Two stages, one start: The star-delta combination
A direct start is often recommended for lower outputs and lower moments of inertia. If an application requires higher outputs such as 5.5 kW or above, a star-delta combination is recommended. With a star-delta combination that is already assembled in the performance range from 5.5 kW to 90 kW or as individual parts for customer assembly up to a performance class of 500 kW, we offer an affordable, space-saving, and highly available option for reliably starting asynchronous motors. Star starting already reduces starting current by a third. An integrated, adjustable time relay is used to set the desired run-up time for the application until the rated speed is reached. The 50 ms interval necessary for switching over from a star to a delta configuration as well as surge suppression (varistor) are already included in the function module.
Reduced starting currentThe purpose of a star-delta configuration is to reduce the starting current of an asynchronous motor in order to reduce load peaks in electrical power distribution. The specifications and limit values (Technical Connection Conditions (TCC)) of the particular power supply company must be observed.
In normal operation, the motor is connected in a delta configuration, where the phase-element voltage corresponds to USTR=UL. In concrete terms, a higher voltage also means a higher making current.
In contrast to the delta configuration, the phase-element voltage in a star configuration is only USTR=UL/√3, where the starting current and starting torque are reduced to about 1/3. After the star startup, the configuration is switched over to delta.