The Back-to-Back Test, also known as the Sumpner's Test or Regenerative Test, is an essential method used to determine the temperature rise, iron loss, copper loss, efficiency, and voltage regulation of a transformer under full-load conditions. This test is especially beneficial for large transformers, where conducting a direct full-load test is impractical due to the lack of an adequate load and the associated high power wastage.
Why Back-to-Back Test is Needed?
In small transformers, full-load testing is easy and manageable. However, in large transformers, arranging a suitable full-load is expensive and energy-inefficient. Thus, Sumpner's test allows us to simulate full-load conditions without actual energy wastage.
Key reasons:
- Suitable full-load may not be available
- High cost of energy wastage in dummy loads
- Efficient testing with minimal power input
Circuit Diagram of Sumpner's Test
Connection Setup
To perform this test, two identical single-phase transformers (T1 and T2) are connected as follows:
- Primary Windings: Connected in parallel and supplied with rated voltage and frequency.
- Secondary Windings: Connected in series opposition. The terminals are arranged such that the induced voltages cancel out, resulting in zero net voltage, i.e., they act as an open circuit initially.
Instruments Used
- Voltmeter V1: Measures the input voltage
- Ammeter A1: Measures the no-load current
- Wattmeter W1: Measures the iron/core losses
- Voltmeter V2: Checks polarity of secondary winding (should be 0 if connected in opposition)
- Ammeter A2: Measures secondary current under full-load condition
- Wattmeter W2: Measures full-load copper losses
Polarity Check for Secondary
To ensure the correct series opposition:
- Connect terminals B and C.
- Measure voltage between A and D using V2.
- If V2 = 0, polarity is opposite (correct setup).
- If V2 = 2 × rated secondary voltage, reverse connections to get series opposition.
Working of Back-to-Back Test
Step 1: Iron Loss Measurement
- Primary connected to supply at rated voltage.
- Secondary in series opposition → no circulating current.
- Wattmeter W1 records iron loss (core loss) of both transformers.
- Ammeter A1 shows total no-load current (2I0).
Step 2: Copper Loss Measurement
- A low voltage is applied to secondary windings using a regulating transformer.
- Voltage is adjusted until Ammeter A2 shows full-load current.
- This causes full-load current to circulate in both primary and secondary.
- Wattmeter W2 records full-load copper loss (2Pcu).
Observations in Sumpner's Test
| Instrument | Reading Measured |
|---|---|
| Ammeter A1 | No-load current of both transformers (2I0) |
| Voltmeter V1 | Rated input voltage |
| Wattmeter W1 | Iron loss of both transformers (2Pi) |
| Ammeter A2 | Full-load secondary current |
| Voltmeter V2 | Total voltage across series connected secondaries |
| Wattmeter W2 | Copper loss of both transformers (2Pcu) |
Temperature Rise Test (Heat Run Test)
The transformers are operated back-to-back for 36 to 48 hours, and oil temperature is measured periodically (e.g., every hour). This helps determine:
- Temperature rise in transformer oil and windings
- Withstand capacity under continuous full-load conditions
Calculation Formulas
Efficiency of Transformer
η = Output Power / (Output Power + Total Losses)Where total losses = iron loss + copper loss
Per Transformer Losses
- Iron loss per transformer = W1 / 2
- Copper loss per transformer = W2 / 2
Equivalent Resistance and Reactance
Based on voltage and current readings, the equivalent parameters can also be calculated using test results.
Advantages of Sumpner's Test
- Tests full-load conditions without real load
- Low power consumption (only losses are supplied)
- Determines:
- Iron loss
- Copper loss
- Efficiency
- Equivalent circuit parameters
- Temperature rise
Disadvantages of Sumpner’s Test
- Requires two identical transformers
- Not suitable if only a single unit is available
Conclusion
- The Back-to-Back Test or Sumpner’s Test is an efficient, economical, and practical method to test large transformers under full-load conditions without wasting real power. It provides critical insights into transformer performance, including iron and copper losses, temperature rise, and efficiency.
If you're preparing for competitive exams like SSC JE, RRB JE, or GATE EE, understanding this test is crucial for both theoretical and practical knowledge.
FAQs on Transformer Back-to-Back (Sumpner's) Test
1. What is the back-to-back test in transformers?
- The back-to-back test, also known as Sumpner’s test, is a method to test transformer performance under full-load conditions without wasting real power. It uses two identical transformers to measure temperature rise, iron loss, and copper loss.
2. Why is the back-to-back test also called the Sumpner’s test?
- It is called Sumpner's test after the engineer who developed it. The test simulates full-load conditions using two identical transformers connected in a regenerative loop, minimizing power wastage.
3. What are the main parameters determined by the back-to-back test?
The test helps determine iron loss, copper loss, efficiency, equivalent circuit parameters, and temperature rise under full-load conditions.
4. Why is the secondary winding connected in series opposition in Sumpner’s test?
- The secondary windings are connected in series opposition to cancel the voltages, ensuring no circulating current flows initially. This helps measure iron loss accurately under no-load conditions.
5. How is temperature rise calculated in Sumpner’s test?
- The test is run for 36–48 hours and the transformer oil temperature is recorded periodically. The highest stable temperature indicates the transformer's thermal performance under full-load.
6. What is the advantage of Sumpner's test over OC and SC tests?
- Unlike open-circuit and short-circuit tests, Sumpner's test simulates real full-load conditions, enabling accurate measurement of both losses and thermal performance simultaneously.
7. What is the main limitation of the back-to-back test?
- The primary limitation is the requirement for two identical transformers, which may not always be available for testing.