Frequently Asked Questions
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How is primary power related to secondary power?
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They will be the same. The power into the transformer is about equal to the power out of the transformer because of the high efficiency of the transformer (95–99%).
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If the voltage of the secondary coil is half as large as the voltage applied to the primary coil, how will the secondary coil current be related to the primary coil current?
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To keep the power in equal to the power out, the secondary coil current must double:
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To have a larger voltage on the secondary coil of a transformer, will there be more turns than the primary coil or fewer turns?
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Therefore, if voltssec is to be larger, there must be more secondary turns and fewer primary turns.
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If the voltage applied to the primary coil of a transformer is 55 volts AC and the secondary coil has a voltage of 165 volts AC induced in it, what is the ratio of turns between the primary coil and the secondary coil?
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Voltage is proportional to the turns of the coils. Therefore:
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If a transformer secondary coil has a power of 350 VA, what is the power of the primary coil?
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350 VA power in = 350 VA power out.
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How is a transformer primary winding different from a transformer secondary winding?
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The primary winding has the voltage applied to it, while the secondary winding has the voltage induced in it.
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What happens to the transformer secondary voltage as the load increases?
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The voltage will decrease slightly because of the voltage drop across secondary winding caused by the increased current flow through the winding.
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What are the two main types of transformer losses?
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Core and coil (Copper or I2R) losses.
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What is done to compensate for secondary voltage drop?
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Extra turns are added to the secondary winding, adding induced voltage to compensate for the voltage drop across the secondary winding.
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What are the primary and secondary voltages of a transformer related to the voltage ratio related to the primary-secondary current ratio?
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For the power out to equal the power into the transformer, the two must be inversely proportional.
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What is meant by the term percent voltage regulation?
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It’s the difference between the no-load and full-load voltage expressed as a percentage of the rated voltage. It expresses how much the output voltage changes as the load changes.
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What transformer turns ratio is required if the primary current is 7 amps and the secondary current is 21 amps?
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The ratio of transformer turns is inversely proportional to the current. Therefore:
The ratio is 1 to 3.
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What are the three transformer functions?
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Step up, step down, and isolation.
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Which winding is always the primary winding of a step-up transformer?
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The primary is the winding with the voltage applied to it, and if the transformer is a step-up, then the primary is the lower voltage or the X winding.
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What is the purpose of the voltage correction taps on a transformer?
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Using different turns on the transformer primary winding, the same nominal secondary voltage can be induced with primary voltages that are higher or lower than the nominal primary voltage.
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What are the four considerations to remember when choosing a transformer?
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- Supply and load voltages must match the transformer voltage rating.
- The KVA rating must be sufficient for the load.
- The transformer enclosure must be correct for the environment.
- The transformer temperature rating must be high enough for the environment’s ambient temperature.
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A three-phase transformer has secondary-phase windings that each has 240 volts induced in them. What will the secondary line voltage of the transformer be if the windings are connected in a wye? In a delta?
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In Wye (Y or Star) connection, the voltage would be:
In Delta connection, the voltage would be:
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Which three-phase transformer configuration will not require phase-winding conductors as large as the secondary line conductors?
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The line current in a delta-connected system is 1.73 times more than the current through each phase conductor; therefore, the delta phase conductors do not have to be as large as the line conductors.
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List one advantage and disadvantage of a delta/delta three-phase transformer compared to a delta/wye three-phase transformer.
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The delta/delta transformer will not need phase conductors as large as the same capacity delta/wye transformer. Unlike the delta/wye transformer, the delta/delta transformer secondary cannot supply more than one voltage.
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One winding of a three-phase delta transformer secondary has failed. What percentage of the original transformer three-phase KVA is still available?
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Because one winding has failed, there will not be two windings to supply current to the load on two of the line connections. Line current is thus limited and is the same as the phase current rather than the 1.73 x phase current. With the line current now 1 ÷ 1.73, or 57.8%, of the original line current, only 57.8% of the original transformer KVA is available to supply the load.
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A delta transformer secondary must supply both three-phase, 480-volt loads and single-phase, 480-volt loads. Describe how to size the transformer.
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Because each winding of the transformer must be able to supply the load connected to it, balance the single-phase loads as evenly as possible across all three phases. Next, take the three-phase load and then put one-third on each phase winding. Finally, choose the phase with the largest total load and then multiply the KVA of that phase by 3. This will be the minimum transformer KVA.
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Describe how to size a three-phase transformer that supplies both single-phase line to neutral and three-phase loads.
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Remember that each of the three windings of the transformer must be able to supply the load connected to it. This means the largest single-phase load plus one-third of the total three-phase load. Balance the amperage of the single-phase loads as evenly as possible between each of the three lines and the neutral. Next, because the line amperage is the same as the phase amperage in a wye system, add the full load amperage of the three-phase loads to each of the three lines. Choose the largest line amperage, and using the three-phase power formula, calculate the total KVA required.
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Which three-phase transformer configuration is rarely used?
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The wye/wye connection. It transfers harmonics and noise from the load on the secondary out to the primary because the common point or neutral on both primary and secondary are connected together through the ground. The phase voltages are also often unstable as loads change.
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List the four types of three-phase transformer connections.
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Delta/delta; delta/wye; wye/delta; and wye/wye.
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How is an autotransformer different from a regular isolating transformer?
