Patent Publication Number: US-5838077-A

Title: Control system for switching loads on zero crossing

Description:
FIELD OF INVENTION 
     The invention pertains to the field of AC load control. More particularly, the invention pertains to switching systems for controlling sources of illumination, such as florescent fixtures. 
     BACKGROUND OF THE INVENTION 
     Building management systems and lighting control systems are designed to switch many types of high voltage AC loads from 120 VAC to 480 VAC in building applications. Such applications include HVAC and lighting controls. 
     These high voltage AC loads can be either capacitive or inductive by nature and, as such, current spikes in the form of inrush currents occur at turn on or turn off times, respectively. These inrush currents can substantially reduce the life span of a mechanical switching element. In severe cases, the contacts of the switching element can be welded together. 
     In modern building control systems, large numbers of florescent tubes need to be switched on and off in accordance with normal work day schedules. 
     The use of electronic ballasts in connection with florescent lights results in lower overall operating costs due to the fact that such ballasts can function properly at lower power levels than conventional ballasts. Electronic ballasts however, generally have a capacitive input impedance. 
     One of the characteristics of a capacitive input impedance is that voltage across the input terminals of the device cannot change instantaneously but current can. As a result, when an AC voltage is switched across a capacitive input impedance, high inrush currents often result as the capacitive input impedance instantaneously behaves like a short circuit. As the voltage builds up across the capacitive input impedance, the current returns to normal operating levels. 
     The desire to use electronic ballasts, as opposed to older conventional ballasts to achieve lower operating costs, has resulted in a need for switching systems which can be used with large numbers of electronic ballasts and which can cost effectively deal with the inrush currents associated with large numbers of electronic ballasts which may be switched on or off at the same time. 
     Testing has shown that switching these loads at the voltage zero cross of the phase being switched, reduces or eliminates the inrush current. What further complicates the implementation of voltage wave zero cross switching in a 277 VAC lighting system is that all 3 phases of the 277 VAC power system are controlled from the same panel. Each phase would have to be monitored and synchronized to provide the necessary time reference for the voltage wave zero cross. 
     Prior attempts to solve the above-identified problems have met with only limited success. One known prior solution is to use solid state switching devices. However, in view of the high currents and voltages involved, along with the inrush currents, appropriate semi-conductor switches tend to be too expensive to be cost-effective in this application. Another attempted solution has been to combine a solid state switch, such as a triac in parallel with an electromechanical relay for the purpose of absorbing the inrush currents. 
     Yet another solution which has only been partially successful is to use heavy-duty relays which are rated and intended for use with lighting control systems where high inrush currents are present due to the use of electronic ballasts. 
     Thus, it would be desirable to be able to provide for switching of all types of loads whether resistive, capacitive or inductive in response to a zero crossing of the associated voltage. Preferably, such systems could be used with all forms of electromechanical switching elements including mechanical latching relays, normally open relays, electronic relays or solenoid actuated breakers. Further, it would desirable if such switching systems address the inrush current associated with highly capacitive electronic ballasts so as to minimize contact welding in electromechanical switching elements. 
     SUMMARY OF THE INVENTION 
     A system for switching a varying voltage to a load wherein the switching element is an electromechanical device, such as a relay, includes a control unit coupled to the switching element. The control unit can include a programmable processor. 
     A circuit is coupled to the control unit wherein a first parameter of the switching element can be stored. For example, one parameter of particular interest is the time delay between when an electrical signal is applied to the switching element and when the contacts first close. 
     A second parameter of interest is the bounce time interval. 
     During the bounce time interval the load switching contacts of the switching element may open and close for short periods of time. In one aspect of the invention, the second parameter is also stored. 
     A circuit is coupled to the control unit for detecting when the varying voltage exhibits a zero crossing. When the control unit detects the zero crossing event, assuming that a load circuit associated with that varying voltage is to be switched on, the control unit determines when the next zero crossing is to be expected and subtracts from the intervening time interval the device delay time and one-half of the device contact bounce time interval. 
     The switching element is then energized by the control unit prior to the next zero crossing such that the switching element is half-way through its bounce time interval when the next zero crossing takes place. This will then minimize the inrush current to which the current carrying contacts of the switching element are exposed. 
     In the event that a plurality of loads are energized from three phase alternating voltage and current, where a multiple phase load is to be switched, assuming all three switching elements are identical, the second and the third phases can be switched similarly by simply adding the appropriate phase delay. This delay is on the order of 5.5 milliseconds for a three phase, 60 hertz system. 
