The voltage is changed by moving the cursor from one TAP to the
next one. The problem with the changer is that it must cope with
two opposite mandatory requirements:
. During TAP change, the circuit must not be opened; and
. During TAP change, there must not be a situation where two
TAPs are short-circuited.
The solution to the above is the use of two resistors, physically
spaced as shown, that operate only during the TAP change.
Thanks to these resistors R, the circuit is not opened, even if the
resistor is put in series to the load during the transient, and the
two TAPs are not short-circuited: between the TAPs the
resistance is 2*R.
The following figure shows in detail what occurs during a single
TAP change.
We start from TAP N; we have to move to TAP N+1. Before the
movement, position 1, there is a direct connection between TAP N
and the input; the primary winding resistance is RN.
As the movement starts, up to position 2 nothing happens; from 2
on, the resistor R is put in series to TAP N, and the resistance of
the primary windings grows to R + RN.
As the movement continues, the second resistance touches TAP
N+1; between the TAPs there is a resistance of 2*R. Towards the
input, the total resistance is now R/2, as the two resistors are
practically in parallel. The total resistance is (RN + RN+1+R)/2.
On a further movement, R is put in series to TAP N+1; the total
resistance is now R + RN+1.
Last, the main contact is on TAP N+1; the total resistance is RN+1
(slightly more or less than RN), ant the movement is completed.
The diagram below shows the total primary resistance change as
the TAP changer operates.
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