Method and device for starting motors in weak grids

摘要

Method and device for starting a motor (12) in weak grids, especially asynchronous motors, which motor after start-up is supplied by a supply grid (13). The device (11) includes a frequency converter (14), which is rated in relation to the size of the motor (12) and arranged to bring the motor (12) up in speed, arranged to synchronize output voltage with voltage from the supply grid (13), and arranged for connecting the motor (12) to the supply grid (13) after the voltage of the motor is synchronized with the voltage of the supply grid (13). The main object is to do this with as low costs as possible and in the most space-saving manner.

主权项

1. A method for starting one or more electrical motors (12) in weak grids, which motor after start-up is powered by a supply grid (13) through a first switch (18a), comprising the steps of:
a) verifying that the motor (12) is not connected to the supply grid (13); b) connecting the motor (12) to a frequency converter (14) through a second switch (18b), wherein the frequency converter (14) is connected to the supply grid (13); c) using the frequency converter (14) to raise the rotational speed of the motor (12) by running output frequency from 0 Hz to the frequency of the supply grid; d) removing magnetizing current of the motor (12) with a capacitor battery (17) arranged in parallel with the motor (12), so that the frequency converter (14) supplies the motor (12) with active current only; e) reading voltage and phase angle of the supply grid (13), and thereafter synchronizing the voltage of the motor (12) to a phase angle in front of the phase angle of the voltage of the supply grid (13) to compensate for a potential drop in motor speed as a consequence of switching time from frequency converter (14) to supply grid (13); f) when the voltage of the motor (12) is synchronized to a phase angle in front of the phase angle of the voltage of the supply grid (12), switching the powering of the motor (12) from frequency converter (14) to supply grid (13) by disconnecting the second switch (18b) and connecting the first switch (18a), such that the frequency converter (14) and supply grid (13) are not connected to the motor (12) concurrently.