     In yet another aspect, a method of switching a load, being energized by an AC-type signal, includes the steps of: 
     determining the characteristics of the load switching element including a time delay between when a signal is applied to the switching element to cause it to close and when the element initially closes as well as a second parameter which defines a bounce time interval; 
     detecting a zero crossing of the applied AC-type signal; 
     energizing the switching element a sufficient time before the next zero crossing occurs such that the switching element will begin to close at or about the time of the next zero crossing; and 
     switching any other phases in accordance with the offset between phases. 
     In yet another aspect, the programmable processor can be implemented as a commercially available microcomputer. The switching element parameters can be stored in a read-only memory or a read-write memory and accessed as required. 
     The programmable processor can be energized off of a reference phase of a multiple phase system. Each of the other phases can be switched, based on detecting zero crossings of the reference phase, by adding to the switching time of the reference phase, a phase delay corresponding to the phase difference between each of the subsequent phases and the reference phase. Hence, in accordance with this embodiment of the invention, the programmable control unit need only be coupled to the reference phase and detect zero crossings thereof in order to be able to provide controllable zero crossing switching for all phases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an overall block diagram of a system usable to control a three-phase load; 
     FIG. 2 is a plurality of graphs illustrative of operation of the system of FIG. 1; and 
     FIG. 3 is a flow diagram of a method implementable by the system of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     While this invention is susceptible of embodiment in many different forms, there are shown in the drawing, and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
     FIG. 1 illustrates, in block diagram form, a system 10 which is in accordance with the present invention. The system 10 includes a control unit 12. The control unit 12 can be implementable as any one of a plurality of commercially available programmable microprocessors. The control unit 12 could also be implemented as a hard-wired, programmed logic array without departing from the spirit and scope of the present invention. 
     The control unit 12, when implemented as a microprocessor, has associated therewith read-only memory 14a wherein control programs can be permanently stored and read-write memory 14b wherein parameters, current data and intermediate results can be stored. If desired, those parameters which are needed on a long-term basis can be stored in programmable read-only memory 14c which can be implemented as electrically erasable programmable read-only memory. For purposes of operator control, a terminal including key board and display unit 18 can be provided, coupled through an appropriate interface, to the control unit 12. 
     The system 10 can be energized off of a single, reference phase, P A  of a three-phase system which could be for example 60 hertz, 227 volts AC or the like. Three-phase loads can be switched using the system 10 and energized off of the three available phases P A  P B  P C . The system 10 however need only be coupled directly to one of the phases, such as P A , notwithstanding it may be controlling switching for the other two phases as well. 
     The system 10 can be used to switch single phase loads or three-phase loads, depending on the requirements. Irrespective of the type of load being switched, the system 10 is always energized off of a single, reference, phase. Coupled to the control unit 12 is a zero crossing detector circuit 22. The detector circuit 22 generates an interrupt at the control unit 12 each time the reference phase, P A , crosses zero. Since each of phases P B  and P C  are 120 degrees apart from each other, zero crossings for those phases occur, in a 60 hertz system, on the order of 5.5 and 11.11 milliseconds respectively after a zero crossing has been detected on the reference phase, P A . 
     Coupled to the control unit 12 is a switching interface 24 which could be implemented as a one of 64 decoder. The switching interface 24 converts a multiple bit, such as an 8 bit, binary code to one of 64 output lines indicated generally at 26. 
     Each of the decoded output lines can be coupled to a control signal input for a switching element indicated in each of pluralities 30, 32 and 34. 
     The members of the plurality 30 could for example, be latching relays or solenoid actuated breakers. In either event, the respective switching element, such as the element 30a has a control input 30a-1, a high power AC input, such as 30a-2 and a switched output 30a-3. 
     The control input 30a-1 could be connected, via appropriate interface, as would be known to those of skill in the art to a selected output line of the decoder 24. When the element 30a has been selected, an appropriate pulse of electrical energy is applied at the control input 30a-1, so as to cause the element 30a to change state and electromechanically correct the input AC power, illustrate connected to the reference phase P A , to the switched output 30a-3. The switched output 30a-3 is in turn connected to a respective load L-30a. 
