BACK TO BACK TEST OF TWO SIMILAR SINGLE PHASE TRANSFORMERS
OBJECT : To determine the % voltage regulation of a single phase transformer.
APPRATUS :
Sr No. Item Req. No. Capacity Remarks
1. Transformer 2 230v/115v
115v/230v To perform an experiment
2. Voltmeter (a.c.) 1 0-300v To measuer the voltage
3. Voltmeter (a.c.) 1 0-15v To measure the voltage
4. Ammeter (a.c.) 1 0-10A To measure the current
5. Load Resistance 1 53??10A To very the current
Theory : Back to back test is used to find out % reglution & efficiency of two similar transformers. In back to badk connections primaries of the two transformers are connected in parallel across the same a.c. supply. The secondaries are so connected that their potentials are in oppossitions to each other. This type of connections is called back to back connections.
Two transformers can be connected in parallel. If their voltage ratio, KVA ratung and % impedance and polarity are same. The same polarity meaning their by the instanteneous positive terminal of the other transformer is connected with instanteneous negative terminal of the other.
This can be checked by a double range (double the range of secondary rated voltage) voltmeter called polarity test. If the voltmeter reads double the voltage of secondary voltage of any one transformer, then the polarity is not connected. To make the polarity correct inter change any terminal of any one side of the transformer ,and again see volt meter reading. If the voltmeter shows zero readingthen we can say that the transformers are connected in same polarity.
When exactely two similar transformers are connected in parallel on their secondary side with the potential difference betweentwo transformers is zero and hence no secondary current flows. So no flux to primary flux i.e. secondary m.m.f. is i.e. in other words the transformers be have as their secondary open circuited. Now, if we connect a load on secondary side of any transformer, as shown in figure and if we load it, then only one transformer is loaded and its voltage gets reducded, as the load increase. While the other transformer behave as open circuited on the secondary side. This happens because of very haigh resistance of voltmeter which is connected between two terminals of secondary side of the transformers and therefore the voltmeter reads directely the voltage difference between two secondaries. This voltage differnce is same as the difference of voltage between no load and load condition of the transformer. And hence the regulation can be find out by dividing this voltage difference by secondary rated voltage.
Let, E0 = voltmeter reading = V =voltage drop
E = secondary rated voltage
% reglution = voltmeter reading 100
Rated voltage
PROCEDURE :
1. Set up connection as per diagram.
2. First perform polarity test and check that two similar transformers are connected by same polarity or not with double range voltmeter.
3. After gatting polarity, replace double range voltmeter by a very low range voltmeter.
4. Increase load up to full load of any one transformer .
5. Note down reading of ammeter ,voltmeter etc. Calculate regulation of transformer.
6. Draw graph of % regulation v/s load current.
OBSERVATION TABLE :
Sr No. Load current in amps.
IL Voltage drop in volts
E0 � V = V % Reglulation
= V 100
115
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
CALCULATIONS :
CONCLUSION :
The experiment is performed and it is seen that during back to back test as we increase load current, then the voltage drop is also increased and hence %reglution is also increased.