GB2198895A - Motor control circuit - Google Patents

Abstract

A circuit for controlling the energisation of an AC electric motor one terminal of which is directly connected to a live conductor of a single phase power supply, the circuit comprising a first terminal 5 for connection to a neutral conductor of the power supply, a second terminal 6 for connection to another terminal of the motor, a solid state switch 7 connected in series between the first and second terminals, and a monitoring and control circuit connected between the first and second terminals for controlling the switch. The monitoring and control circuit comprises means for sensing the load on the motor (capacitor 13 for phase and resistors 10, 17 for magnitude of motor voltage) and means responsive to the sensed load to control the solid state switch to open for a constant time further from the voltage peak to reduce the power supplied to the motor when the motor is operating at reduced load. The circuit may be arranged to control a plurality of motors connected to terminal 6 (Fig. 6) <IMAGE>

Description

MOTOR ENERGISATION CONTROL CIRCUIT The present invention relates to a motor energisation control circuit and in particular to a circuit for controlling the energisation of a plurality of AC electric motors from a single phase power supply.

The efficiency of electric motors is generally expressed as a percentage derived by dividing the useful power that the motor delivers by the electrical power that the motor consumes. In many applications of standard AC induction motors the maximum motor output power must be sufficient to accommodate "worst case" conditions, for example at start-up. Unfortunately these worst case conditions do not often apply and accordingly for much of the time the motor may be running at reduced load.

Whereas the efficiency of a standard induction motor of the type used for example to drive the compressor of a refrigerator is typically of the order of 80% the efficiency reduces rapidly, for example to less than 70% at half load. This is because losses due to electrical current through the windings (proportional to the current squared) and losses due to magnetic currents in the metal cores of the motor stator and the motor rotor (proportional to the applied voltage) reduce very little in response to reductions in the load on the motor.

The above described problems associated with AC motors are well known and various proposals have been made to overcome these problems. In particular electronic controllers have been produced which incorporate triac or other solid state switching devices which are controlled to modulate the voltage, current, phase angle or frequency of the power supplied to the motor. The known controllers do reduce energy losses in the motor but may disrupt the operation of secondary control circuits with which motors are normally associated, for example radio frequency suppression circuits,. capacitive correction circuits and on/off controls. This disruption occurs because the known controllers operate on both the power supply connections to the motor.Disruption problems can be overcome by carefully tailoring the controller to the particular motor and auxillary circuits found in individual cases but it makes it extremely difficult to produce a controller which can be used in a wide variety of circumstances and in association with a wide variety of motors.

A further problem with the known energy loss controllers is that it is not possible to provide a single control unit which can be connected to a number of motors so as to improve the efficiency of each of them. For example, in the case where there are four refrigeration compressor motors in a particular location, each associated with a respective separate cold room or cabinet, each motor will be turned on and off in dependence upon the cooling required for the respective cold room or cabinet. The motors will be turned on and off by respective thermostats, pressure switches or other control devices. An independent energy loss controller could be provided for each motor, but this would be relatively expensive as compared with a single unit adapted for connection to all of the motors.Unfortunately a single conventional control unit connected to each of the motors could not be sited on the motor side of the normal controlling devices such as thermostats and then feed back to the four compressor motors as the motors would not then be individually controlled by the respective normal controlling devices, and a single controller could not efficiently supply energy to a series of motors the loads on which did not vary in unison. In most applications, the load applied to each motor varies independently of the load applied to the other motors.

In one known motor controller the energy supplied is controlled by reducing the supplied voltage to zero for a period the duration of which corresponds to the required reduction in the supplied energy. With such an arrangement two or more motors supplied by a single controller each receive substantially the same energy regardless of the loads applied to that motor. It has also been proposed however to provide a motor controller in which the energy supplied is controlled by reducing the supplied voltage to zero for a predetermined constant period, the energy supplied being adjusted by adjusting the position of the period during which the supplied voltage is zero relative to supplied waveforms. No proposals have been made however to use such a controller in a simple manner to supply more than one motor such that each motor receives energy appropriate to the load on that motor.

It is an object of the present invention to obviate or mitigate the above problems.

According to the present invention there is provided a circuit for controlling the energisation of an AC electric motor one terminal of which is directly connected to a live conductor of a single phase power supply, the circuit comprising a first terminal for connection to a neutral conductor of the power supply, a second terminal for connection to another terminal of the motor, 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 supplied to the motor when the motor is operating at reduced load.

The monitoring and control circuit is powered by the power supply to the motor regardless of the condition of the solid state switch. Thus, secondary features which may be incorporated in the monitoring and control circuit, and equipment incorporating the motor, such as radio frequency suppression, capacitive correction, and on/off controls for example, are not disrupted by the reduction in the power supplied in light load conditions.

The connection of the controlling circuit in the neutral conductor of the power supply significantly reduces installation problems as compared with connection in the live conductor.

