The speed of an asynchronous motor rotating field and thus its rotor speed depends on the number of pairs of poles in the stator field .the rotor speeds n are lower than the table values by the amount of the relevant slip speed ns. Motors are designed with up to three stators windings, which have different number of pairs of poles. They can therefore run at up to pairs of poles. They can be in any desired ratio to one another. A pole-changing switch is used to changing poles for the desires speed, hence the term pole-changing.
The Dahlander pole-changing circuit is a particularly important type of two "speed pole "changing control device. The stator winding of the motor consists of six coils. By combing these in different ways, the winding can be given in different numbers of pairs of poles. The motor has two speeds, the only available ratio being 1:2.the stator winding is delta-connected for the lower speed and higher number or pairs of poles, two coils being connected in series to make up each leg of the winding. At the higher speed, two coils are connected in parallel each leg and the winding is double-star connection. The starting characteristics of the two types of circuit differ, as do their run-up curves. Manufacturers design the windings of the Dahlander pole-changing motors to produce a power ratio: Pn ,n max Pn, n min Pole-changing motors with two separate winding may have one or both connected as Dahlander circuits. Asynchronous motor of this type have up to four different speeds. The disadvantage of the pole-changing method is that only fixed speeds are obtained and that these are restricted to range. Since the speed of a three-speed asynchronous motor depends on the frequency of the voltage supplied to it, we can make use of frequency to control speed. The motor is fed from the supply line via a frequency converter; Machine converters or electronic frequency converters are used.
Asynchronous motor are constructed for fixed frequencies, her...
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