Sumpner's Test (Back-to-Back Test) of Transformer — Circuit, Procedure & Calculations
What is Sumpner's Test?
Sumpner's Test, also known as the back-to-back test or regenerative test, is a method used to determine the efficiency and temperature rise of large power transformers without actually loading them to full capacity. The test uses two identical transformers connected in a specific configuration where the power drawn from the supply equals only the total losses of both transformers combined.
This test was developed to overcome the practical difficulty of conducting full-load tests on large transformers (100 kVA and above), where the energy wasted during testing would be enormous and expensive. By using the back-to-back arrangement, the transformers are effectively loaded to full capacity while consuming only a fraction of the rated power.
Why Sumpner's Test is Needed
For large power transformers rated at several MVA, a direct full-load test is impractical because:
- Energy waste: A 10 MVA transformer at full load would consume enormous power just for testing
- Load arrangement: Arranging a suitable load bank of matching capacity is expensive
- Temperature rise: The test must run for several hours to determine thermal performance
- Cost: The electricity bill for a prolonged full-load test would be prohibitive
Sumpner's test solves all these problems by requiring only enough power to supply the losses (typically 2-5% of rated power), while still subjecting both transformers to full-load conditions.
Circuit Diagram & Apparatus
The test requires the following apparatus:
- Two identical transformers of same rating
- One wattmeter, voltmeter, and ammeter for the primary circuit
- One wattmeter, voltmeter, and ammeter for the secondary circuit
- A regulating transformer (variac) for injecting voltage into the secondary
- Single-phase AC supply at rated voltage and frequency
Fig: Circuit diagram of Sumpner's Test (Back-to-Back Test) showing two identical transformers with primary windings in parallel and secondary windings in series opposition.
Test Procedure
Step 1 — Primary Connection: Connect the primary windings of both transformers in parallel across the rated supply voltage V₁. This ensures both cores are magnetised at rated flux density.
Step 2 — Secondary Connection: Connect the secondary windings in series such that they are in phase opposition. A voltmeter connected across the series combination should read zero (or near zero), confirming opposing polarities.
Step 3 — Verify Phase Opposition: If the voltmeter reads twice the secondary voltage (2V₂), the connections are aiding — reverse one secondary winding to achieve opposition.
Step 4 — Measure Iron Losses: With no voltage injected in the secondary, record the wattmeter reading on the primary side (W₁). This gives the combined iron losses of both transformers.
Step 5 — Inject Secondary Voltage: Using the regulating transformer, inject a small voltage into the secondary circuit. Gradually increase it until the ammeter on the secondary side reads the rated full-load current (I₂).
Step 6 — Measure Copper Losses: Record the wattmeter reading on the secondary side (W₂). This gives the combined full-load copper losses of both transformers.
Measurement of Iron Losses
When the primary windings are energised at rated voltage with secondaries in phase opposition, the net EMF in the secondary loop is zero. Therefore, no current flows in the secondary windings, and the transformers behave as if they are on no-load.
The primary wattmeter reads:
Iron loss per transformer = W₁ / 2
Since both transformers are identical and operate at the same flux density, the iron losses are equally divided.
Measurement of Copper Losses
When voltage is injected via the regulating transformer into the secondary loop, current circulates through both secondary and primary windings (reflected). The injected voltage only needs to overcome the impedance drops of both transformers.
The secondary wattmeter reads:
Copper loss per transformer = W₂ / 2
The injected voltage required is typically 5-10% of the rated secondary voltage, as it only needs to drive current through the combined leakage impedances.
Efficiency Calculation
Once both losses are determined, the efficiency of each transformer at any load fraction can be calculated:
Where:
x = fraction of full load (0 to 1)
S = rated kVA of transformer
cosφ = power factor of load
Pᵢ = W₁/2 (iron loss per transformer)
Pcu = W₂/2 (full-load copper loss per transformer)
Maximum efficiency occurs when iron losses equal copper losses:
Advantages & Disadvantages
Comparison with Open Circuit & Short Circuit Tests
Applications
- Factory acceptance testing: Manufacturers use this test to verify efficiency guarantees on large power transformers before dispatch
- Temperature rise testing: The test can run for 6-8 hours to determine maximum temperature rise under full-load conditions as per IS 2026 / IEC 60076
- Heat run test: Used to validate cooling system design (ONAN, ONAF, OFAF) by monitoring oil and winding temperatures
- Loss segregation: Accurately separates iron and copper losses for efficiency certification
- Research and development: Comparing core materials and winding designs under identical loading conditions
Frequently Asked Questions
1. Why is Sumpner's test also called the back-to-back test?
Because the two transformers are connected with their secondary windings in series opposition — effectively "back-to-back." The opposing EMFs cancel out, creating a closed loop where current circulates through both transformers without delivering power to any external load.
2. Can Sumpner's test be performed with non-identical transformers?
No. The test requires two transformers of the same rating, turns ratio, and impedance. If they are not identical, the iron losses won't divide equally, the secondary EMFs won't perfectly cancel, and the results will be inaccurate. For non-identical transformers, individual OC/SC tests are preferred.
3. What is the total power drawn from the supply during Sumpner's test?
The total power drawn equals the sum of iron losses (from primary supply) and copper losses (from regulating transformer). For a typical transformer with 2% losses, testing two 1000 kVA units would require only about 40 kW instead of 2000 kW for a direct load test.
4. How does Sumpner's test help in determining temperature rise?
Since both transformers carry full-load current in their windings and operate at rated flux density, they generate the same heat as under actual full-load conditions. By running the test for several hours and monitoring winding/oil temperatures, the maximum temperature rise can be determined without wasting full-load power.
5. What is the difference between Sumpner's test and Swinburne's test?
Swinburne's test is a no-load test for DC machines that predetermines efficiency from measured losses. Sumpner's test is specifically for AC transformers using two identical units in back-to-back configuration. Both are indirect methods that avoid full-load power consumption, but they apply to different machines and use different principles.