Transformers must be tested under conditions as close as possible to real operation to accurately evaluate their performance. Sumpner’s Test, also known as the Back-to-Back Test, allows two identical transformers to operate under simulated full-load conditions without requiring a large external load. It helps determine iron loss, copper loss, efficiency, voltage regulation, and heating behaviour. This article provides information about its working, circuit setup, procedure, and calculations.
Sumpner’s Test Overview
Sumpner’s Test, also known as the Back-to-Back Test, is a practical method for testing a transformer under simulated full-load conditions without an external load. It is used to determine basic performance values such as iron loss, copper loss, efficiency, voltage regulation, and heating behaviour.
In this test, two identical transformers are connected so that full-load current circulates between them, while the supply provides only the power needed to cover the losses. Because of this arrangement, Sumpner’s Test is useful when direct loading would be expensive, inefficient, or impractical.
Why Sumpner’s Test Is Economical for Large Transformers
For large transformers, direct loading is often uneconomical because a large external load is needed to absorb the full output power during the test. This leads to high power waste, higher cost, and more difficult test arrangements. Sumpner’s Test avoids this by creating full-load conditions while drawing only the power required to supply the transformer losses. This makes it a practical and economical method for testing large-capacity transformers.
Working Basics of Sumpner’s Test
Sumpner’s Test reproduces near full-load operating conditions by making two identical transformers experience rated voltage and rated current at the same time, while the supply delivers only the total losses. When the primaries are energized at rated voltage, both transformers develop normal core flux, so iron loss occurs as in normal service. When rated current is circulated through the windings, full-load copper loss is also produced. In this way, the test simulates actual operating conditions more closely than open-circuit and short-circuit tests performed separately.
Requirements and Circuit Setup of Sumpner’s Test
Requirements and Circuit Setup of Sumpner’s Test
Main Requirements for Sumpner’s Test
For accurate results, Sumpner’s Test requires two transformers with the same rating, the same voltage ratio, correct polarity, and closely matched impedance and operating characteristics.
Standard Circuit Setup of Sumpner’s Test
In the standard arrangement, the primary windings of the two transformers are connected in parallel across the rated supply so that both operate at rated voltage. The secondary windings are connected in series opposition. An adjustable low voltage is then injected into the secondary loop to circulate the rated current. Measuring instruments are connected to record the applied voltage, circulating current, and wattmeter readings during the test.
Polarity Check in Sumpner’s Test
Before the test begins, the secondary connections must be checked carefully. If the voltage across the free terminals of the series-connected secondaries is nearly zero, the polarity is correct and the opposition connection is proper. If the measured voltage is approximately twice the rated secondary voltage, the connection is incorrect and the terminals must be rearranged before continuing the test.
Procedure and Meter Readings in Sumpner’s Test
Before starting the test, check all instrument connections and confirm correct polarity of the secondary windings. After that, apply the rated voltage to the primary windings of both transformers. Under this condition, both transformers operate at rated flux, and the primary-side wattmeter W1 measures the combined iron loss of the two transformers.
Next, inject a small adjustable voltage into the secondary loop through the regulating transformer. Increase this injected voltage gradually until the secondary ammeter shows the rated full-load current. At that point, the wattmeter W2 measures the combined full-load copper loss of the two transformers.
If heating performance is to be observed, the test can be continued for a longer period under the same conditions. The recorded readings are then used to determine the losses, efficiency, voltage regulation, and temperature-rise behaviour of the transformers under simulated full-load operation.
| Instrument | What It Measures |
|---|---|
| Primary Voltmeter | Rated input voltage |
| Primary Ammeter | Total no-load current of both transformers |
| Wattmeter W1 | Combined iron/core loss of both transformers |
| Secondary Ammeter | Full-load circulating current |
| Secondary Voltmeter | Secondary condition for polarity and testing |
| Wattmeter W2 | Combined full-load copper loss of both transformers |
Performance Calculations from Sumpner’s Test
Loss Calculation in Sumpner’s Test
If:
• W1 = combined iron loss of both transformers
• W2 = combined full-load copper loss of both transformers
Then for one transformer:
• Iron loss per transformer = W1 / 2
• Full-load copper loss per transformer = W2 / 2
These two values represent the principal losses under normal operating conditions and are used for further performance evaluation.
Efficiency and Voltage Regulation in Sumpner’s Test
If one transformer delivers an output of:
Output = V2 × I2 × cosφ
then the efficiency at full load is:
Efficiency = Output / (Output + Iron Loss + Full-load Copper Loss)
Using the values obtained from the test, this becomes:
Efficiency = Output / [Output + (W1 / 2) + (W2 / 2)]
At a load fraction x, the copper loss varies approximately as x², so:
Efficiency at x load = Output at x load / [Output at x load + (W1 / 2) + x²(W2 / 2)]
This allows efficiency to be estimated at different load levels and power factors.
Voltage Regulation from Sumpner’s Test
Sumpner’s Test does not measure voltage regulation directly with a load bank, but it provides the loss data needed for estimating full-load performance. In practice, voltage regulation is usually determined with the aid of equivalent resistance and reactance obtained from transformer test data. For this reason, Sumpner’s Test is mainly valued for realistic loss and heating evaluation, while regulation is commonly estimated from the corresponding equivalent parameters.
Advantages, Limitations, and Applications of Sumpner’s Test
Advantages of Sumpner’s Test
• Allows testing under near full-load conditions without a large external load
• Produces iron loss and full-load copper loss at the same time
• Makes temperature-rise observation possible under realistic operating conditions
• Uses much less input power than direct loading
• Gives more practical performance data than separate open-circuit and short-circuit tests
Main Limitation
• Requires two transformers with the same rating and voltage ratio
• Requires correct polarity and closely matched impedance characteristics
• May give inaccurate circulating-current conditions if the two transformers are not well matched
Applications of Sumpner’s Test
• Requires two transformers with the same rating and voltage ratio
• Requires correct polarity and closely matched impedance characteristics
• May give inaccurate circulating-current conditions if the two transformers are not well matched
Conclusion
Sumpner’s Test is an effective method for evaluating transformer performance under simulated full-load conditions. When a rated current is circulated between two identical transformers, both iron and copper losses occur simultaneously, while the supply provides only the power needed to cover them. This allows accurate measurement of efficiency, voltage regulation, and heating behaviour. Because of its economical operation, the Back-to-Back Test remains used in transformer testing.
Frequently Asked Questions [FAQ]
Why are two transformers used in Sumpner’s Test?
To circulate full-load current between them while the supply provides only the power needed for losses.
Can Sumpner’s Test use non-identical transformers?
It is best performed with transformers that have the same rating, voltage ratio, and similar impedance to maintain accurate results.
What transformers are tested using Sumpner’s Test?
It is mainly used for medium- and large-capacity transformers where direct loading is impractical.
What safety checks are needed before starting Sumpner’s Test?
Verify correct polarity, check instrument connections, and gradually increase the injected voltage.
How long can Sumpner’s Test run?
It can run for a long period because the supply power equals only the transformer losses.
How is temperature rise evaluated in Sumpner’s Test?
Both iron and copper losses occur simultaneously, producing heating conditions similar to those in real operating conditions.
