## Example

A **13800V/4160 V** transformer has **five taps** on the primary winding giving **-5%**, **-2 1/2 %, nominal**, **+2 1/2 %** and **+5 % turns**. If, on-load, the secondary voltage reduces to **4050 V** then, which tap, should be used to maintain **4160 V on-load** (assuming the supply voltage remains constant)?

**The following answer results:**

**Examining the relationship:**

**V _{1}/V_{2} = N_{1}/N_{2}** or

**V**indicates that, to keep the equation in balance with primary voltage and secondary winding turns fixed, either V

_{1}·N_{2}= V_{2}·N_{1}_{2}or N

_{1}must be adjusted. Since the objective is to raise V

_{2}back to nominal, then N

_{1}must be reduced.

To raise V_{2}from 4050 to 4160V requires an increase in secondary volts of:4160/4050 = 1.027or102.7 %. N_{1}must be reduced to 1/1.027 =0.974

_{1}must be reduced by (1 – 0.974) =

**0.026 or 2.6 %**.

**Reducing N**will accomplish the increase in secondary voltage output.

_{1}by 2.6 %### The nearest tap to select is -2 1/2% (see Figure 1).

### How tap-changer works (VIDEO)

**Reference:** Science and Reactor Fundamentals – Electrical / CNSC Technical Training Group

Amit

in 33kV/433V tranformer if secondary voltage reduce from 433V to 405V ,Any consumption reduce .

Anil

You need to reduce your tap by 6.46%, nearest tap is at -7.5%.

Subodh Prakash

Taps on HV winding are provided for variation of HV volts. It may not be good practice to change the taps for reduced LV volts. Though , the extent to which the variation is proposed is Ok. The transformer should be designed suitable for variation of LV volts with taps on HV.

kazim

Nice article but I don’t understand the need to take the inverse of the turns ratio (1.027). Pls explain n thanks

Jeff D

Kazim, the ratio of secondary windings to primary windings needs to increase by 1.027. If the transformer in question had multiple taps on the output, we would select the tap closest to +2.7%. Since the secondary doesn’t have multiple taps, we’re stuck with 1/1.027 times as many windings as we’d like on the secondary, so the number of primary windings has to be reduced accordingly.

Put another way, we want N2actual/N1actual = 1.027*N2nominal/N1nominal, and we’re constrained by N2actual=N2nominal. Solving this system of equations for the ratio of actual to nominal primary windings yields N1actual/N1nominal = 1/1.027.

Steffen

What would be th off-nominal transformation ratio of the transformer shown in the example?