Electrical news and engineering . aSystem.—The large starting current taken by a squirrelcage induction motor at starting, limits the size of motorthat can be run on a given system. It can readily be seenthat it is impossible to start :i :.(){l h |). motor which has astarting current of .i^ , full loail currcnl. from a 200kilowatt alternator which can only give :i times full loadcurrent on short circuit. Consider the following i)r((in: I lie normal loadon the alternator is A amp. at 807o power factor, the largestsii full load, and 3J/2 B amp. at 40/(ipower factor on .starting. li
Electrical news and engineering . aSystem.—The large starting current taken by a squirrelcage induction motor at starting, limits the size of motorthat can be run on a given system. It can readily be seenthat it is impossible to start :i :.(){l h |). motor which has astarting current of .i^ , full loail currcnl. from a 200kilowatt alternator which can only give :i times full loadcurrent on short circuit. Consider the following i)r((in: I lie normal loadon the alternator is A amp. at 807o power factor, the largestsii full load, and 3J/2 B amp. at 40/(ipower factor on .starting. lig. 7 shows the relation between the ratio A/B and/r voltage drop on the alternator, the alternator rheostatbeing tixed. Under these conditions the normal currentof the motor should not exceed 59o of the normal currentrating of the alternator. Most alternators are now sup-plied with automatic regulators if operating on a large. 20 80 too £ 90 60 Fig. 7 motor load. The regulators vary the excitation as the loadvaries so as to keep the terminal voltage of the alternatorconstant, up to the point where there is full excitation onthe alternator. Curve 8 shows the maximum current atdifferent power factors which can be handled by such aregulator, without any drop in generator voltage. Withthe amount of field margin shown in Fig. 8. the ratio of B/A is 30%. Variable Speed Operation of Induction Motors.—Forvariable speed service there is no choice of type of motor,the wound rotor motor must be used. The squirrel cagemotor is essentially a constant speed motor. The speed ofa wound rotor motor can be reduced by increasing therotor resistance, because if the rotor resistance is increased,a greater voltage will be necessary to send a given currentthrough the rotor, and a greater slip will be necessary togive this voltage. One objection to this method of speed variation is thatthe speed varies grea
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Keywords: ., bo, bookcentury1800, bookdecade1890, bookidelectricalnewsen20donm