The second terminal may be arranged for connection to a terminal of each of a plurality of motors, each motor having one terminal connected to the said live terminal and another terminal connected to the said second terminal, the monitoring and control circuit comprising means for sensing the combined load on the motors, the control means being responsive to the sensed combined load on the motors to control the solid state switch to reduce the power supplied to the motors when one or more of the motors is operating at reduced load, and the control means being operative to open the solid state switch for a predetermined substantially constant period during each cycle of the power supply, wherein the timing of the opening of the solid state switch relative to the zero crossing point of the power supply waveform is dependent upon the sensed load.

The control circuit described above is capable of efficiently energising more than one motor because of the basic characteristics of AC motors. An AC motor is a non-linear impedance device which when running creates its own internal back EMF which together with the motor losses and the motor load balances the power input to the motor. This balance results in a speed of rotation which does not vary much with load, a variation of 5% of design speed being typical between high load, i.e. minimum speed, and low load, i.e. maximum speed. The phase slip angle of the motor varies with the load, and the change in slip angle affects both the magnitude and phase of the internally generated EMF. Under high load conditions the back EMF is reduced in magnitude and the phase difference is reduced.

When two or more motors are connected in parallel to a single controller in accordance with the invention, the signal sensed by the controller is a combination of the back EMF's generated by each motor. Although these signals are seemingly combined through the common connections to the motors their effect is to provide a common magnitude and phase change signal to the controller. If those motors are operating under different load conditions the high load motor or motors will produce relatively small back EMF's and phase slips whereas the low load motor or motors will produce relatively large back EMF's and phase slips.

The controller switch-off time is of substantially constant duration but the phase angle at which the switch turns off varies according to the sensed load signal. This means that under low load the switch is off near to the peak of the voltage supply waveform and at high load the switch is off near to the zero axis. Thus for example, if there are a number of controlled motors running at low load the voltage waveform is reduced near its peak. If one of those motors is then subjected to a high load, the reduced portion of the voltage waveform is moved away from the peak and the apparent RMS voltage increases. The position of the period of reduced applied voltage moves such that at the relative phase angle of the low load motors the power supplied to them is- still being reduced. Relative to the phase of the high load motor however the power supplied is increased.The high load motor is still supplied with peak voltage at its relative zero phase. Of course in practice there are combinations of conditions where motors are being supplied with slightly more or less power than optimum, thus causing a small degradation in efficiency. However there is an additional effect within the motors which compensates for this to some extent. When a motor comes onto high load, its back EMF decreases so the effective impedance of that motor falls below that of a low load motor. Thus the high load motor can take more of the available power from the controller. The low load motors will have relatively high impedances in phase, and thus the increase in power supplied has relatively little effect on the power consumed by the low load motors.

In contrast with the present invention, if the period during which the supply waveform is reduced were to be fixed in phase but varied in duration, the resulting effect would be the same for each of the motors irrespective of their load condition.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which Fig. 1 is a schematic illustration of an arrangement known from the prior art; Fig. 2 is a schematic illustration of an embodiment of the present invention indicating the differences between the basic approach of the present invention and that of the prior art as shown in Fig.

1; Fig. 3 is a detailed circuit diagram of an embodiment of the invention; Fig. 4 is a schematic illustration of an arrangement known from the prior art for energising a series of four motors; Fig. 5 is a schematic illustration of an arrangement incoporating a single energy controller which could be used using conventional techniques but which would not provide acceptable results; and Fig. 6 is a schematic illustration of an arrangement in accordance with the present invention indicating the basic approach adopted when using a controller of the type illustrated in Fig. 3 to control a series of four motors.

Referring to Fig. 1, in a conventional controller of the type used for reducing the power supply to a single AC electric motor when that motor is operating with a reduced load, it is conventional practice for a controller 1 to be connected on one side to both the terminals of a motor 2 and on the other side to both the neutral conductor 3 and live conductor 4 of an AC mains power supply. The controller will incorporate a solid state switch and circuitry for monitoring the load on the motor and controlling that switch so as to reduce the power supplied as the load reduces. In addition however the equipment of which the motor forms part, (e.g. a refrigeration system), will incorporate auxilliary circuits the operation of which can be disrupted as a result of the control of the power supplied to the motor.

In accordance with the present invention as illustrated in Fig. 2 the controller is interposed in only the neutral conductor so that one side of the motor is permanently connected to the live conductor. The controller ncorporates a solid state switch and associated control circuitry and in addition auxilliary circuitry all of which is powered by the live conductor waveform which is received by the controller through the motor 2.

Referring now to Fig. 3, the illustrated circuitry comprises a first terminal 5 which is connected to the neutral conductor of the power supply and a second terminal 6 which is connected to one of the terminals of the motor. A triac 7 is connected between the motor and power supply.

