Star-Delta Starters are used to start larger ac induction motors used in industries, such as Manufacturing and Marine.
Like ‘Direct Online’ (DOL) starters, which are also used for ac induction motors starting (though smaller, lower current motors), Star-Delta starters only turn the motor on and off at a single speed.
With the growth in the internet of things, industries, such as Mining and Marine (who use Star-Delta starter systems) need to consider how to keep their business efficiency as good as their competitors.
Competitors are always seeking a competitive advantage. This can be achieved through cost and efficiency savings, and IOT (Internet of Things) connectivity allows close monitoring of system processes, and so-called ‘machine learning’.
In the case of a Star-Delta motor starter, their area number of outputs that we may wish to measure using sensors.
Firstly, the current being drawn by the Star-Delta starter. This includes both the starting (or Inrush) current, and the FLC (Full Load Current).
Monitoring of the Full Load Current (FLC) for example is useful because an increase in current been drawn through a phase might be due to a break in one of the other phases. This causes what is known as ‘Single Phasing’ to occur.
Single Phasing can potentially damage both the motor windings and cabling from the starter to the motor, due to the current doubling in the two phases still connected.
The motor windings, for example, would not be designed to handle the excess current flowing through the two (still) connected ones, potentially causing damage, expense, and downtime.
The Star-Delta Starter normally has a current limiting device installed, called an Over Current Relay (OCR).
There are three types of OCR, Electronic, Magnetic, and Thermal.
The OCR will detect higher than normal currents, caused for example by ‘Single Phasing’.
When single phasing is detected by the OCR, it will disconnect the three-phase supply going to the induction motor (after a short delay, dependent on overcurrent size).
To be continued………
(c) 2016 Craig Miles