When bypass contactor is used, the motor current is forwarded to flow via contactor and not via Altistarts thyristors after successful motor start.
Therefore when bypass contactor is switched on, the thyristors are not flown by the current and so no power dissipation is generated in altistart and its thermal state is decreasing while motor is running. If no other motor start is executed the Altistart will gradually reach cold state. Being in cold state means that next motor start can be done with greater current or with longer starting time. Alistart without bypass will start decreasing its thermal state after the motor is stopped.
Advantage of using bypass contactors is:
- Less power dissipation inside altistart
- altistart can manage more starts per hour or start with greater current or with longer starting time without need to oversize the drive. To understand what greater current and longer time means see extract at the end by comparing start from cold state and start from hot state.
- additional functionality for example cascading
If bypass contactor is not used, there is no problem for altistart or the motor providing that:
- continuous motor current is lower than altistart ICL current.
- no need for high number of starts per hour, starting with high current or with longer time
- not intention to use cascading (consecutive starts of several motors)
Extract from ATS48/ATS480 catalog:
In standard applications, the Altistart 48, Altistart 480 are designed to provide:
- Starting at 4 In for 23 seconds or at 3 In for 46 seconds from cold state (corresponding to motor duty S1).
- Starting at 3 In for 23 seconds or at 4 In for 12 seconds with a load factor of 50% (*1) and 10 starts per hour or an equivalent thermal cycle (corresponding to motor duty S4)
(*1) Note, for altistart the interval when thyristors are being bypassed by the contactor is considered as no-load.
