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.

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