The estimation of fire danger of the asynchronous electric motors exploited
at the industrial enterprises
The analysis of the fire-prevention condition of
various purpose assignment objects; destination shows, that their fire safety in many respects depends on of an
electric equipment technical condition on the whole. This fact underestimation quite often leads to occurrence of
fires (including large ones) with a significant material damage.
On statistical data about failure of asynchronous electric
motors by voltage up to 1000Â, collected for the period 2000 - 2004 at some industrial enterprises in Donetsk and
Donetsk region, approximately 62 - 71 % of damages of asynchronous engines are caused by their work on two phases.
This abnormal condition is the most frequent reason of three-phase asynchronous engines failure.
The overheat of asynchronous engines windings can cause
ignition of their isolation, that quite often leads to a fire when on cases of these machines cars there are
adjournment of fire-dangerous dust, and means protection thus are in a non-working condition, or don’t react
to the given operating mode.
Most often having overheated windings the asynchronous
electric motors arise at the braked rotor (jamming), breakage of a stator phase, a deviation rejection of voltage of
the network from normalized values, asymmetry of the feeding voltage.
One of the widespread modes, causing overloads the
asynchronous electric motors, is asymmetry of the primary voltage. Value of asymmetry factor at breakage of a
linear wire on the party side of the feeding voltage as in the place of breakage - 28,7 %,
and in nearby units - 15 - 28,3 %, considerably exceed the values established installed by GOST 13109 - 97.
Asymmetry of the feeding voltage leads to occurrence of currents of return sequence which is imposed on currents of
direct sequence and cause additional heating a rotor and stator that leads to fast isolation ageing.
The fire at operation of the asynchronous engine can occur
at concurrence in space and time of following casual events: an inadmissible overheat of the case of the engine
(for example, at breakage of one of the phases feeding the asynchronous engine); refusal in operation of
corresponding meeting means of protection; presence of a combustible material on cases of considered examined
asynchronous engines.
The purpose of work is the definition of probability of
occurrence of fires during time t at operation the asynchronous electric motors at the industrial enterprises;
through what time interval it is necessary to check presence of combustible material (dust) on cases the asynchronous
electric motors that probability of fires from fire-dangerous unit «protection - the asynchronous electric motors -
a combustible material» was up to standard, recommended GOST 12.1.004-91, i.e. Q0(8760) = 1*10-6.
For achievement of the object in view in work analytical methods of researches which are based on the basic concepts
of the theory of probability and reliability were used.
In the event that intervals of time between: breakages of
one of phases of the lines, submitting the electric power to the asynchronous engine; a casual operating time of the
asynchronous engine on two phases; intervals of time between refusals of means of protection; duration of work of the
asynchronous engine with the damaged protection; intervals of time between occurrence of combustible material on cases
of the asynchronous engine; duration of a finding of combustible material on cases of the asynchronous engine don’t
contradict exponential to functions of distribution of probabilities then average time up to the first fire can be
defined by means of the formula:
 , (1)
where  ;  ;
here:  - an average interval of time between occurrences of breakage of a
phase of the asynchronous engine and average duration of existence of the given operating mode;
 - an average interval of time between refusals of means of
protection (a safety fuse, the automatic switch);  - an average
interval of time between occurrences of combustible material on cases of the asynchronous engine and average duration
of a finding of a combustible material (dust) on cases of the asynchronous engine;
 - an interval of time between checks of the condition of
protection means;  - an interval of time between stock-takings of
combustible material on cases of an electric equipment.
The formula (1) is fair when:  ,
 and  ,  .
Probability of fires, in shop with sufficient for
practical purposes accuracy, at operation of the asynchronous engine it is possible to define as follows:
 . (2)
As a result of research dependence between probability of
occurrence of a fire of the asynchronous engine maintained at the enterprise and intervals between prophylactic of
electric equipment has been received.
Changing terms of preventive maintenance
also it is possible to achieve that the probability of fires from
operation of the asynchronous engine will correspond to norm rate (GOST 12.1.004-91),
i.e. Q0 (8760) = Q0(8760) = 1*10-6.
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