Electrical Equipments

What ratio test?

Divide primary voltage by secondary voltage? Or just look it up on the nameplate?

There are many methods. read below

Types of Turns Ratio Tests

When considering the range of elements shown in the transformer
schematic and considering also the varying requirements of different transformer
applications, it can be seen that no single measurement technique
will fully satisfy all turns ratio questions. For this reason, Voltech AT series
transformer testers offer five different turns ratio measurement techniques,
which can be selected individually to meet specific needs.

1-TR (turns ratio). This test energizes any chosen winding at a specified
voltage and measures the induced voltage on any other winding. The
results are then presented as a ratio (e.g. 2:1, 5:1, etc.) Voltech AT testers
do this by dividing one voltage by the other while compensating for winding
resistance. Phase is also measured: 'in-phase' (positive polarity) and 'antiphase'
(negative polarity).

2- TRL (turns ratio by inductance). This test separately energizes two
selected windings and measures the inductance value of each winding.
The results are then presented as a ratio of turns (e.g. 2:1, 5:1, etc.)
calculated from the squared root of the inductance values. Phase is also
measured: 'in- phase' (positive polarity) and 'anti-phase' (negative polarity).

3- LVOC (low voltage open circuit). This test applies a voltage to the primary
winding, reads the voltage induced in the secondary winding and presents
the results as a secondary voltage (e.g. 2.545V). Phase is also measured:
'in- phase' (positive polarity) and 'anti-phase' (negative polarity).

4- VOC (voltage open circuit - AT3600 only). This test uses the same
principal as LVOC but by using a high-power generator, capable of energizing
a winding at voltages up to 270V, the test is suitable for testing
low-frequency power transformers. Phase is also measured: 'in- phase'
(positive polarity) and 'anti-phase' (negative polarity).

5- VOCX (voltage open circuit with external source - AT3600 only).
This test, which is used in conjunction with the Voltech AC Interface
Fixture, will control an external AC source or step-up transformer for
testing higher power and higher voltage transformers up to 600V and 10A.
Phase is also measured: 'in-phase' (positive polarity) and 'anti-phase'
(negative polarity).

RATIO TEST:

Ø Perform the test on all taps

Ø Apply 3 Phase, 50Hz, Low Voltage, say, 415V to the HV Winding with the LV winding open circuited and measure the voltage in the secondary winding

Ø Use good quality voltmeters for measurement; use same meter for measuring HT & LT Volts

Ø Check with the ratio declared in the transformer name plate/routine test certificate

Ø Caution: Do not apply voltage to LV Terminals, as this would induce dangerously high voltages at the HV Terminals