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Starting Chain Limitation

Whenever necessary the starting of a three-phase electric motor of squirrel cage, it should be direct in the network or full voltage, by means of vacuum contactors and / or circuit breakers. We should know that for a given motor alemite 8072-a the curves of conjugates and current are fixed regardless of the difficulty of starting, for a constant voltage. In cases where the motor starting current is high, harmful consequences such as high voltage drop in the mains supply system can occur, which will cause interference in equipment installed in this system, the power system (cables, switches, protection, transformer ) should be about dimensioning, increasing costs, determination of the electric utilities that limit the voltage drop of the network, as well as the limitation in case of isolated networks operating with generator sets.

If direct starting in the network is not possible, because of the problems mentioned above, we can use indirect starting system to decrease the starting current. These indirect starting, low voltage, frequency variation, and resistance systems are the star-delta key, the compensating or self-locking switch, the static or soft start starter, the frequency inverter and the rheostat for motors rings.

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LIMITING THE STARTING CURRENT

Whenever possible, the starting of a three-phase cage motor shall be direct, using contactors. In cases where the engine starting current is high some damaging consequences can occur as if direct starting is not possible, the indirect starting system is used to reduce the starting current. These indirect starting systems are: the protection system must be oversized, resulting in high cost and high voltage Leeson mrosupply drop in the mains power system. Due to this, it causes interference to equipment installed in the system.

The drive of an electric motor is only admissible via star-delta switches if it has six accessible terminals and has a nominal double voltage. The advantages are the reduced cost; high number of maneuvers; starting current reduced to 1/3 of nominal; relatively small dimensions. The disadvantages are the specific application to motors with double rated voltage and having six accessible terminals; starting torque reduced to 1/3 of nominal; the mains voltage must match the motor’s triangle voltage; the motor must achieve at least 90% of its speed regime so that, during switching, the peak current does not reach high values.