Physical Process Modeling

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ELECTROMAGNETS


part II

   Contactors are mechanical switching devices, having only one position of rest and operated otherwise than by hand. They are capable of making, carrying and breaking currents under normal circuit conditions, including operating overload conditions.
   The driving force that actuates the contact system(s) of a contactor to switch ON may have different origin. The electromagnetic (E.M.) contactors are members of a very large and extremely widely used group, where the force is developed by means of a magnetic circuit, excited either by AC or DC power source.


mechanical switching - theory


   When energized (connected to a source voltage U), the contactor coil w produces magnetic field. It develops a force Fd which pulls down the armature to the core. In the case presented, the contact systems are in the form of a contact-bridge which comes into contact with the stationary (fixed) contact body-pairs (A1 ,a1 and B,b). Such contact systems, designated as CS1, and CS2 are known as contactors poles. By definition pole is: a portion of a switching device associated exclusively with one electrically separated conducting part of its main circuit and excluding those portions which provide a means for mounting and operating all poles together. A switching device is called single-pole if it has only one pole. It is called two-pole, three-pole device, etc. provided poles are coupled in such a manner as to operate together. When the coil w is de-energised, a spring provided will push the armature back to its initial position. Thus the coil is considered part of the control circuit of a contactor.
   The contact system CS2 has dual action - in one operating cycle it opens A2, a2 and closes A1, a1. The first contact pair (A2, a2) is called normally closed, while the second one - normally opened. Besides the main contact systems, contactors may have auxiliary contact systems which are mechanically operated simultaneously with the main ones.
   It is a great advantage of a contactor that by means of a small amount of power Pc =Uc.Ic (control circuit power) a very large power PM = UM . IM in the main circuit can be controlled. The ratio PM/PC may reach 103 - 104 or even more. A second important advantage is that a remote control becomes available - the auxiliary switch (button, contact system) may be far away from the place where the contactor is fixed. What is more, contactors may be considered elements of large systems of automatic control and follow a complex algorithm of operation, based on the auxiliary switch opening and closing.
   E.M. contactors are specially designed to operate frequently. Their frequency of operation is practically limited by the transient process of the armature motion, which usually takes some time (15-30 ms one direction). An additional consideration is that electric arcs, initiated on the main contacts, also need some time to be cleared.


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