Methods of drying of windings of electric motors


What methods of drying of windings of electric motors exist?

In use or, on the contrary, in the course of inaction of electric motors insulation resistance of windings can fall to the critical mark. There is it because of saturation of windings moisture.

Схема обдува изоляции обмоток

Figure 1. Scheme of blowing of isolation of windings.

The lower limit of insulation resistance is set on the mark of 0,5 megohms. At values below of it operation of the electric motor can lead to undesirable effects - breakdown of the electric motor. In such situation drying of the winding is made.

There are several ways, will stop on the most popular. In case of considerable decrease in insulation resistance of the winding of the engine it needs to be dried external heating, by method of losses in steel or the short-circuit current. External heating is applied in that

case if the machine strongly dampened. For this purpose blow in isolation of windings the hot air (fig. 1), using blowers with heaters, glow lamps and heating resistance. Power of heating elements - 3-10 kW. It is at the same time possible to pass current through windings. Current size is supported at the same time within 0, 4-0, 7 rated currents of the electric motor. For high-speed engines (it is higher than 1000 RPM) take lower limits of current, and for low-speed (it is lower than 1000 RPM) – higher values of current.

Схема сушки изоляции обмоток током короткого замыкания

Figure 2. Scheme of drying of isolation of windings short-circuit current.

The necessary amount of air in the minute has to be to equally one-and-a-half volume of the camera in which dry the electric motor. Power of the heating element in kilowatts has to be equal to camera volume in cubic meters. If camera volume for drying of the engine is equal to 8 m ³, then the volume of the hot air which should be passed one minute via this camera has to be 12 m ³, and electric heating element power – 8 kW.

For drying of isolation of windings the short-circuit current (fig. 2) of the winding of separate phases close short and give to them the lowered tension. At the same time usually welding transformers are the voltage source.

From above the electric motor is covered with heat-insulating material. Current in windings of the stator is brought to 50% of rated and support him at this level of 2-3 h. During the subsequent 3 h (bucketed 20-30 min.) current is brought to 90% of rated. In the first 3-5 h temperature of windings should not exceed 40-50 °C, after 8-10 h drying – 60-70 °C. At the same time temperature of the coming-out air should not be above 50 °C, and temperature of isolation of the winding should not be above 70 °C. In each 2 h check the thermometer temperature of windings and measure by the megohm meter resistance of their isolation.

Process of drying of the electric motor can be considered finished if at the temperature of hot air of 50-60 °C insulation resistance remains invariable during 3-5 h.

Схема сушки изоляции при помощи потери мощности на вихревые токи

Figure 3. The scheme of drying of isolation by means of the power loss on whirling currents.

For drying of isolation of windings of the stator of the electric motor of any power it is possible to use power losses on whirling currents in active steel. These currents are formed as a result of creation in steel of the stator of the alternating magnetic field by means of the special winding (fig. 3). The magnetizing current is chosen within 60-200 A, and by number of rounds of the winding from 6 to 28. Tension on one round of the winding - 3-4, 5 Century. Welding transformers are the power source. At the beginning of drying it is necessary to accelerate rise in temperature, and then to lower it to such level which is necessary only in order that losses in steel covered heat waste. For this purpose usually reduce the brought tension or increase number of rounds of the magnetizing coil.

It is possible to apply lamps of infrared radiation with mirror reflectors to drying of isolation of windings of the electric motor or normal electric lamps. Lamps mount in the drying cabinet. Air temperature in it is maintained in limits 100-110 °C.

It is possible to apply alternating current of the lowered tension to drying of windings (3-5 times less than rated). Current in the winding of the stator is regulated so that temperature did not exceed it 60-75 °C. Duration of drying of small electric motors - 8-12 h.

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