     Other members of the plurality 30, such as 30b, 30c out to 30n can be connected to respective outputs from the interface element 24 and to respective loads, such as the load L-30n. Members of the plurality 32 can be respectively connected to output lines from the plurality 26b as well as to respective loads such as the L-32a . . . L-32n. Similar comments apply to members of the plurality 34 which in turn can be coupled to members of the lines of the output plurality 26c as well as respective load members L-34a . . . L-34n. 
     The switching elements, members of the pluralities 30, 32, and 34 could be for example implemented as Touch-Plate relay model No. 3000PL or Aromat model No. JT1AG-DC24. Other electromechanical switching elements can be used without departing from the spirit and scope of the present invention. 
     FIG. 2 illustrates various switching wave forms associated with the system 10. FIG. 2(A) illustrates the reference phase, P A , which is in turn coupled to the system 10. For exemplary purposes only, and not by way of limitation, the wave form of FIG. 2(A) is illustrated as a single phase (out of a possible 3 phases) of a 60 hertz AC-type electrical wave form which could be 110 volts RMS or 220 volts RMS without departing from the spirit and scope of the present invention. 
     FIG. 2B illustrates a contact closing control signal of a type which might be applied to control input 30a-1 of switching element 30a for the purpose of switching the AC-type electrical energy to the load L-30a. As illustrated, and without limitation, the contact closure electrical signal of FIG. 2B is applied for on the order 10 milliseconds for purposes of causing the switching element 30a, which could be a relay, to go from an open circuit state between lines 30a-2 and 30a-3 to a closed circuit state there between. In the closed circuit state, electrical energy is to be provided to the load L-30a. 
     The members of the plurality 30-34, being electromechanical devices, do not change state instantaneously. Rather, there is a delay interval, the device delay D.sub.Δ  associated with each of the switching elements, such as the element 30a, between when electrical energy causing that element to change state is applied to the control line 30a-1 and when contacts close between the lines 30a-2 and 30a-3. This delay D.sub.Δ  is illustrated on FIG. 2B. 
     There is a second parameter which is useful to know with respect to the switching elements 30-34. This parameter, B.sub.Δ, is the contact bounce time during which the respective contacts open circuit and close circuit intermittently before they settle down to a closed circuit condition. 
     In accordance with the graphs of FIG. 2 and the method of FIG. 3, control element 12 stores for subsequent uses, the values of the two parameters, the delay delta D.sub.Δ  and the bounce delta B.sub.Δ. It has been found that each type of switching element useable as a member of the pluralities 30-34 exhibits relatively constant values of the characterization parameters D.sub.Δ  and B.sub.Δ. Using only these two parameters, multi-load, multi-phase zero crossing switching can be carried out. Either a unique set of parameters is stored for each of the switching elements of the pluralities 30-32, or if the elements are all the same type, within normal variations they will, exhibit the same two parameter values for the parameter values D.sub.Δ  and B.sub.Δ. In this case only two parameter values need be stored for all the elements of the pluralities 30-34. 
     For example and without limitation, the characterization parameters for the two relay models noted above follow: 
     
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                D.sub.Δ                                             
                          B.sub.Δ                                   
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TOUCH PLATE 3000PL                                                        
                  6      Msec.    2    Msec.                              
AROMAT MODEL JT1AG-DC246                                                  
                  8.9    Msec.    .75  Msec.                              
______________________________________                                    
 
    
     Assuming for the moment that the system 10 incorporates one or the other of the above two noted switching elements, only those two parameter values need be stored, provided all the switching elements are identical. 
     As illustrated in FIG. 2B the control unit 12, upon sensing a zero crossing on the reference phase P A  and determining a delay time X 1  will then energize the switching element 30a such that the element 30a will close or short circuit lines 30a-2 and 30a-3 when the next zero crossing occurs which will be centered at the middle of the bounce delay B.sub.Δ. Without further connection to the other phases, namely P B  and P C  corresponding switching elements 32a and 34a can be switched at respective zero crossings of the respective voltage phase at subsequent times, namely: X 1  +5.55 MSEC for phase P B  and X 1  +11.11 MSEC for phase C, P C . 
     As a result, a system and a method in accordance with the present invention use characterization parameters associated with each of the switching elements, to repeatedly and reliably carry out zero voltage cross-over switching for single phase or three-phase loads as desired. Switching at the zero voltage cross-over point as illustrated in the graphs of FIG. 2 minimizes inrush current making it possible to extend the life of the switching elements and also to use less expensive switching element, which in turn are more cost effective. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitations with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.