MEASUREMENT OF NO-LOAD AND CURRENT
   
 
PURPOSE OF THE MEASUREMENT
 

In the no-load measurement the no-load losses P0 and the no-load current I0 of the transformer are determined at rated voltage and rated frequency. The test is usually carried out at several voltages below and above the rated voltage UN, and the results are interpolated in order to obtain the values corresponding at nominal voltage and if any at other voltage values.
   
 

APPARATUS AD MEASURING CIRCUIT
The measurement is performed by means of volt-ammeter method.
 

Fig. 4-1 Circuit for the no-load measurement.
G1 supply regulators, T1 transformer to be tested, T2 current transformer, P2 ammeters, P3 voltmeter (r.m.s. value), P4 voltmeter (mean value x 1.11), P1 wattmeters.
   
 
PERFORMANCE
 

The following losses occur at no-load:

  • iron losses in the transformer core and other constructional parts.

When carrying out the no-load measurement, the voltage wave shape may somewhat differ from the sinusoidal form. This is caused by the harmonics in the magnetizing current which cause additional voltage drops in the impedance of the supply. The reading of the mean value meter and r.m.s. will be different.

Because the losses have to be determined under standard conditions, it is necessary to apply a wave shape correction whereby the losses are corrected to correspond to test conditions where the supply voltage is sinusoidal.

In the test the voltage is adjusted so that the mean value voltmeter indicates the required voltage value. At the same time the voltage value on the voltmeter at r.s.m. value has to be read.
The ratio is:
 

(4.1)
U = voltage value read on r.s.m. voltmeter
U' = voltage value read on mean value voltmeter
   
 
The loss value corresponding to sinusoidal voltage is obtained from the measured value Pm as follows:
 

(4.2)
  It is assumed that for oriented magnetic steels P1 = P2 = 50%.
   
  The current and power readings of different phases are usually different (the power can even be negative in some phase). This is due to the asymmetric construction of the 3-phase transformer; the mutual inductances between different phases are not equal.
   
 
RESULT
 

The report shows the corrected readings at each voltage current and losses value.

A regression analysis is carried out on the corrected readings. From the no-load curve thus obtained no-load losses and no-load apparent power corresponding to voltage values from 90 to 110% of UN.