This is a one from several types of transformer.It usually used for motor starter,we recognize it by name “auto transformer starter”.Basically design for auto transformer only have a single winding and provided with tap connection for step up or down.
It have magnetic core with single winding which common primary and secondary circuits.The primary connection is connected with main AC power supply and secondary connection is connected with load.
If the primary is the total winding and is connected to a supply and the secondary circuit is connected across only a portion of the winding,It mean this is for stepped down voltage.
But if primary winding portion is the primary and is connected to the supply voltage and the secondary includes all the winding, then the voltage will be stepped-up.
What is the auto transformer?
Auto transformer application
In market,the price for auto transformer is much cheaper than the two-coil transformer of the same rating,but the disadvantage of the auto transformer is that the primary and secondary circuits are electrically connected and could not safely be used for stepping down from high voltage to a voltage suitable for loads.
The auto transformer main purpose is for reducing line voltage for step increases induction motors starting. Generally it have four or five tapping connection that are controlled by timers or plc sequence.It control in each step until the full voltage is applied across the induction motor.
The main purpose is to avoids the large inrush current when starting induction motors at full line voltage.For more detail about auto transformer starter,please read my last post :auto transformer starter