Switching current

The following are common methods to start a squirrel cage motor, depending upon the limitation, if any, on the magnitude of switching current, / . 

When a capacitor circuit is compensated through a series reaetor. either to suppress the system harmonics or to limit the switching inrush currents (Section 23.11) or both, it will require suitable adjustment in its voltage and capacitive ratings, fhe series reactor will dampen the switching currents but consume an inductively reactive power and offset an equivalent amount of capacitive kVAr. and require compensation. The following example w ill elucidate this. 

This is a generalized formula to simplify the equation. In fact, the actual switching current will be much less than this, due to circuit impedance. For more accurate... 

Table 23.5 Contactor rating for capacitor switching at different transient (switching) currents...

However, in no case will the switching current exceed the short-circuit kA of the switching circuit. 

Consider an inductance L pH introduced into each capacitor circuit as shown in Figure 23.27(a) or (b) to limit the switching current within the making capacity of the switching device. Then... 

For limiting inrush currents Generally, it is not required for the LT systems. For larger banks, however, say, 500 k VAr and above, when they are installed close to the supply source, so that the line reactance up to the bank is too small to limit the switching currents, the use of reactor would be advisable. 

The switching currents must be limited to as low a value as possible. However, in general, capacitors are designed for switching cuirents up to lOO/f (r.rn.s.) for milliseconds. If necessary, a series resistance or indtictance can be introduced into Ihe switching circuit to limit them to a desired level (Seetion 23.1 I ). 

In this part the author provides all relevant aspects of a reactive control and carries out an exhaustive analysis of a system for the most appropriate control. Harmonic effects and inductive interferences as well as use of filter and blocking circuits are covered. Capacitor switching currents and surges and methods of dealing with these are also described. 

Fixed ramp added to switch current sense signal (crude but popular method of introducing slope compensation)... 

The average of the switch current waveform, evaluated over the full cycle, is /, x I) = 0.67A (Block 1). 

Note that in a Buck, the average input current is NOT the load current (as the engineer was rather intuiAvely visuaflzing), but the average switch current. The average switch current is of course related to the load current by 7SW = Io x D (for a Buck). 

Note that while prototyping, it is a bad idea to insert a current probe (through a loop of wire), anywhere in a critical trace section. The current loop becomes an additional inductance that can increase the amplitude of the noise spikes dramatically. Therefore practically speaking, it can often become virtually impossible to measure the switch current or the diode current individually (especially in the case of switcher ICs). In such cases, only the inductor current waveform can really be measured properly. Sometimes we can place a small sense resistor instead of a current loop, because a good resistor will not create inductive kicks at least. 

The voltage at the anode of diode D6 in Fig. 10.29 was compared to the gate voltage waveform, shown below in Fig. 10.30a. The voltage (a representation of the switch current) across the sense resistor R2 was... 

Figure 10.32 IsSpice results (switch current ISwitch and source current I In).

Figure 10.33b PSpice results (switch current) using an IRF150 model that avalanches.

Figure 10.34b Micro-Cap results (switch current). The Micro-Cap model does not avalanche.

A triode is made from two closely situated pn junctions. These devices are also known as transistors, which is a circuit element that is useful for amplifying or switching currents. Solid-state transistors have revolutionized the electronics industry. Today, very complicated integrated circuits can be made of thousands of semiconductor elements, all contained in a very small package. Such integrated circuitry is vital to the computer, as well as scientific, telecommunication, and many other businesses. Our age is truly the age of the semiconductor ... 

Figure 1.27 Polarization switching current of TGBF single crystal with decreasing electrical field.

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