Calculation And Measurement Methods; Turns Ratio Measurement; Principle; Voltage Ratio And Turns Ratio - Raytech mini-ATOS Instruction Manual

4 Calculation and Measurement Methods

4.1 Turns Ratio Measurement

4.1.1 Principle

The transformer turns ratio test is used for measuring the ratio of turns of wires of the two sets of
windings e.g., the primary side and the secondary side of a transformer. The turns ratio test does
not tell exactly how many turns of wire are in each coil, but rather, it measures and displays the
ratio of the number of turns of two coils.
This is a useful test to check for shorted turns and also to find probable incorrect settings of tap
changers during manufacturing or repairing.
The mini-ATOS performs the turns ratio by applying voltage on the high voltage side of each phase
and measuring the applied voltage and transformed voltage. Figure 2 represents the very simple
schematic of the measuring circuit.
Figure 2: Very simple schematic of mini-ATOS turns ratio measuring circuit
The excitation current is also measured during the turns ratio test. The value of the excitation
current has a direct relation to the test voltages. A higher test voltage leads to a higher excitation
current. Additionally, the phase angle between the applied voltage and the transformed voltage is
also measured and is included in the results.

4.1.2 Voltage Ratio and Turns Ratio

It is important to understand the differences between transformer voltage ratio and transformer
turns ratio. The voltage ratio is the ratio of rated voltages of two windings but the turns ratio is
exactly the ratio of the number of turns of two sets of windings. For a single-phase Transformer,
the Turns Ratio is the same as the Voltage Ratio, however for a three-phase transformer they may
be different.
As an example, consider a single-phase transformer schematic with a primary side and a
secondary side. The primary side nominal voltage which is written on the nameplate is 6900V and
from the manufacturing data, we know it consists of 15000 turns. And secondary side nominal
Preliminary
90297-0.24 Instruction Manual mini-ATOS Page 11 of 125