the three phases of an A.C. supply, A,B,C, and the windings of a motor 30.

Prior to connection of the A.C. supply with the motor 30 the triacs 1, 2 and 3 will be in the "off" condition with an overload cut out 21 and stop button 24 both being in their "closed" position.

In order to connect the supply to the motor 30 to start same, start button 25 will be closed thus placing a positive voltage on the gate of SCR 29 turning it on and latching same. This provides a path for current from phase A through the primary winding of a transformer 1 9 through the overload cut out 21 and through a full wave bridge rectifier 22 to phase B.

This current flow develops three isolated voltages on the three secondary windings of the transformer 19. These voltages are rectified by three bridge networks 13, 14 and 1 5 and smoothed by capacitors 16, 1 7 and 1 8.

The respective current limiting resistors 7, 8 and 9 connect the respective voltages so developed across the gate electrodes of the triacs 1, 2 and 3 thus turning them on simultaneously regardless of the phase angle of the supply voltages appearing on phases A, B and C. The switching on and, as hereinafter described, the switching off, of the triacs 1, 2, 3 can alternatively be effected at zero phase angle voltage.

With the triacs 1, 2 and 3 conducting, current can now pass to the windings of the motor 30 via resistors 10, 11 and 1 2. These resistors can be heating resistances for the overload cut-out 21 such that in the event of the motor 30 drawing slightly excessive current through its windings due to a mechanical overload then eventually cut-out 21 is heated to a critical temperature and open-circuits.

This will cause SCR 29 to turn off and remain off. The primary winding of transformer 1 9 is now open-circuited and the three triacs 1, 2 and 3 will be simultaneously turned off and hence also the motor current supply.

Upon the resistors 10, 11 and 1 2 cooling, the over-load cut-out 21 will reclose such that the circuit is now once again ready for restarting by the closure of push button 25 momentarily.

Instead of using resistors 10, 11, 1 2 the heat sinks of the triacs 1, 2, 3, could alternatively be monitored and used to operate the cut-out 21.

The turning off of SCR 29 and hence the motor current supply will normally be achieved of course by the operation of the "off" button 24.

It will be seen that an inherent "no-volt release" is also provided in that in the event of a temporary failure of the supply, SCR 29 will again turn off, switching off triacs 1, 2 and 3. The start button 25 will then have to be pressed and this accordingly avoids any unexpected start up of the motor upon the mains supply being re-established.

A capacitor 23 connected across the SCR 29 provides a smoothed current therefor to permit latching whilst a resistor 26 connected in series with the SCR limits the maximum current drained from the capacitor 23 thus avoiding self interruption.

A resistor 27 provides a current limiting control for the gate of SCR 29 whilst a resistor 28 provides a negative bias for the gate to avoid spurious triggering.

A voltage dependent resistor 20 is connected across the primary winding of the transformer 1 9 to absorb the inductive voltage occurring when SCR 29 is turned off to protect same from receiving over voltage at any time. Voltage dependent resistors 4, 5 and 6 connected across the triacs 1, 2 and 3 serve a similar function in absorbing transient over voltages that are developed by the motor windings when the motor 30 is turned off thus protecting the triacs 1, 2 and 3 from any overvoltages.

Referring now to Fig. 2 of the accompanying drawings the same reference numerals have been used herein as were previously used in Fig. 1 where appropriate.

Thus the switching of the three phase power supply on terminals A, B, C, to the windings of the motor circuit is achieved by thyristors in the form of triacs 1, 2, 3. Voltage dependent resistors 4, 5, 6 may be provided as shown shunting the respective triacs 1, 2, 3.

However in this embodiment of the invention the gate control circuit for the respective gates of the triacs 1, 2, 3 does not utilize the primary and secondary windings of a transformer (19) but instead utilizes respective optical isolator circuits, 33, 34 and 35 with zero voltage switching.

Again the gate control circuit includes an input connected with the supply on phase A but instead of the primary winding of a transformer as in the embodiment of Fig. 1 resistors 31 and 32 are instead utilized. The turning on of the SCR 29 will result in current flow through the light emitting circuitry of the respective isolators 33, 34, 35. This current flow and light emission is detected by the respective light sensitive devices incorporated therein and resulting in the application to the respective gate electrodes of the triacs 1, 2, 3 of a trigger voltage appearing at the respective gates and determined by the respective resistors 7, 8 and 9. The capacitor 36 charged during the switching on of the SCR 29 serves to maintain the potential across the respective input circuits of the isolators 33, 34 and 35.

Referring now to Fig. 3 of the accompanying drawings, numerals corresponding to those used in the previous figures are again used where appropriate.

In this embodiment of the invention use has again been made of a transformer-type gate control circuit for a number of thyristors connected or connectable in series between the alternating current supply and the motor windings. However this embodiment is also directed towards enabling the reversal of the phase in the supply to respective terminals of the motor.

