Hot start — the answer to the question
A question
According to the documentation, the electric motors we have installed in the enterprise can start 2 times in a row from a cold state and 1 time from a hot state. Suppose the electric motor is started from a hot condition and after 5 minutes it is stopped due to malfunctions of the process equipment. After what is the minimum time, in this case the electric motor can be switched on again? What is the hot condition of the engine? In fact, after disconnecting it from the network, the temperature of the motor gradually decreases to the ambient temperature.
Answer
Electric motors are calculated based on the possibility of two starts from a cold state or one from a hot state. This means that after long-term operation of the motor at rated load, when the temperature of its winding has already reached the maximum permissible, it is allowed to restart once after shutting down such a hot motor.
This hot start can be seen as overloading the motor, resulting in a short-term increase in the coil temperature above the maximum continuous temperature. Such an excess of winding temperatures is allowed, since such regimes are adopted rarely enough, in exceptional cases.
The degree of temperature rise of the motor windings during a hot start depends on the current density in the motor winding and the duration of the start.
If, after starting from a hot condition, for some reason it was necessary to stop the motor again, then it can be started a second time when the temperature of its winding drops to the value of the corresponding long-term permissible temperature at rated load, that is, to the temperature when again short-term overloading of the motor windings is allowed.
However, such starts cannot be abused, as this leads to accelerated aging of the insulation.
The minimum allowable interval between hot starts depends on the constant heating of the motor (depending on the current density in the windings), which is different for different types of motors, and on the magnitude and duration of the motor load before the hot starts.
If the engine is operating at rated load, then the allowable hot start interval may be in the range of 80 - 60 minutes, and the engine load in the range of 0.75 - 0.80 of the rated start interval may be reduced to 15-30 minutes…
See also on this topic:
Heating and cooling of electric motors
Thermal conditions and rated power of engines
Current overloads and their effect on the operation and service life of electric motors