Inrush current

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... 

To contain the start-up inrush current, as a result of low start-up impedance and to control the same as needed through external resistance in the rotor circuit. 

The starting of an induction motor does not relate to simple switching alone. It also involves its switchgears to control its starting inrush current, starting torque, or both, and its overload and short-circuit protection. 

Since the starting inrush current is kept moderate for all types of drives, it can economize not only on ratings of the switchgears and cables but also on the size of the generator when a captive power is required to feed the load. 

The coil ends must be rigidly supported and adequately braced with binding rings or tapes to prevent their movement and also absorb shocks and vibrations during excessive overloads, starting inrush currents (/, ), and voltage surges. 

Check selection of fuses for the type of load and switching. Perhaps they are under-rated for the starting inrush current and its duration. For selection of fuse ratings see Section 12.10.4. 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

The electromagnetic unit, however, as used in a residual VT (Section 15.4.3) or a capacitor VT (Section 15.4.4) should be suitable for carrying the heavy discharge or inrush currents during a capacitor discharge or switching respectively. 

The control transformer to be selected may have a rating nearest to the maximum hold-on VA burden so calculated and must be suitable to feed the required inrush current at the p.f. so calculated without affecting its regulation.. So long as these two points fall below the inrush curve of the control transformer, its regulation... 

The same theory will apply in capacitor switching except that the interrupting current will now be leading the voltage by almost 90° instead of lagging. See also Section 23.5 to determine the amount of surge voltages and inrush currents. 

In capacitor switching, introduction of an inductor coil (Section 23.I1) can contain not only the inrush current but also tame the current phasor to shift closer to the voltage and thus limit the TRV on an interruption. 

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. 

The c ipacitor inrush current is a function of steady state and the transient components of the current, i.e. 

For simplicity and to obtain a close approximation, the value of/ may be considered to be negligible and therefore ignored. It may give a higher value of inrush current, which is on the safe side to select the switching device. 

The figures of Li i tind may be substituted in equiitinn (22.13) for Lt and C respectively to derive a more accurate switching inrush current. 

At the instant of switching the surge impedance, Z, (equation (23.12)), and the natural frequency of the switched circuit, i.e. the transient frequency, /i, (equation (2.2.14)). determines the amount of inrush current. 

This extremely high inrush current at a frequency of almost 3-7 kHz will release an enormous amount of let-through energy during contact making,... 

Let us refer to equation (23.13) for the inrush current, / on parallel switching. If we can increase the impedance of the switching circuit by introducing a resistance R or inductance L, or both, we can easily control this current to a desired level. 

If switching is effected when the incoming capacitor is already charged, then the inrush current, will be nearly twice this, i.e. 

In HT systems, where a series reactor is already being used, to suppress the system harmonics this would also serve to limit the switching inrush currents and no separate reactor would be necessary,... 

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