Heating of electric motors on the isolation class

During the operating time of electric motors there is their heating. Heating temperature can be different, i.e. one engines heat up less, others - it is more. Admissible heating of electric motors depends on the class of isolation of windings.

Схема электродвигателя в разрезе

The scheme of the electric motor in the section.

On the plate of the electric motor with all data also the parameter, the called isolation class is specified.
It is necessary to know that with temperature increase of windings of electric motors over admissible values, isolation service life is sharply reduced.

 Ambient air temperature at which the electric motor can work with rated power is considered 40ºС. At temperature increase of the ambient air more 40ºС, load of the electric motor has to be lowered so that temperature of its separate parts did not exceed admissible values.

Limit admissible exceeding of temperature of active parts of electric motors (at ambient temperature 40ºС):

  1. Class Y: admissible heating temperature is up to 90 °C.
  2. Class A: admissible heating temperature is up to 105 °C.
  3. Class E: admissible heating temperature is up to 120 °C.
  4. Class B: admissible heating temperature is up to 130 °C.
  5. Class F: admissible heating temperature is up to 155 °C.
  6. Class H: admissible heating temperature is up to 180 °C.
  7. Class C: admissible heating temperature is over 180 °C

At asynchronous engines, together with reduction of tension of the power line, in
square power on the engine shaft decreases. Besides, reduction of tension gives 95% of rated to significant growth in current of the engine and heating of windings below. Growth of tension is higher than 110% from rated also leads to growth of current in engine windings, heating of the stator at the expense of whirling currents increases.

Irrespective of decrease in ambient air temperature to increase current capacities more than for 10% rated it is not allowed.

  1. N.I

    What harmful substances are emitted when overheating or burning windings of powerful electric motors of the direct current on sea vessels? (Excuse for unpleasant details — the close relative Poka Died under not clear circumstances. Worked starmekhy on the foreign vessel + got to resuscitation 2 more persons pulled out it from the room so far. Poisoning with carbon dioxide fire extinguishing is excluded.)

    To answer
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