GB2253531A

GB2253531A - Energy saving motor control device

Info

Publication number: GB2253531A
Application number: GB9104667A
Authority: GB
Inventors: Douglas Balls
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-02-22
Filing date: 1991-03-06
Publication date: 1992-09-09
Anticipated expiration: 2011-03-06
Also published as: GB2253531B; GB9104667D0; GB9103786D0

Abstract

The device is interposed between the mains neutral and the neutral connection to an A.C. induction motor. The operation of the device is as follows:- Triac 6 completes the circuit when energised by triac 7. Capacitor 8 is charged through resistor 9 and variable resistor 10, by the voltage appearing across the terminals 3 and 5, which is the mains voltage less motor back e.m.f. When the capacitor 8 has reached sufficient voltage to overcome the two zener diodes 11 and 12, triac 7 is switched on. Current determined by resistor 13 then flows to triac 6, switching it on, to complete the supply to the motor. A series choke 14, and capacitor 15 provide a reduction in emitted interference. When the motor is running under light loads, the back e.m.f. is high, so the charging rate of capacitor 8 is low, and the time taken to overcome the zeners 11 and 12 and consequently turn on triac 6 is longer than when the motor is running under heavy loads, when triac 6 is switched on sooner. The effect of this is to save unwanted current at light loads and allow it to be used at heavy loads. The device is suitable for use with 3-phase induction motors by inserting a respective device (4, Fig 3) in each phase. <IMAGE>

Description

ENERGY SAVING UNIT FIELD OF THE INVENTION The present invention relates to an energy saving unit for controlling the energisation of an AC operated device.

BACKGROUxTD TO HF INVENTION It has long been appreciated that in environmental control systems such as refrigeration systems where an AC induction motor is placed under inconstant load there is scope for economy in use of electricity to power the motor. A number of different energy saving units for strategically cutting supply of electricity to induction motors and similar AC operated devices are believed to exist. One example of such a unit, believed to be the most relevant prior art, is described in UK Patent Application No 2198895.

SUMb OF THE INVENTION According to the present invention there is provided an energy saving unit for controlling the energisation of an AC powered device, said unit comprising an electrical circuit having a first terminal for connection to one of the conductors of the power supply, a second terminal for connection to a terminal of the powered device, a solid state switch connected in series between the first and second terminals, and a monitoring and control circuit connected between the first and second terminals, the monitoring and control circuit comprising means for sensing the load on the motor, and means responsive to the sensed load to control the solid state switch to reduce the power supply to the motor when the motor is operating at reduced load, characterised in that the control means is operative to open the solid state

switch at the zero crossing point of the power supply wave for the timing of the shutting of the solid state switch relative to the zero crossing point of the power supply wave form being dependent upon the sensed load.

Preferably the control means comprise a pair of zener diodes According to a second aspect of the present invention there is provided a method for controlling the energisation of a threephase induction motor wherein an energy saving unit of the present invention is inserted in each of the three live phases of the induction motor.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings wherein; Figure 1 is a schematic diagram illustrating how a unit embodying the present invention may be deployed in use; Figure 2 is a circuit diagram embodying the first aspect of the present invention; Figure 3 is a schematic diagram illustrating one embodiment of the second aspect of the present invention; and Figure 4 is a voltage-time graph of the sinusoidal power supply wave form as might result from use of the present invention.

DESCRIPTION OF THE PREFERRED EMEODTMENT Referring to Figure 1, an AC induction motor 2 is connected by one terminal 1 to the live supply wire of an AC electric power supply and an energy saving unit 4 embodying the present invention is connected at 3 to the neutral terminal of the motor 2 and at 5 to the neutral wire of the power supply.

The circuitry of the energy saving unit 4 is illustrated in Figure 2. Between the terminals 3 and 5 of the unit is positioned a first triac 6 to control the flow of power through the unit 4.

