This invention relates to the control of electrical power such as the automatic control of in-rush currents that may occur upon the application of power to a load and/or the elimination of transient interferences caused by the switching off of power to a load.

For the sake of convenience, the invention will be described, in the main, in relation to the control of in- rush currents but it is to be understood that the invention is not limited thereto. BACKGROUND ART

Most electrical equipment is subjected to in-rush current transients when power is connected to the equipment. These in-rush currents are a cause of reduced life and premature failure of the equipment and can creat temporary overloading transients in the electrical system For example incandescent lamps usually rely upon the increase in resistance that occurs due to heating of the filament to stabilise the current through the lamp. When cold, the lamp has a reduced resistance and thus the in¬ rush current is higher than the normal running current. The higher starting in-rush current is a cause of reduced life of incandescent lamps. Similar considerations apply in respect of other electrical heating elements.

Electrical motors usually rely upon the back e.m.f. stabilise the current through the motor. At start up, th back e.m.f. is zero and therefore the motor in-rush current is the stalled motor current. When the motor run

up to speed, the motor current reduces from the stalled motor current to the running load current. The higher starting in-rush current can cause overheating, higher than normal magnetic forces as well as higher than normal torque which can cause motor failure as well as failure of equipment driven by the motor due to the application of the higher than normal torque. Electrical transformers also suffer from in-rush currents.

A further example of the problems associated with in- rush currents occurs with capacitive loads such as power factor corrective capacitors where the in-rush current is dependent upon the instantaneous value of the voltage applied to the capacitors.

In general terms, most electrical systems are a combination of incandescent lamps, heating elements, motors, transformers, inductive and capacitive loads and thus difficulties arise in controlling the starting in¬ rush currents. DISCLOSURE OF INVENTION It is the object of this invention to provide an automatic power controller which may be inserted between the source of electrical power and the load to minimise the starting in-rush currents.

According to one aspect of the invention there is provided a power controller adapted to be inserted between a source of electrical power and an electrical load, said power controller comprising means for providing a reduced level of the power to the load which is below the normal level of the power source whereby the transients or

starting currents imposed upon the load are reduced and means for increasing the level of power to the load to a predetermined level of power.

Preferably, the power controller includes circuit means for setting the level of the initial power to be supplied to the load, the final level of power to be supplied and the time to elapse between the initial and final power levels. As indicated above, the controller may be operated in reverse to eliminate the transient interferences caused by the switching off of power to a load. BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings in which:

Fig. 1 is a block diagram of a power controller according to one embodiment of the invention which utilises amplitude control. Fig. 2 is a schematic circuit diagram of one form of the power controller shown in Fig. 1,

Fig. 3 is a block diagram of a power controller according to another embodiment of the invention which utilises phase angle control. Fig. 4 is a schematic circuit diagram of one form of the power controller shown in Fig. 3, and.

Fig. 5 is a schematic circuit diagram of a modification of the power controller shown in Fig. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The power controller shown in block form in Fig. 1 utilises amplitude control to effect control o.f the level of power applied to the load. The load 10 is connected between terminal AL and terminal NL. Power is supplied to the terminals A and N by an on/off switch (not shown).

Power to the load 10 is supplied through load transistor networks 11 and 12 which are driven alternatively by transistor drive network 22. Two separate signals are used to activate and to control the operation of the transistor networks 11 and 12. A square wave is used to overcome the non-linear regions of the various.devices in the networks and an alternating voltage of increasing amplitude is used to regulate the conduction of the transistor networks 11 and 12.

The square wave signal is generated by the transistor conduction hysteresis network 13 and applied to the combining network 14. The variable amplitude alternating voltage signal is generated by the network 15 under the influences of parameter setting adjusters 16, 17 and 18 which respectively set the initial level, the final level and the rate of increase of amplitude of the alternating voltage signal. The rate of decrease of the alternating voltage signal (when operating the controller in reverse) can be set by adjuster 19.

The alternating voltage signal from the network 15 is applied to a variable gain amplification network 21 which receives a synchronising and wave form signal down line 20

from the power source. The alternating voltage output from the amplification network 21 is applied to the combining network 14 along with the square wave signal generated by network 13 and the output of the combining network 14 is applied to the transistor driver network 22.

In the specific embodiment of the amplitude controlled, power controller shown in Fig. 2, the power is connected to terminals A and N and the load 10 is connected to terminals LA and LN. The load transistor networks 11 and 12 comprising transistors 33 and 34 and diodes 31 and 32 are connected in series with the load 10.

The amplitude of the load current is controlled by transistors 33 and 34. When line A is at a positive voltage relative to line N, the flow of current is from line A to the load 10 through diode 31 and then to line N through transistor 34.

When line A is at a negative voltage relative to line N, the flow of current is from line N to the load 10 through diode 32 and then to line A through transistor 33. The conductor paths could be arranged in alternative ways to achieve a similar result.

