Emax Low voltage air circuit-breakers - ABB Download Center

Switching and protection of asynchronousmotorsA low voltage automatic air circuit-breaker can, by itself, guaranteethe following functions in power supply circuits of threephaseasynchronous motors:– switching– overload protection– short-circuit protection.1SDC200208F0001t [s]MA = Circuit-breakerB = Overload protection (inverselong time-delay trip)C = Short-circuit protection(instantaneous)M = Asynchronous motorDiagram showing direct startingof an asynchronous motor usingjust the circuit-breaker fitted withan electronic overcurrent trip unit.I [A]Ie = Rated current of the motorIa = Initial value of the transient starting currentIp = Maximum instantaneous value of the subtransient starting currentta = Starting timets = Duration of subtransient phase1SDC200209F0001Trend of current values in the starting phase of a three-phase asynchronousmotor.This solution is particularly suitable if the switching frequencyis not high, as it is normally the case for large motors. In thiscase, using only the circuit-breaker for motor switching andprotection represents a highly advantageous solution thanksto its competitive cost-efficiency, reliability, ease of installationand maintenance, and compact overall dimensions.The circuit-breakers in the SACE Emax selective (not currentlimiting)series are able to provide the motor switching and protectionfunction by virtue of their high breaking capacities andthe wide range of possible settings offered by the electronictrip units.SACE Emax circuit-breakers are suitable for use with motorswith rated powers within the range between 355 kW and 630kW. For power ratings up to 355 kW, the moulded-case circuitbreakersin the SACE Isomax and Tmax range are also available.Medium voltage power supplies are normally used forpowers above 630 kW.6ABB SACE 6/35