The solid state switch formed by the triac 6 closes to complete the circuit allowing power to flow through the unit 4 when energised by a second triac 7. This second triac 7 is, in turn, energised by a pair of zener diodes 11, 12. The controlling action of the zener diodes 11, 12 to instigate switching on of the solid state switch, triac 6, is effected in response to the voltage arising across the terminals 3 and 5 of the unit 4. The potential difference across the unit 4 is equivalent to the mains supply voltage minus the back EMF from the motor 2. The potential difference across the unit 4 is indicative of the load on the motor since the back EMF from the motor is inversely proportional to the load on the motor.

The potential difference across the unit 4 is "sensed" by charging of a capacitor 8 through a preset resistor 9 and variable resistor 10. When the capacitor 8 has charged above the threshold voltage to fire the zener diodes 11, 12 these will energise firstly the second triac 7 and then the first triac 6 to supply power to the motor 2. The current supply to triac 6 is determined by a resistor 13 between triac 7 and terminal 5 of the unit 4.

Necessary refinements to the minimalist circuit design include a series choke 14 and a capacitor 15 to suppress emitted interference from the unit 4.

Referring to figure 4, the unit 4 operates to truncate the sinusoidal wave form of the AC power supplied to the motor 2.

The triac 6 switches on, ie closes to complete supply to the motor 2, when the voltage across the unit 4 rises above a predetermined level. The triac 6 opens to shut off power supply as the voltage of the AC power supply passes the zero crossing point of the power supply wave form. The voltage to the motor is interrupted for a period t5 beyond passing of the voltage through the zero crossing point. In the Figure 4 illustration this period t5 lasts for 2 milliseconds. Should the load on the motor decline then the back EMF from the motor will rise ,impeding charging of the capacitor 8 and thereby further delaying switching on of the triac 6 such that the period t5 is lengthened from 2 milliseconds to, for example, 3 milliseconds.

The unit of the present invention need not necessarily be used with an AC induction motor. Various other AC devices which generate a back EMF may provide suitable alternative applications for the unit 4. The unit 4 of the present invention need not be used on the neutral side of the motor 2. Referring to figure 3, three units 4 are connected to a three-phase induction motor 16, each unit 4 being connected to a respective one of the three live phase terminals of the motor 16.

Claims

10. CLAIMS. What we claim is that by use of a simple circuit as described, inserted into the mains neutral supply, the cost of running an electric induction motor may be reduced. Also, we claim that this simple circuit may be inserted into each phase of a 3-phase electric induction motor, and the running cost will be reduced. Claims

1. An energy saving unit for controlling the energisation of an AC powered device, said unit comprising an electrical monitoring and control circuit connected in series with the powered device between two conductors of an AC power supply, the monitoring and control circuit comprising a capacitor which is charged through a resistor means dependently on the load on the powered device, zener diode means fired when the charge on the capacitor exceeds a threshold voltage, and a triac means energised when the zener diode means is fired, thereby to supply power to the powered device through said triac means, wherein the triac means opens to shut off the power supply to the powered device when the AC power supply crosses the zero crossing point of the power supply waveform and closes to supply power at the end of a period immediately following shut off which is dependent upon the rate of charging of the capacitor, which is in turn dependent on the load on the powered device.

2. An energy saving unit according to claim 1, wherein the triac means comprises a first triac controlling the flow of current between the two supply conductors and a second triac which controls energisation of the first triac, the energisation of the second triac being controlled by the zener diode means.

3. An energy saving unit according to claim 2, wherein the supply current to the first triac is determined by a resistor connected between the neutral supply conductor and the second triac.

4. An energy saving unit according to claim 1 or claim 2 or claim 3, wherein the zener diode means comprises two zener diodes connected back to back in series.

5. An energy saving unit according to any of claims 1 to 4, wherein the resistor means comprises a preset resistor and a variable resistor.

6. An energy saving unit according to any of claims 1 to 5, wherein for interference suppression a choke is connected in the neutral supply conductor and an associated capacitor is connected between the two supply conductors.

7. An energy saving unit according to any of claims 1 to 6, when used to control energisation of an AC induction motor.

8. An energy saving unit according to claim 7, wherein an energy saving unit in accordance with any of claims 1 to 6 is connected in each of the three live phases of the motor.

9. An energy saving unit substantially as hereinbefore described with reference to Figures 1, 2 and 4 or to Figure 3 of the accompanying drawings,