When power is connected to the power controller through lines A and N, resistors 35 and 36 operate as a voltage dropping network which provides a low voltage signal to resistors 66 and 71. The transformer 37 provides a low voltage source to the control network. The network formed by rectifying diodes 38, 39, 40 and 41, capacitors 42 and 43 resistors 44 and 45 and zener diodes 46 and 47 provide a smooth direct current positive voltage

at line 48 and a smooth direct current negative voltage at line 49. Both these voltages are relative to the line 50 which runs from the junction of the centre tap of the secondary winding of transformer 37 which constitutes the ground of the power controller.

The positive and negative voltages at lines 48 and 49 supply power to the operational amplifiers 51, 52, 53, 54 and 55. For the sake of simplicity, the power circuit connectors are not shown. Upon application of power to the controller, the capacitor 57 is charged through variable resistor 56 and, as a consequence, the voltage to resistor 58 increases. The rate of increase of voltage to the resistor 58 can be adjusted by variable resistor 56. This "slow ramping" voltage could be generated in a variety of ways. The variable resistor 59 provides a set (but adjustable) negative voltage to the resistor 60.

The voltages applied to the resistors 58 and 60 are combined by the operational amplifier 51. Feedback to the operational amplifier 51 is provided through resistor 61. The output of the operational amplifier 51 is an initial fixed voltage that is set by variable resistor 59 and a slow ramping voltage the rate of which is set by variable resistor 56. The output voltage of operational amplifier is applied to the resistor 62 which is connected to the operational amplifier 52.

The network formed by the resistor 62, operational amplifier 52, diode 63 and transistor 64 provides a precision voltage to current converter which supplies

current to resistor 65 to control the gain of the operational amplifier 55 (which is a gain controlled amplifier) .

Variable resistor 66 and resistors 67 and 68 provide a synchronised alternating current voltage to the input of the operational amplifier 55. The output of the operational amplifier 55 is an alternating current voltage the amplitude of which increases after the application of power to the controller. The initial value of output is set by the variable resistor 59 and the rate of rise is set by the variable resistor 56. The output from operational amplifier 55 passes through resistor 69 to the operational amplifier 54. Resistor 70 is a load resistor for the output of operational amplifier 55. _^ The operational amplifier 53, resistors 71 and 72 and the variable resistor 73 provides a variable amplitude square wave to the resistor 74. The inputs to resistors 69 and 74 are combined by the operational amplifier 54. Feedback to the operational amplifier 54 is provided through resistor 77.

The output of operational amplifier 54 is the sum of a square wave and an alternative voltage. The initial value of the alternating voltage is set by variable resistor 59 and the rate of increase, after the power has been applied to the controller, by the variable resistor

56. The output of the operational amplifier 54 is applied through diodes 78 and 79 to the diodes 80 and 81 of the optical transistor couplers 82 and 83.

The diodes 80 and 81 of the optical transistor

couplers 82 and 83 control the transistors 33 and 34 respectively. The power to drive transistors 33 and 34* is derived from the networks formed by resistors 84, 85, 86 and 87 and zener diodes 88 and 89. In operation of the controller, the square wave component of the voltage output of operational amplifier 54 is used to overcome the diode voltages in the diodes 78 and 79, the diodes 80 and 81, the optical transistor couplers 82 and 83 and the transistors 33 and 34. As the square wave overcomes the diode voltage (or non linear regions) of the various devices, the alternating voltage component of the output of operational amplifiers 54 is used to regulate the conduction of the transistors 33 and 34. The conduction of the transistors 33 arid 34 is controlled by the increasing amplitude of the alternating voltage component of the operational amplifier 54 and the resultant power to the load is an alternating voltage and alternating current which increases in amplitude after the application of power to the controller.

The power controller shown in block form in Fig. 3 utilises phase angle control to effect control of the level of power applied to the load.

The load 100 and the load control 101 (which may contain a triac or silicon controlled rectifiers) are connected in series. The power source is connected to terminals A and N and the load is connected to terminals AL and NL.

Two separate signals are used to activate and control

the operation of the control unit 101. A trigger network 102 provides a trigger signal to combining network 103. A ramp-up network 104 provides a ramping signal to the combining network 103. The initial level of the trigger signal from the combining network 103 is set by the network 106 and the final level by network 107. The ramp down network 105 controls the ramp when the controller is operated in reverse. In this instance networks 104, 105, 106 and 107 are all adjustable.

In the specific embodiment of the phase angle controlled power controller shown in Fig. 4, the source of power is connected to the terminals A and N and the load 100 is connected to the terminals AL and NL. The load current is controlled by manipulation of the firing angle of a load triac 121 connected between terminals NL and N. When power is connected to the controller, resistor 135, 136 and 137 operate as a voltage dropping network. The network formed by rectifying diodes 138, 139, 140 and 141, capacitors 142 and 143, resistors 144 and 145 and zener diodes 146 and 147 provide a smooth direct current positive voltage at line 148 and a smooth direct current negative voltage at line 149. Both the voltages are relative to the line 150 which runs from the junction of resistors 136 and 137 (which constitutes the ground of the power controller). Lines 148 and 149 provide power to the operational amplifier 151.

