Patent Publication Number: US-6671157-B1

Title: Method for effecting an electronic drive control

Description:
BACKGROUND 
     The present invention relates to a method for controlling a drive coil of a contactor. 
     An electronic drive control for a magnetic drive is known from European Patent Application EP 0 789 378 A1. 
     If the control voltage of a contactor is drawn from one of the three phases, synchronization effects can ensue between the closing or opening angle of the main contacts in the load circuit and the AC control voltage. 
     This is due to the fact that a certain voltage build-up of the supply voltage for the electronics has to take place. 
     If the control voltage is switched on at voltage zero, then the supply voltage does not build up immediately but only at a later point in time. Around this point in time, an unwanted synchronization takes place as a result of which one of the three switching contacts of the contactor is always subject to higher wear. This means, that one contact has a switch-on point at which the voltage level is approximately identical, independently of the instant of the control signal. Since the service life of the contactor depends on the service life of the most eroded contact, the synchronization results in shortened service life of the overall device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for electronic drive control of a drive coil of a contactor with which the service life of the contactor is increased. 
     The present invention provides a method for controlling a drive coil of a contactor having main contacts for a load current. The method includes providing an electronic drive control apparatus including a microcontroller. A randomly selected constantly varying time delay is applied in the microcontroller after a time at which a supply voltage has built up at the electronic drive control apparatus and before a time at which the supply voltage is measured. 
     Via the present invention, an irregular erosion of the switching contacts is prevented and, consequently, the service life of the contactor is increased. 
     With reference to the drawings, embodiments of the present invention will be described and illustrated in greater detail. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be elaborated upon below with reference to the drawings, in which: 
     FIG. 1 shows a timing diagram in accordance with the present invention; 
     FIG. 2 depicts a timing diagram without delay time in accordance with a prior art system; 
     FIG. 3 shows a schematic representation of an electronic drive control system; 
     FIG. 4 depicts a flow chart of a first embodiment of a method for controlling a drive coil of a contactor; and 
     FIG. 5 shows a flow chart of a second embodiment of a method for controlling a drive coil of a contactor. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a timing diagram in which the rectified supply voltage is shown in a lower graph, the supply voltage being an AC voltage. 
     Subsequent to switching on the supply voltage at instance t 0  which is lies at the voltage zero crossing, the voltage of the electronics builds up only after 3 to 5 ms. Subsequent to this voltage build-up, a reset signal occurs for a microcontroller. The program is started. After this instant t 1 , the state is detected in which the contactor is operated. It is checked which type of control is concerned. The detection refers to the conventional operation, programmable control operation or low-power operation. This input recognition takes approximately 5 ms, the end of recognition being denoted by t 2 . If a voltage level is present, then the conventional operation is detected. 
     After this point in time, to prevent synchronization effects, provision is made for a randomly determined delay time t x  ranging between zero and half the period duration of the control voltage. 
     The supply voltage is measured only after this delay time, the point in time being denoted by t 3 , it being possible for the supply voltage to be both a DC and an AC voltage. In this connection, it is checked which supply voltage is concerned (AC or DC). The level of the applied voltage is ascertained at the same time. 
     After t 3 , it is checked, moreover, whether the supply voltage lies within a predetermined voltage window Umin, Umax, i.e., whether the determined value lies in the permissible range. 
     These processes take approximately 12.5 ms, it being possible for instant t 4  at which the contactor drive is activated, that is, the pickup process is initiated, to lie at any point of the sine curve of the supply voltage due to delay time t x . The time of the pickup process is different, depending on the contactor type. Depending on the contactor type, the pickup takes place after 50 to 100 ms. 
     Switching point t 5  of the contacts is distributed at random for the other phases as well, resulting in a uniform loading of the switching contacts. 
     The unwanted synchronization effect will be explained in greater detail with reference to FIG.  2 . The broken lines show different switch-on points with which different voltage values can be associated. Since the voltage for starting the microcontroller must first be built up, the reset signal lies always at the same point of the curve nearly independently of the switch-on time so that the switch-on times are virtually synchronized to the same reset time. 
     If the control voltage is switched on, for example, at the voltage zero point, then the supply voltage for the electronics does not build up immediately. 
     Using the explained method, unwanted synchronization effects between the closing angle or opening angle of the main contacts in the load circuit are prevented. 
     To implement a timing element having a variable time which is set at random, a random-number generator is required. The value delivered by the random-number generator is subsequently processed in such a manner that, via the subsequent processing of the value, a time from zero to maximally half the period duration of the control voltage ensues. 
     A random value is obtainable in two ways. 
     FIG. 3 shows a schematic representation of an electronic drive control system. 
     In a first embodiment, a value in a RAM storage location  11  in the microcontroller is read out, the content of this cell subsequent to switching on the voltage being undefined. 
     In a second embodiment, a value is obtained form a storage cell, or location, 1 of an EEPROM, the value being manipulated appropriately. The old value is utilized for determining the new value. The old value is then replaced with the new value in the EEPROM. 
     FIG. 4 shows the appertaining flow chart for the implementation with a RAM. 
     In a second embodiment, a value is obtained from a storage cell, or location, 1 of an EEPROM, the value being manipulated appropriately. The old value is utilized for determining the new value. The old value is then replaced with the new value in the EEPROM. 
     Subsequently, the manipulated value is written into a register  2 . Then, the timer is started. When the timer reaches the value zero, then a timer underflow ensues which is forwarded to the CPU, and the program is continued. 
     FIG. 5 shows the corresponding flow chart. 
     Due to this additional time loop, the closing and opening angles of the main contacts with respect to the control voltage are not synchronized.