Emax Low voltage air circuit-breakers - ABB Download Center

Switching and protection of asynchronousmotorsOperation with the loss of a phaseThe IEC 60947-4-1 Standard prescribes that a trip unit, withcompensated temperature and sensitive to phase losses, must:– not trip in less than two hours at 20°C, when one phase carries90% of In and the other two carry 100% of In– trip in less than two hours at 20°C, in the event of the loss ofa phase when the current in the energized poles reaches1.15 times the rated current In.With the PR122 and PR123 trip units by activating the Unbalancefunction it is possible to check the losses of phase.Selecting the circuit-breakers to be used for motorprotectionThe tables in the next pages show the rated characteristics forlarge motors, from 355 to 630 kW, with circuit-breakers in theSACE Emax series for switching and protecting motors in categoryAC-3 at 415/690 V - 50 Hz.The tables show the choice of current transformers able to ensurea sufficiently high value for the instantaneous trip thresholdsetting (I): in the absence of experimental data, it is advisableto verify that the ratio between the threshold of protectiondevice I (I3) and the threshold of protection device L (I1) is:I3/I1 =12 ... 15.The PR122 and PR123 electronic trip units conform to the internationalIEC 60947-4-1 Standard. In particular, the devices ensureprotection of class 10A, 10, 20 and 30 of motors. PR122and PR123 protection trip units are compensated in temperature,and their operation is not negatively affected by the loss ofa phase.6Advantages of earth fault protection GThe earth fault protection (G) is recommended in order to:– improve safety against fire hazards– improve protection of motors and personnel in the event ofmachine faults.Advantages of thermal memoryThe advisability of enabling the thermal memory (option offeredby PR122 and PR123 trip units) must be evaluated in relation tothe type of load. Enabling the thermal memory (which makesthe electronic protection similar to the one provided by a thermomagneticdevice) increases the protection level of the motorwhen restarting after tripping due to an overload.Undervoltage protectionThe undervoltage protection device in control systems for asynchronousmotors demands special attention, performing,amongst other things, two important functions:– it prevents simultaneous restarting of all the motors on returnof the power supply, with the risk of making the entire installationgo out of service by tripping the main circuit-breakerovercurrent protection devices– it prevents the motor from restarting without a control signal,which could be a hazard for maintenance personnel or coulddamage the processing cycle.6/38ABB SACE