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The secondary winding is part of the primary winding. The two windings are not isolated from each other.
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How is a K-rated transformer different from a normal transformer?
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The K-rated transformer has conductors that are sized to withstand the extra heat generated by harmonic currents and special core designs that minimize harmonic flux effects. The neutral conductors are also sized up to 200% larger to carry the extra currents resulting from triplen harmonics.
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What are the two types of transformer cooling?
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Cooling with air or with liquid.
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Which type of core construction is most common with three-phase transformers?
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The core type in which the windings are placed around the transformer core. This reduces the size and weight of the transformer.
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What is transformer temperature rise?
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The average temperature increase within the transformer winding above the ambient temperature when the transformer is fully loaded.
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List the forces that affect transformer windings.
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Axial forces that tend to push or telescope the windings apart in the direction of the transformer core. Radial forces that tend to force conductors in the windings out at right angles to the core.
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The transformer insulation temperature rating is based on what three factors?
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An ambient temperature of 40ºC, the temperature rise the transformer was designed for, and an extra temperature value to compensate for hot spots within the transformer winding.
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What is the transformer BIL rating?
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The Basic Impulse Level is a measure of the level of transient voltage spikes, which the transformer windings can withstand without damage to the insulation.
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Why is a routine transformer maintenance program important?
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The maintenance program can extend transformer life and identify problems that may cause transformer failure in time to plan for orderly replacement or repair with minimal disruption of service.
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If a transformer has a temperature rise of 80ºC if it is fully loaded and the ambient temperature is 39ºC, what is the average winding temperature?
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The average winding temperature is the sum of the ambient temperature and the temperature rise of the transformer: 80ºC + 39ºC = 119ºC.
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Describe the non-energized tests that can be performed on an air-cooled transformer.
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Test the insulation with a megohmmeter; the high-voltage winding should have a resistance of more than 1,000 meg-ohms and the low-voltage winding more than 100 meg-ohms. Use a turns ratio tester to test for shorted turns in the winding. Visually inspect the windings and the blocking that holds them in place to make sure they are secure.
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How do large short-circuit currents affect transformer windings?
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Short-circuit currents may cause forces 100 to 250 times greater than normal to deform transformer windings. This may cause the insulation to crack and fail or windings to come into contact with each other or the core, which will cause the failure of the transformer.
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List the advantages of a liquid-cooled transformer.
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It is more efficient, has a longer life, and has a higher overload capacity.
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How do voltage transients affect transformer windings?
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They weaken the insulation and may cause cracks, resulting in insulation break- either instantaneously if large enough or over time.
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If a dry transformer is to be located outdoors, what is the special requirement for the enclosure?
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Art. 450.22 requires that it must be a weather-proof enclosure.
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What are the main types of liquid-cooled transformers?
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Arts. 450.23 through 450.26: Oil-cooled, less flammable liquid-cooled (with a flashpoint not less than 300ºC), Askarel insulated, and nonflammable liquid-cooled.
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What are the basic requirements for all transformer installations?
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Arts. 450.8 through 450.13. The transformer must be provided with a case or enclosure to protect it from damage. The ventilation must be sufficient to keep the temperature rise within the transformer specifications. The transformer must also be located, so it is readily accessible for maintenance.
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Can a 100 KVA nonflammable-liquid-filled transformer be located indoors?
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Art. 450.24 allows such an installation but only if it is less than 35,000 volts and the gases that might result from arcing are vented outside. In addition, there must be a containment area for the liquid.
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What must be done if a 500 KVA oil-cooled transformer is to be located indoors?
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Art. 450.26 requires that the oil-cooled transformer must be installed in a proper transformer vault.
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What is always required when installing an oil-filled transformer indoors or outdoors?
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Art. 450.26 states that an oil-filled transformer located indoors must be in a vault. Art. 450.43(B) requires that the sill should confine the oil to the vault. In Art 450.27, there is also a requirement for an oil containment area sufficient to hold the transformer oil.
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Can a nonflammable liquid-filled transformer be installed indoors without a vault?
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Yes, according to Art. 450.24, as long as the transformer voltage is 35,000 volts or less, a liquid confinement area is provided—along with a pressure relief vent—and gases generated by arcing inside the tank are either absorbed or vented to the outside.
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What is the main difference between KNAN and ONAN in Oil-Type Distribution Transformers?
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In oil-type distribution transformers, the designation of cooling types depends on the fire point of the oil. Mineral oils are denoted by the letter “O” since their fire point is below 300°C. Non-mineral oils like synthetic ester, silicone oil, and natural ester (vegetable oils) are denoted by the letter “K” as their fire point is greater than 300°C. While non-mineral oils are costlier, they offer various benefits depending on the application of the transformer.
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What is the difference between radial and loop feed in a transformer?
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Radial feed Transformers have three bushings (H1, H2 and H3) for high voltage. There is the bushing for each phase to accommodate the incoming high voltage cables. A Grounded Wye connected Transformer will also have an H0 bushing.
Loop feed transformers contain six bushings (H1A, H2A, H3A, H1B, H2B, and H3B) for high voltage. For each phase, there are two bushings. This enables the user to wire all the transformers in a loop configuration. The customer can also connect two feeds to the transformer this way. The A and B feeds can be switched using a four-position switch.
