## Introduction to test

The no-load losses are very much related to the operational performance of a transformer. As long as the transformer is operated, **these losses occur**. For this reason, no load losses are very important for operational economy. No-load losses are also used in the heating test.

The no-load loss and current measurements of a transformer are made while one of the **windings** (usually the HV winding) **is kept open** and the other winding is supplied at the rated voltage and frequency.

During this test the **no-load current (Io)** and the **no-load losses (Po)** are measured.

The measured losses depend heavily on the applied voltage waveform and frequency. For this reason, the waveform of the voltage should be **very sinusoidal** and **at rated frequency**.

Normally, the measurements are made while the supply voltage is increased at equal intervals** from 90% to 115% of the transformer rated voltage (Un)** and this way the values at the rated voltage can also be found.

### No-load losses and currents

**The no-load losses of a transformer are grouped in three main topics:**

**Iron losses**at the core of the transformer,**Dielectric losses**at the insulating material and- The
**copper losses**due to no-load current.

The last two of them are very small in value and can be ignored.

**So, only the iron losses are considered in determining the no-load losses.**

### Measuring circuit and performing the measurement

In general according to the standards, if there is less than 3% difference between the **effective (U) value** and the **average (U’) value of the supply voltage**, the shape of the wave is considered as appropriate for measurements.

**effective (r.m.s.) value**and

**the average (mean) value**of the voltage are

**different**. If the readings of both voltmeter are equal, there is no need for correction.

During measurements, the supply voltage U´ is supplied to the transformer by the average value voltmeter. In this way, the foreseen induction is formed and as a result of this, the hysteresis losses are measured correctly. The eddy-current losses should be corrected according to equation below.

P_{m} = P_{0} · (P_{1} + k · P_{2})

**P _{m}**: Measured loss

**P**: No-load losses where the voltage is sinusoidal

_{0}Here: **P _{0} = P_{h} + P_{E} = k_{1} · f + k_{2} · f^{2}**

**k = [ U / U’ ] ^{2}**

**P _{1}**: The hysteresis loss ratio in total losses

**(P**

_{h}) = k_{1}· f**P**: The eddy-curent loss ratio in total losses

_{2}**(P**

_{E}) = k_{2}· f^{2}At 50 Hz and 60 Hz, in cold oriented sheet steel, **P _{1} = P_{2} = % 50**.

**So, the P**

_{0}no-load loss becomes:P_{o} = P_{m} / (P_{1} + k · P_{2}) where **P _{1} = P_{2} = 0,5**

**According to IEC 60076-1:** **P _{m} = P_{0} · (1 + d)** where

**d = [ (U’ – U) / U’ ]**

**average of the three phase currents**.

Before the no-load measurements, the transformer might have been **magnetised by direct current** and it’s components (resistance measurement or impulse tests).

For this reason, the core has to be demagnetised. To do this, it has to be supplied by a voltage value (increasing and decreasing between the maximum and minimum voltage values for a few minutes) higher than the rated voltage for a certain time and then the measurements can be made.

The no-load currents are neither symmetrical nor of equal amplitude in three phase transformers. The phase angles between voltages and currents may be different for each of three phases.

**For this reason, the wattmeter readings on each of the three phases may not be equal.** Sometimes one of the wattmeter values can be **0** (zero) or **negative** (-).

**Resource:** Transformer Tests – BEST Transformers