LOAD TEST OF A SINGLE PHASE TRANSFORMER
OBJECT: TO determine the percentage regulation & efficiency of a given transformer by carrying out a load test.
APPARATUS:
Sr. no. Name
Range No.Regd. Remarks.
1 Single phase transformer
2 Single phase variac.
3 Single phase watt meter.
4 Volt meter.
5 Ammeter.
6 Load resistance.
7 Single pole switch.
THEORY: A transformer is a static apparatus used to transform a.c. electrical power from one voltage to another voltage. It works on the principle of mutual induction.
In a transformer there are two windings primary winding & secondary winding. Both these windings are having their internal resistance & leakage current. When the transformer is loaded there will be voltage drop in the transformer due to the resistance & reactance of the windings.
REGULATION: If E0 is the load voltage of the secondary side & V, is the terminal voltage of the secondary side.
When it is loaded: Then,
E0 � V = Voltage drop in the transformer when it is loaded.
=Change in the terminal voltage of the transformer when it is loaded.
Voltage regulation of a transformer is the ratio of change in the terminal voltage of the secondary from no load to load condition.
To its rated voltage, at a particular load current.
i.e % Regulation = E0 -V �100
V ra ted
As the load current increases, the voltage drop in the transformer windings also increases & hence the voltage regulation increases.
The Regulation of a transformer should not more then 8%.
In a transformer lower percentage regulation is a better regulation.
EFFICIENCY: The ratio of output power to the input power of a transformer is called the efficiency of the transformer up to certain amount of load, efficiency increases with the increases in load and the onward slightly decreases.
EFFICIENCY= OUTPUT =V2 I2 COS?
INPUT IN PUT POWER
The above formula shows that efficiency depends not on the value of load current, but also on the power factor (COS?) of the load current.
If the load is purely resistive ,the power factor i.e. cos? =1.
.
. . EFFICIENCY = V2 I2
IN PUT POWER
The efficiency of a transformer is generally more. Compared to other rotation machines, as it is a static apparatus, involving no frictional losses. Generally efficiency of a good transformer is more then 90% at full load.
A load test is performed in the laboratory to check its performance before it is actually used on site.
For getting the values of regulation & efficiency at different power factor, the different types of loads that is inductive or capactive load should be used.
PROCEDURE:
1. Set up the connections as per circuit diagram.
2. At first, open single pole switch, which is connected in load circuit & note down no load voltage, current & input power.
3. Now, close single pole switch & gradually increases the load current up to rated value.
4. Note down readings of volt meter, watt meter & am meter.
5. Make calculations.
6. Draw graph percentage efficiency V/S load current.
7. Percentage regulation V/S load current.
OBSERVATION TABLE:
Sr. no.
Secondary load current in amp. I2 Secondary voltage in Volts V2 In put power in watts Win Out put power in watts Wout= V2I2 % Efficiency = V2 I2/Win % Reg.
E0 �V/V
X 100
1
2
3
4
5
6
7
8
9
10
CALCULATION:
CONCLUSION: The experiment is performed & seen that as load current increases voltage drop in transformer winding is also increases, voltage and hence voltage regulation is increased.
Also we increase the load current the efficiency is also increase up to certain limit after this efficiency decreases with increase load current.