When the electric motor operates directly connected to the mains, that is, with a regular three-phase power supply, the sum of all the vectors corresponding to the phases is zero, the neutral being stationary and is usually maintained in the ground. For a given voltage, the characteristic curve of the motor will be fixed, independent of the load. Being connected directly to the electrical network will present a certain starting current peak, which normally presents its maximum value and that will cause a voltage drop in the network, interfering with the devices where they are connected. If so. Then the direct start must be replaced by another possible connection in order to reduce the starting current.
The connection types commonly used are serial-parallel connection, star-delta connection, static-start connection or electronic starter connection using a frequency inverter or variable speed drive. Frequency inverters have been increasingly used to control squirrel cage three-phase induction electric motors because of their efficiency and competence to regulate working speed as well as other parameters.