Types of protection for electric motors


Electric protection of asynchronous electric motors

Reliable and smooth operation of the engine obespeyochivatsya first of all by the right choice of its rated power, observance of necessary treyobovaniye at design of the circuitry, mounting and operation of the electric drive. However even for correctly designed and operated electric drives there is always the opasyonost of emergence of the modes, emergency and abnormal for the engine. On this case means for restriction of development of avayoriya and prevention of premature damage have to be provided.

Схема подключения трехфазного асинхронного электродвигателя

Scheme of connection of the three-phase asynchronous electric motor.

The main thing and the most efficient means являет­ся electric protection of engines, vypolyonyaemy according to Rules of the device of elektroyoustanovka.

Depending on character of possible povrezhdeyoniya and abnormal operating modes, distinguish some of main most widespread types of electric protection of asynchronous engines.

The maximum and current protection which is hereinafter referred to as for brevity by the maximum protection. The devices which are carrying out the maximum protection (safety fuses, electromagnetic relays, avtoyomatichesky switches with the electromagnetic releaser), almost instantly, i.e. without endurance of the vreyomena, are disconnected by the engine from network at emergence in the main chain or in the control circuit of short-circuit currents or abnormally big pushes of current.

Protection against the overload, or thermal protection, protects the engine from inadmissible overheating at rather small in size, but long overloads. Devices of the thermal zayoshchita (thermal relays, automatic cuts with the thermal releaser) at emergence of the peregruzyoka switch-off the engine with the certain endurance of the vreyomena, subjects bigger, than less overload.

Схема тепловой защиты

Scheme of thermal protection.

Protection against work on two phases protects the engine from inadmissible overheating which can occur owing to wire break or burn-out of the safety lock in one of phases of the glavyony chain. Protection affects shutdown of the engine. As devices of protection both teployovy, and electromagnetic relays are used. In the latter case protection can not have excerpts of time.

Protection of minimum voltage (zero protection) is carried out by means of one or several devices, affects shutdown of the dviyogatel at undervoltage of network below the established value, preventing possible overheating of the engine and danger of its "capsizing", i.e. the ostayonovka owing to decrease in the electric moment. Zero protection protects also the engine from samoyoproizvolny inclusion after the power supply break.

Besides, there are also some other, the found types of protection are more rare (from increase the napryazheyoniya, single-phase ground faults in networks with the izoliyorovanny neutral, increases in rotational speed of the drive, etc.).

Devices of electric protection can осуще­ствлять one or at once several types of protection. So, some automatic cuts with the kombiniroyovanny releaser provide the maximum zayoshchita, protection against the overload and against work on two phases.

Таблица выбора теплового реле

Table of the choice of the thermal relay.

One of protection devices, for example safety fuses, are devices of single action and demand replacement after everyone the srabatyvayoniya. Others, such as electromagnetic and thermal relays, - devices of repeated action. The last differ on the way of return to the ready state on devices with self-return and with the manual vozyovrat.

The choice of this or that type of protection or several is at the same time made in each specific sluyocha taking into account degree of responsibility of the drive, its moshchyonost and working conditions. Big advantage priyonest the analysis of data on accident rate the elektrooborudovayoniya in workshop, on the building site, in the workshop, etc. definition of the most often repeating nayorusheniye of normal operation of engines and the tekhnologicheyosky equipment can.

The right choice and setup of devices of protection have essential value. For example, the increased failure of engines because of work on two phases owing to combustion of the fusible insert in one phase nablyuyodatsya sometimes. But in many cases combustion of the insert results not from single-phase korotyoky short circuit (breakdown on the body), and is caused by the neprayovilny choice of inserts, installation in different phases of accidentally found safety locks with different currents of melting of inserts.

Experience of many enterprises pokayozyvat that at high quality of repair of engines, careful performance of mounting, appropriate care of contacts of actuators and contactors and the right choice of fusible inserts operation of engines on two fayoza is practically excluded and installation of special protection is not required.

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