Sumpner’s Test, Back to Back for two identical 1-phase Transformers, Regenerative Test of Transformer

Sumpner’s Test
Back to Back (Sumpner’s Test) (for two identical 1-phase Transformers) (Regenerative Test)
Q. Explain the procedure for back to back test on transformer with neat circuit diagram. State advantages and disadvantages of this method.
Q. Draw the experimental setup of the Sumpners test and state its advantage.
Q. Draw the experimental set up or the Sumpners test on 1 phase transformer. Also write its procedure.

• If large transformer are to be tested for determining their efficiency, temperature rise and regulation then a load of high capacity, may not be available and if available, the energy consumed in testing will be wasted. To overcome this difficulty back to back test is adopted.
• It is a regenerative test and hence economical and very useful.
• T₁ and T₂ are two identical transformers T₃, autotransformer 
• S₁ ,S₂ andS₃, switches A-ammeters, V-voltmeters, W-wattmeters.
  

• Two similar transformers are required to carryout this test. As shown in Figure the primaries of two transformers (T_{1 and} T₂ ) are connected in parallel across the supply at rated voltage of primary.
• Their secondary’s are connected in phase opposition or back to back fashion.
• When primaries of two transformers are energized by switching on switch S₁, the emfs induced in secondary windings come in phase opposition. Since the two transformers are identical, there is no circulating current in the local circuit formed by secondary’s even, if primaries energized.
• To ensure that the secondary’s are connected in phase opposition, a voltmeter (V₃) and a switch is connected in parallel as shown. V₃ should be of the double range of that of secondary voltage.
• This is because, if the polarities are not connected in phase opposition the voltmeter may receive twice the voltage of secondary i.e. secondary voltage of first pulse secondary voltage second transformer.
• If voltmeter indicates zero it ensure that secondary’s are Le. connected in phase opposition, then switch V₃ is closed. If voltmeter does indicate more voltage then secondary connections are interchanged.
• To circulate the necessary current one auto transformer (T₃) is used in the secondary circuit as shown.
• Voltage is injected by switching on S₂ and by varying the voltage with the help of T₃ full load current is circulated in the secondary’s.
• The current corresponding to this circulating current also flows in the closed circuit formed in the primaries, however it does not appear in the ammeter and wattmeter W, connected in primary side. So the current taken from supply side is only the total no load current of two transformers.
• The wattmeter reading (W₁) connected in the primary side indicates total no load loss or iron loss of two transformers.
• The wattmeter connected in secondary side (W₂) indicates total copper loss or load loss of two transformers caused by the circulating current. Since both the losses are known efficiency of the transformer can be easily determined.

Determination of regulation

• The injected voltage (V₂) supplied to secondary side circulates the full load current in the secondary’s of both transformers. The current corresponding to this circulating current also flows in the primaries (in the local circuit formed). Thus this injected voltage (V₂) supplies the voltage drops (IZ drop) of both transformers. Hence voltage drop of one transformer will be V₂ / 2

       i.e. injected voltage / 2

Determination of circuit Constants for drawing equivalent circuit

• The voltage drop of each transformer = V₂ / 2 but voltage drop is the impedance drop i.e. = I₂Zs
• Where Zs = Equivalent impedance of one transformer referred to secondary side.

• Magnetic circuit constants are found from primary side readings as explained in open circuit test of transformer. Thus equivalent circuit can be drawn.