Inrush

Vacuum is applied to the chamber and vapor is removed through a large pipe which is connected to the chamber in a manner such that, if the vacuum is broken suddenly, the inrushing air will not greatly disturb the bed of material being dried. This line leads to a condenser where moisture or solvent that has been vaporized is condensed. The noncondensable exhaust gas goes to the vacuum source, which may be a wet or diy vacuum pump or a steam-jet ejector. 

Reduced-voltage starting. A reactor, resistor, or transformer is temporarily connected ahead of the motor during start to reduce the current inrush and limit voltage dip. This is accompanied by reduced starting torque. For reactor or resistor start, the torque decreases as the square of current for transformer start, the torque decreases directly with line current. The reactor, resistor, or transformer can be adjusted to give a proper balance between torque and 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. 

Because of heavy start-up inrush eurrents, the use of LT motors should be preferred up to a medium sized ratings, say, up to 160 kW, in squirrel eage motors and up to 750 kW in slip-ring motors. For still higher ratings, HT motors should be used. 

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. 

Starting current (/, ) Can be controlled to any desired level subject to the minimum required Cannot be controlled. Although duration of the starting inrush is very short because of no-load start Can be controlled to any desired level. Normally up to 1.5 times the rated current... 

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... 

Maximum inrush occurs when five steps of the PFCR are ON and the sixth is switched ON... 

Computing the maximum hold-on (steady state) and inrush burden values and their cos 0 ... 

Maximum hold-on values Maximum pick-itp (inrush) values ... 

Hold-on for five steps already ON Inrush for the sixth step ... 

The actual values us worked out above must fall below the inrush curve of the selected control transformer of 500 VA. as illustrated in Figure 15,12,... 

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. 

Excessive charging currents (switching inrush or making currents) 23/750... 

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. 

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