The triac 7 is turned on and off by a first stage triac 8 which is in turn triggered by an avalanche signal produced across a potential divider formed by resistors 9 and 10. Diacs 11 and 12 produce an avalanche signal when the potential created between capacitor 13 and resistors 14 and 15 exceeds a reference signal by a predetermined value.

The reference signal is generated by resistors 16 and 17, capacitor 18, and diodes 19, 20, 21 and 22.

Capacitor 13 senses phase, and resistors 17 and 10 sense magnitudes. Resistor 14 is used to adjust the avalanche point. Resistor 23 and capacitor 24 provide standard snubbing for the triac 7 and resistor 25 provides a restricted supply to the load in-phase signal. Capacitor 26 and resistor 27 in combination with a series diode 28 provide a time delay before the triac 7 can be triggered off to ensure full power for start up. Effectively the time delay inhibits circuit operation for a short time.

An LED 29 is progressively activated depending on the effective root means square voltage difference between the input 5 and output 6. A zener diode 30 provides a low threshold with the resistor 31 and diode 32 providing a progressive increase in illumination up tc a maximum at 50 volts root mean square differential.

Thus the entire circuit is powered by the live waveform of the power supply which is always present at terminal 6. There is no direct connection to the live terminal of the motor, the live waveform being delivered through the motor itself. The signal feedback from the motor indicating the motor load is derived from the neutral terminal of the motor.

The described embodiment of the invention is applied to a single phase AC motor. Alternative configurations are possible providing the motor is supplied by a single phase supply (that is a live and neutral connection plus in most cases an earth. An example of such an alternative configuration is a so-called two phase motor which uses some means for providing a phase shift between separate windings for example by means of capacitors, inductors or a floating neutral.

Referring to Fig. 4, a conventional arrangement for controlling four motors 1 of the type used in refrigeration compressors is shown. Each motor 1 is an AC electric motor supplied from live (L) and neutral (N) terminals through demand control circuits 2 which may comprise for example thermostats, pressure switches and the like. It would be possible to connect a separate energy controller to each motor but this would be uneconomic in many applications. A single energy controller 3 could be connected to both the live and neutral supplies of each motor as shown in Fig. 5 but if this was done the control circuits 2 would not be able to control the respective motors.

Thus the arrangement of Fig. 5 is not an available option to reduce the costs of providing efficient energy supplies to the motors. In addition equipment incorporated in the motors will include auxilarV circuits the operation of which can be disrupted as a result of the control -of the power supplied to the motors.

In accordance with the present invention, as illustrated in Fig. 6, a single controller 3 is interposed in only the neutral conductors of the motors so that one side of each motor is connected to the live conductor by its associated control circuit 2. The controller 3 incorporates a solid state switch and associated control circuitry and in addition auxilliary circuitry all of which is powered by the live conductor waveform which is received by the controller through the motors 2.

The controller 3 of Fig. 6 is identical to the circuit described above with reference to Fig. 3.

The first terminal 5 is connected to the neutral terminals of the control circuits 2 and the second terminal 6 which is connected to one terminal of each of the motors 1. The triac 7 is connected between the terminals 5 and 6.

For any given setting of the resistor 14, the time which elapses between the waveform on terminal 6 passing through zero volts and the avalanche signal being produced is a function of the waveform on terminal 6 which in turn is a function of the load on the motors connected to terminal 6. Thus the timing of the turning on of triac 7 is a function of motor load. The triac 7 is turned off for a period the duration of which is substantially constant and is a function of the capacitance of capacitor 13.

Thus, as in the case of the single motor embodiment of the invention described with reference to Figs. 2 and 3, the entire circuit is powered by the live waveform of the power supply which is always present at terminal 6. There is no direct connection to the live terminal of the motors, the live waveform being delivered through the motors. The signal feedback from the motors indicating the motor loads is derived from the neutral terminals of the motors.

Claims (4)

CLAIMS:

1. A circuit for controlling the energisation of an AC electric motor one terminal of which is directly connected to a live conductor of a single phase power supply, the circuit comprising a first terminal for connection to a neutral conductor of the power supply, a second terminal for connection to another terminal of the motor, a solid state switch connected in serbs between the irt 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 te reduce the power supplied to the motor when the motor is operating at reduced load.

2. A circuit according to claim 1, wherein the second terminal is arranged for connection to a terminal of each of a plurality of motors, each motor having one terminal connected to the said live terminal and another terminal connected to the said second terminal, and wherein the monitoring and control circuit comprises means for sensing the combined load on the motors, the control means is responsive to the sensed combined load on the motors to control the solid state switch to reduce the power supplied to the motors when one or more of the motors is operating at reduced load, and the control means is operative to open the solid state switch for a predetermined substantially constant period during each cycle of the power supply, the timing of the opening of the solid state switch relative to the zero crossing point of the power supply waveform being dependent upon the.sensed load.

3. A circuit substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.

4. A circuit substantially as hereinbefore described with reference to Figs. 3 and 6 of the accompanying drawings.