Motor-circuit breakers are devices widely used in electrical projects of machines and industrial drives, and if you got here it is because you painted that doubt about its operation or you can even ask yourself what motor-breaker is. We need to understand what is its main application to be able to take advantage of everything that this electrical component can offer, so let’s learn a little more about this device and what is the function of the motor circuit breaker?
These devices are adapted thermomagnetic circuit breakers, so we speak because it has special characteristics that ordinary circuit breakers do not have. They are specially used for electric motor controls, applied to motors, and always three-phase. In many cases the motor-circuit breaker is connected to the output of the power contactor itself, basically, they are used when starting three-phase electric motors.
Motor circuit breaker characteristic
The main characteristic of motor circuit breakers occurs at the moment of opening in abnormal current conditions, that is, an opening occurs in the event of a short circuit or overload of the motor because at this point there is a large amount of current flowing to the motor, this is the great difference between motor circuit breakers and common circuit breakers, motor circuit breakers manage to open without damaging their internal components, the electric arc formed at the time of opening the circuit breaker is extinguished in your extinguishing chamber, on the other hand at this level of current circuit breakers cannot open without damaging their internal contacts.
Advantage of motor circuit breaker
A great advantage in the use of motor circuit breakers is that they provide the possibility of starting at full voltage, yes they support a start of 8x to 10x the rated current of a motor. Even if this motor is subjected to a high starting torque, the motor circuit breakers are manufactured to withstand this momentary overload during the motor start, without jeopardizing the efficient protection of the circuit. A common circuit breaker would open the circuit at the start, which would require an over dimensioning of the circuit, causing the circuit not to be well dimensioned, losing the protection efficiency.
Many motor-circuit breaker manufacturers offer the same with current adjustment, enabling more precise thermomagnetic regulation for the protection of the motor, therefore, it is often not necessary to use NH protection fuses, thus the cost-effective use protection device may be more advantageous.
The motor circuit breakers have a control handle totally isolated from the energized parts, as it favors even more protection during the maneuvers. It is still possible to perform the blocking (Consignment) of the equipment in the handle of the motor circuit breakers, meeting the regulatory standards NR12 and NR10.
The protections against phase failure and overloads are enhanced when the installation of the motor circuit breakers in series with a thermal relay is performed. This electrical connection is easy to perform without the need for cables or other components because when they are from the same manufacturer they have their own busbar structure to make the connection.
Main protection points of the motor circuit breaker:
Through the magnetic trip, the motor circuit breaker offers protection against short circuit, since it will cut the electric current whenever it reaches 13x the regulated trip current.
The thermal trigger is responsible for protection against overloads and lack of phase, its principle of action is through bimetallic materials of different expansion coefficient. When the bimetallic blades overheat, they deform, causing the circuit breaker to open the circuit.
In electrical projects, a motor circuit breaker has a specific symbology, note that below the electrical contacts there are two symbologies in series, the first says that the motor circuit breaker has a thermal trigger protection and the second refers to magnetic trigger protection:
For the correct specification of the motor circuit breakers, it is important to make sure the service and rated current of the motors to be protected. These data can be identified on the motor nameplate and the current setting on the motor circuit breaker must be in accordance with the service current, to ensure the protection efficiency.
These circuit breakers can be installed and fixed easily on a 35mm DIN rail which facilitates their installation on electrical panels. It is very important to highlight, a mistake that many electricians make! When applying motor-circuit breakers to the output of frequency inverters where they operate below the 60Hz grid frequency, we must look for a circuit breaker with this technical specification on the market.
Motor circuit breakers that act as their protective mechanisms at multiple working frequencies are generally more expensive. When this point is not observed and a motor circuit breaker that operates only at mains voltage is installed, the circuit will not be protected at lower frequencies and may cause a surprise from motor burning and losses in the electrical circuit.
As we have seen the application of motor circuit breakers in three-phase power circuits in order to protect electric motors, it offers us greater protection and safety for both equipment and people.
These advantages make these circuit breakers the most used in electrical, drive and industrial electrical projects. To opt for the use of this device, we must take into account all the features discussed here and carry out the cost-benefit balance.