The diode 152 blocks the direct current voltage of capacitor 142 so that a full wave rectified voltage is

supplied to resistor 153 which together with zener diode 154 provides a truncated full wave rectified voltage to resistors 155 and 156.

The network formed by resistors 155, 156 and 157, capacitor 158, and the unijunction transistor 159 generates a saw tooth voltage that is applied to resistor 160. The saw tooth voltage is shaped by the selection of the network components to have a maximum value occurring as late as possible in each half cycle. The positive and negative direct current voltages as well as the saw tooth voltages could be generated in other ways. These voltages are present wherever the supply voltage is applied to the controller.

Upon the initial application of power to the controller, the network formed by the variable resistor 161 and capacitor 162 starts to charge the capacitor 162 and as a result of this the voltage applied to resistor 163 increases with time. The rate of increase of the voltage applied to resistor 163 may be adjusted by the variable resistor 161. This slow ramping voltage could, of course, be generated in many other ways.

The saw tooth voltage to resistor 160 and the slow ramping voltage to resistor 163 are combined by the operational amplifier 151; they could, of course, be combined in other ways. Feedback to the operational amplifier 151 is provided through resistor 172.

The output of the operational amplifier 151 causes current to flow through the diode 164 of the optically coupled triac driver 165. The resistor 166 acts as a

current limiter for the diode 164. The triac 167 of optically coupled triac drive 165 is turned on at the instant the current through diode 164 reaches the firing threshold of the optically coupled triac driver 165. With the correct selection of component values, the current to the diode 164 can be arranged to be just under the firing threshold of the optically coupled triac drive 165 at the instant when power is connected to the controller, that is when the voltage to resistor 163 is zero.

As soon as the voltage to resistor 163 increases by a sufficient amount so that the output of the operational amplifier 151 provides a current through the diode 164 above the firing, threshold, the triac 167 is turned on. The initial turning on of the triac 167 occurs very late in each cycle and as time passes, the phase angle to turning on decreases progressively as the voltage applied to the resistor 163 is "ramped up" or increased. The ramping up of the voltage to the resistor 163 continues until the triac 167 is fully turned on each half cycle. The triac 167 of the optically coupled triac drive 165 turns on the load triac 121 through resistors 168, 169 and 170. The capacitor 171 acts as a snubber for the triacs. The current to the load 100 is thus phase angle controlled from "off" to the full "on" condition over the ramp up time which can be adjusted by the variable resistor 161. In the event that a variable ramp up time is not required the variable resistor 161 can be replaced

by a fixed resistor.

Other triggering devices could be used to turn on the load triac 121 and indeed the triac 121 could be replaced by silicon controlled rectifiers. Fig. 5 shows an enhancement of the phase angle controlled power controller of Fig. 4. Circuit components of Fig. 5 which are common to Fig. 4 carry the same reference numerals.

A variable resistor 173 supplies an adjustable voltage to resistor 174 so that the initial power level of the controller may be varied. The direct current voltage supplied by resistor 173 is applied to the operational amplifier 151 and is added to the saw tooth voltage supplied by resistor 160. This additional voltage causes the threshold trigger current of the optically co pled triac driver 165 to be exceeded upon the application of power to the controller. Thus the firing angle of the controller is variable from "off" to full "on" through any intermediate value. The switch 175 and associated circuitry enables the controller to be ramped down. The switch 175 is connected to both negative line 149 and variable resistor 176 which in turn is connected to resistor 177 and to capacitor 178 through diode 179. The other side of the capacitor 178 is connected to the ground line 150.

When the controller has reached its final level and when switch 175 is closed the variable resistor 176 charges the capacitor 178. The voltage developed across the capacitor 178 is supplied to the resistor 177 and

added to the other voltages being applied to the operational amplifier 151.

As the network formed by variable resistor 176, diode 179 and capacitor 178 is supplied from the negative voltage line 149, the voltage across capacitor 178 is negative relative to ground line 150. This negative voltage cancels the ramped up voltage supplied to resistor 161. The decrease rate may be adjusted by variable resistor 176. The final level of the ramp up voltage may be adjusted by the variable resistor 180. The ramp up voltage supplied to resistor 161 is decreased by the ratio of the voltage dividing effect of variable resistor 180. Thus the final level of the ramp up voltage is adjustable ^' from full on -to any value down to the initial value.

As will be apparent ^' from the preceding description, a wide variety of electrical devices may be employed in the power controller to achieve the desired effects.

Power controllers according to the invention may be applied to extra low voltage, low voltage, medium voltage and high voltage loads and may be used with alternating current, variable frequencies and direct current.

Direct current power controllers would ramp up the voltage after the application of power. Power controllers according to the invention may be located in electrical systems wherever switching type functions are performed.

Various modifications may be made in details of design and implementation of the power controller without departing from the scope and ambit of the invention.