Thermal relay for motor phase failure protection function

The motor star connection, it can protect the general relay 3. However, the motor should be selected for the triangular connection of the motor with a phase failure protection device. Because of a delta connected motor phase disconnection operation, current flowing through the motor winding through the thermal relay is different, one of the most serious overload of the phase winding current than the other two phase winding series will be 1 times, increased higher than that of the increase of line current ratio, with 3 thermal relay general no protection.

In the practical application, the relay with no phase protection can also provide the phase protection for the motor in a certain range. For example: when the diode or triode thermal relay for motor protection star connection, because the current phase through the thermal relay is still flowing through the motor winding, still can provide protection for the motor; when the triangle connection of the motor in greater than 67% or less than 50% of rated load operation, thermal relay can also be do not take off phase protection. This is because the large load, after the failure of the fault phase can make the general thermal relay action; and small load, the broken phase current is small, generally will not damage the motivation. In addition, if the thermal relay is installed on the motor, for the triangular connection, can not be broken phase protection of the three poles of the thermal relay are respectively connected to the three-phase winding, also can play the role of phase failure protection. However, it should be noted that: at this time the thermal relay setting current should be 56% of the motor rated current to set. Relay manufacturers