Thus, instead of the three triacs 1, 2 and 3 there are now herein provided for phase A a pair of parallel connected SCR's 1 A, 1 B whilst for the other phases B and C there are two pairs of SCR's 2A, 2B, and 2C, 2D, and 3A, 3B and 3C, 3D respectively. The gates of the SCR's can be controlled by either one of a pair of secondary windings 19C, 19C', 19D, 19D' depending on the position of a reversing switch 37.

Thus, in the position shown in Fig. 3, upon operation of the on button 25 and switching on the SCR 29 current flow will be through the primary winding 1 9A and not through the primary winding 19B. This results in voltages being developed in secondary windings 19C, 19C' . . ., and the switching on of SCR's 1A, 1 B, 2A, 2B, and 3A, 3B. With the reversing switch 37 in the opposite position the current flow will now be through the primary winding 1 9B resulting in the switching on of the SCR's 1A, 1 B, 2C, 2D and 3C, 3D with the result that the phases B and C to the windings of motor 30 are reversed.

It is to be appreciated however that instead of the transformer circuit shown in Fig. 3 incorporating the two primary windings and, for the three phase supply and coupled SCR's shown, the twelve secondary windings, alter natively the use of optical isolators as described in respect of the embodiment of Fig. 2 and/or triacs as described in the embodi ments of both Figs. 1 and 2 will be readily apparent.

Furthermore, it is mentioned that the resistors 10, 11 and 1 2 of the embodiment of Fig.

1 have been omitted in the embodiments of Figs 2 and 3 indicating that in these embodi ments the overload cut-out is being operated by its being fitted to the thyristor heat sinks so as to detect overload by a detection of temperature increase directly. However it is to be appreciated that the resistors 10, 11, 1 2 could again be utilized in the embodiments of Figs. 2 and 3 if desired.

Thus, by this invention, there are provided embodiments of a semi-conductor switching circuit suitable for switching an alternating current supply to an inductive load such as an A.C. motor in an efficient and reliable manner.

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 one possible embodiment 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

Claims (11)

appended claims. CLAIMS

1. A semiconductor switching circuit for switching an alternating current supply to a load, comprising; a thyristor having (a) a first terminal which is connected to the load, (b) a second terminal which is connected with an alternating current supply and (c) a gate electrode which is connected with an output of a gate control circuit; an input of said gate control circuit being connected to said supply and a switching means included in said gate control circuit operable to enable current to flow through said gate control circuit, said gate control circuit being adapted to provide an output signal on said output which turns on said thyristor to connect said supply to said load upon said current flowing through said gate control circuit.

2. A semiconductor switching circuit as claimed in Claim 1 wherein said gate control circuit comprises a secondary winding of a transformer connected with said gate electrode and a primary winding of said transformer connected to said supply through said switching means, said switching means being operable to connect said primary winding to said supply whereby a secondary voltage is developed in the secondary winding connected to said gate electrode to turn on said thyristor.

3. A semiconductor switching circuit as claimed in Claim 1 or Claim 2 wherein said switching means comprises an SCR having its gate electrode connected through a switch to one pair of terminals of a rectifying means a second pair of terminals of which being connected respectively to said primary winding and said supply.

4. A semiconductor switching circuit as claimed in Claim 3 wherein said switching means includes a further switch operable to disconnect said SCR from said rectifying means.

5. A semiconductor switching means as claimed in Claim 4 wherein said switching means further includes an overload cut-out means provided between said switching means and said supply and operable to disconnect said switching means from said supply upon an overload being detected.

6. A semiconductor switching means as claimed in any one of the claims 2 to 5 wherein said secondary winding is connected to said gate electrode through a rectifier and current limiting means.

7. A semiconductor switching circuit as claimed in any one of Claims 2 to 6 wherein said alternating current supply is a three phase supply, three of said thyristors are provided each having a respective second terminal connected with a respective input terminal of said supply, a respective gate electrode of each of said thyristors connected with a respective secondary winding of said transformer.

8. A semiconductor switching circuit as claimed in Claim 7 wherein said primary winding is connected through said switching means across two of said input terminals of said supply.

9. A semiconductor switching circuit as claimed in Claim 7 wherein a pair of said primary windings are provided which can be selectively connected by a reversing switch with said switching means, a pair of said thyristors oppositely poled and connected in parallel provided for a first phase and two oppositely poled parallel coupled pairs of thyristors provided for each of the other two phases, each of said primary windings being mutually coupled with three pairs of secondary windings, each of the secondary windings being connected with a respective gate electrode of a respective thyristor in such a manner that reversal of said reversing switch will reverse two of the phases to the said load.

1 0. A semiconductor switching circuit as claimed in Claim 1 wherein said gate control circuit comprises an optical isolator the output of which provides said output of said gate control circuit and an input of which is connected with said switching means to receive at least part of the current flowing therethrough upon said operation thereof.

11. A semiconductor switching circuit substantially as herein described with reference to Fig. 1, Fig. 2 or Fig. 3 of the accompanying drawings.