Inductive current interruption

Interrupting when the motor is in a locked rotor condition (Section 1,2) or is still accelerating and carrying a highly inductive current of the order of six to seven times the rated current at a p.f. of about 0.2-0,3,... 

Slamecka, M.E.. Interruption of small inductive currents Working Group 13.02 of Study Commiliec 13 (Switching Equipment). Electro, No. 95 (1983). 

Considering the importance of this first technological application of electrochemical discharges, we will present here a simplified theory of Wehnelt s current-interrupter. A resistance R and inductance L is connected to the Wehnelt interrupter. Based on early mathematical models of the device [83,99], the Wehnelt interrupter is considered to be a resistance Rv in series with an electrical switch (Fig. 2.5). 

Inductive kick A voltage surge produced when a current flowing through an inductance is interrupted. Inductive load A load that possesses a net inductive reactance. 

Shunt reactors for other cables are often connected to buses, as the area compensation is applied at these voltage levels. When shunt reactors are connected to buses, the zero-missing phenomenon does not occur. In this case, however, the inductive VT connected to the cable needs to have enough discharge capability, and the line breaker needs to have sufficient leading current interruption capability. 

In all the above cases, whether the load is inductive or capacitive, the voltage and the current phasors are more than 70° apart. At a current zero, the system voltage will reappear almost in full (around 95% or so) across the pasting contacts of the interrupting device (Figure 17.11(c)). This voltage is detrimental to the successful interruption of the circuit at the current zero, unlike in the previous case. 

Interrupting small magnetizing currents, such as interrupting an induction motor or a transformer on no-load, a situation, when the current may lag the impressed voltage by nearly 90°. 

Interrupting an induction motor immediately after a switch on, when the current is large and highly inductive. 

Interrupting fault currents that are mostly inductive (Section 13.4.1) and occur at very low power factors. They are excessive in magnitude, and cause high thermal effects and electromagnetic forces on the arc chamber, the contacts and the contact mounting supports. 

The events are so fine-tuned and the size of chamber, pressure of gas, travel, distance of the moving contact and the size of blast nozzle so designed and adjusted that a near-slrike-free interruption can be achieved for low reactive currents (inductive or capacitive) as well as fullload and very heavy fault currents. The advance compression of gas through the movement of main contact plays an important role by storing a part of the gas even before opening of the arcing contacts. 

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