An elementary book on electricity and magnetism and their applications . We havealready seen that thestationary polyphasecurrents generate arotating magneticfield. This rotatingmagnetic field pro-duces eddy currentsin the pole faces anddamping grids whichare set in the polefaces (Fig. 279).These grids form low-resistance paths for the eddy currents,and so, just as in the squirrel-cage rotor of an induction motor,help to produce a good starting torque. They also tend todamp out oscillations, called hunting, and from this actionget their name. In starting a motor, however, by this methodit is no
An elementary book on electricity and magnetism and their applications . We havealready seen that thestationary polyphasecurrents generate arotating magneticfield. This rotatingmagnetic field pro-duces eddy currentsin the pole faces anddamping grids whichare set in the polefaces (Fig. 279).These grids form low-resistance paths for the eddy currents,and so, just as in the squirrel-cage rotor of an induction motor,help to produce a good starting torque. They also tend todamp out oscillations, called hunting, and from this actionget their name. In starting a motor, however, by this methodit is not practicable to connect it at rest directly to the line,because the starting current would be excessive, perhaps 5 to10 times the normal current. So the motor is supplied withcurrent from transformers fitted with half-voltage taps. 275. Uses of synchronous motor. Small motors of thistype are not used because of the difficulty of starting themand the need of always starting without load and the necessityof supplying the motor with direct current for exciting its field. Fig. 279. Field structure of synchronous motorshowing damping grids. 404 ELECTRICITY AND MAGNETISM coils. But large synchronous motors are used on long-distancepower-transmission lines, because they have an absolutelyconstant speed. Then, too, they can be operated on a linewith a voltage of 6600 volts. Sometimes this type of motor is used to improve the powerfactor of a transmission line. The power-carrying capacityof any electrical circuit depends upon the power factor of thereceiving circuit. For example, suppose a line is rated to carry200 amperes at 60,000 volts and therefore has a power-carryingcapacity of 12,000 kw., provided the power factor is one; thatis, provided the current and voltage are in phase with eachother. But if the load consists of induction motors and lightlyloaded transformers, the current may lag behind the voltageas much as 45°, which means a power factor of , andtherefore the power
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmagnetism, bookyear19
