Heating of electric motors
During the electric motor operating time the part of elektricheyosky energy will be transformed to thermal. It is connected with losses of energy on friction in bearings, on whirling currents and magnetic reversal in steel of the stator and the rotor, and the takyozha in pure resistances of windings of the stator and the rotor. Losses of energy in windings of the stator and the rotor of the proyoportsionalna to the square of size of their currents. Current of the stator and the rotor is proportional
to loading on the shaft. Other losses in the engine almost do not depend on loading.
At invariable loading on the shaft in the engine the certain quantity of heat in vreyomena unit vydeyolyatsya.
Temperature increase of the engine happens unevenly. In the beginning it increases quickly: almost all warmth goes for temperature increase, and only the small quantity it goes to the environment. Peyorepad of temperatures (the difference between temperature of the dviyogatel and ambient air temperature) is still small. However in process of increase in temperature of the dviyogatel the difference increases and heat emission to the environment increases. Growth of temperature of the engine zayomedlyatsya.
Temperature of the engine stops vozrasyotat when all again marked out warmth the polyonost dissipates to the environment. Such tempeyoratura of the engine is called established. The size of steady temperature of the engine zayovisit from loading on its shaft. At the heavy load the large amount of warmth in vreyomena unit, so above the steady temperature of the dvigayotel is distinguished.
After shutdown the engine is cooled. His Temperayotura goes down quickly as difference its big in the beginning, and then in process of difference reduction - slowly.
The size of admissible steady temperature of the engine is caused by properties of isolation the obmoyotok.
At the majority of engines of the general application for isolation of the winding enamels, synthetic films, the impregnated cardboards, the cotton pryayozha are used. Maximum permissible heating temperature of these materials is 105 °C. Engine winding temperature at the rated load has to be 20...25 °C lower than maximum permissible size.
Much lower temperature of the engine corresponds to its work with small loading on the shaft. At the same time the efficiency of the engine and coefficient of its power are small.
Operating modes of electric motors
Distinguish three main operating modes of engines: long, repeated and short-term and short-term.
Prodolyozhitelny is called power setting at poyostoyanny loading duration not less, than it is necessary for achievement of the established temperatuyora at the invariable ambient air temperature.
Such operating mode at which the short-term invariable nayogruzka alternates with shutdowns of the engine is called repeated and short-term, and during loading temperature of the engine does not reach the established value, and during the pause the engine does not manage to be cooled up to the ambient air temperature.
Such mode at which during engine loading temperature it does not reach the established value is called short-term, and during the pause it manages to be cooled up to the temperature of okruzhayuyoshchy air.
In fig. 1 curves of heating and the okhlazhdeyoniya of the engine and the brought capacities P for three reyozhim of work are represented. For the long operating mode are represented three curve heating and cooling 1, 2, 3 (fig. 1, a), corresponding to three different nagruzyoka on its shaft. The curve 3 corresponds to the greatest loading on the shaft; at the same time the brought P3 power> P2> Pi. At the repeated and short-term mode of the engine (fig. 1, b) temperature it during loading does not reach established. Temperature of the dviyogatel would increase on the dotted curve if time of loading was longer. The Prodolzhitelyyonost of turning on of the engine is limited to 15, 25, 40 and 60% of time of the cycle. Duration of one cycle tts is accepted equal 10 min. and is defined by the sum of time of loading of N and time of the pause of R, i.e.
tts = N + R
For the repeated and short-term operating mode engines with period of operation of PV 15, 25, 40 and 60 of % vyyopuskatsya: PV = N: (N + R) * 100%
In fig. 1 in curves of heating and the okhlazhyodeniye of the engine at the short-term operating mode are represented. For this mode engines with the dlitelyyonost of the period of the invariable rated load 15, 30, 60, become 90 min.
Heat capacity of the engine - size considerable therefore its heating to steady temperature can continue several hours. The engine of the kratkoyovremenny mode during loading does not manage to nayogretsya up to the steady temperature therefore it works with bigger loading on the shaft and bigger podyovodimy power, than the same engine of the prodolyozhitelny operating mode. The engine repeated кратковременного the operating mode also works with bigger loading on the shaft, than the same engine of the prodolzhiyotelny operating mode. The prodolzhitelyyonost of turning on of the engine, the more allowable load on its shaft is less.
Asinkhronyony engines of the general application of the long operating mode are used to the majority of machines (compressors, ventilyatoyora, potato peelers, etc.). Engines repeated кратковременного the operating mode are used to elevators, cranes, cash registers. Engines of the short-term operating mode are used for machines, primenyayomy during repair work, for example electric hoists and cranes.Top