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DESCRIPTION:
The motor protection is an electrical device capable of detecting the conditions of overload, and short circuit fault and an electric motor for automatically interrupting the power supply.
Where they are placed, how many they are and how they are connected
Mechanical (fuses)
The mechanical motor protectors are very cheap, but they are also less efficient , especially with three-phase systems and are therefore to be coupled to a magnetic backing , these fuses are inserted upstream and in series with each individual load for each load uses it 1 per phase , so for the single-phase loads or civil one it is sufficient , while for the three-phase loads or industrial need it three .
Electromechanical external to the engine ( or Thermal and leakage )
These devices are inserted upstream of the load or loads , it is sufficient to only one, to be connected in series with the load to ensure safety to the machine or machines , but also to the hook of the electrical installation . These devices have adjustable intervention threshold , so that only one device can be used for different loads or to be able to adjust to a load that has changed its character with the progress of time .
Electromechanical internal engine
This device is no more than a thermal switch " PTC " , which acts with the increase of the resistance ( this device has the form of a stack of clock , but bigger ) , inserted in close contact of the stator winding ( nelMotore asynchronous Single / Three phase ), when the thermal switch is subjected to a temperature higher than that of normal operation, remove power to the motor until it cools this thermal switch prevents the engine for starting.
Such devices are usually more than one for the engine, generally three , this is because the motor may overheat unevenly , especially in three-phase motors in the case of a missing phase power , otherwise there is a risk that the motor protection does not intervene .
These motor protection are generally connected in series between them , because this acts as one of many motor starters, opens the power supply circuit , another case is to connect them to each other in parallel , this solution is less secure , because in If it were just a step too overloaded burning engine , this type of connection is used to reduce the number of motor protection interventions .
electronic
These devices are external , but have a part that must be internal to the engine , are temperature sensors " PTC " (thermistors ) , the presence of the sensors in the motor depends on the manufacturer , while the external device it is only one, the sensors interiors are linked together according to the will of the manufacturer , if they are in series ensures maximum safety, if no parallel , while the outer part of the motor protection device is connected in parallel to the load.
Type of connection to the power supply circuit [
In addition to the motor circuit breakers in parallel ( electric ) , powered by a separate circuit and overtemperature or open signaling the tripping circuit of the contactor to which they are attached (usually also have contacts for possible reporting ) that disconnects the engine, you have also protective motor switches connected in series (electromechanical) to the power supply circuit upstream of the load to be protected.
Type of Manual Motor Starters
When the motor protection ?
The cases in which the motor protection are listed below:
Starting the engine too slow.
Too many start-ups close .
Network is not stable : excessive voltage fluctuations or variations in frequency .
Locked rotor .
The engine struggles too (excessive resistive torque) .





