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110kv Power Transformer
Installation: Outdoor Ambient temperature: -30C-+40C Altitude: 1000m Relative Humidity: 90% (25C) |
| Single and Three Phase Transformer Questions |
| Temperature and Heat Related Transformer Questions |
| Reverse Transformer Questions |
| Type of Transformers Questions |
| Copper and Aluminum Transformer Questions |
| Voltage and Electrical Load Transformer Questions |
| Miscellaneous Transformer Questions |
| Energy Efficiency Transformer Questions |
| About Voltage Converters / Transformers |
| About Power Inverters |
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What is a Transformer? What is a transformer and how does it work? How are transformers classified? What are taps and when are they used? What is the difference between “Insulating,” “Isolating,” and “Shielded Winding” transformers? Can transformers be operated at voltages other than nameplate voltages? Can 60 Hz transformers be operated at 50 Hz? Can transformers be used in parallel? Can Acme Transformers be reverse connected? Can a Single Phase Transformer be used on a Three Phase source? Can Transformers develop Three Phase power from a Single Phase source? How do you select transformers? Why should Dry-Type Transformers never be over-loaded? Are temperature rise and actual surface temperature related? What is meant by “impedance” in transformers? Why is impedance important? What terminations are provided? Can 60 Hz transformers be used at higher frequencies? What is meant by regulation in a transformer? What is temperature rise in a transformer? What is “Class” in insulation? Is one insulation system better than another? Can tr ansformers listed in this catalog be reconnected as autotransformers to increase their KVA rating? What is BIL and how does it apply to transformers listed in this catalog? What is polarity, when associated with a transformer? Can Single Phase Transformers be used for Three Phase applications? Does ACME provide “Zig - Zag” Grounding Transformers? How do you select a transformer to operate in an ambient higher than 40° centigrade? What is exciting current? Will a transformer change Three Phase to Single Phase? Can air cooled transformers be applied to motor loads? How are transformers sized to operate Three Phase induction type squirrel cage motors? Why are Small Distribution Transformers not used for Industrial Control Applications? Can 4-Winding Single Phase Transformer be auto-connected? What about balanced loading on Three Phases? What is meant by “Balanced Loading” on Single Phase Transformer applications? What are typical applications for transformers? How are transformers classified? How are power transformers used? What is the purpose of an audio transformer? What is the simplest type of transformer? What is the purpose of Radio frequency transformers? In transformers what is the core of the coil made of? |
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Dry Type Vented Dry Type Non-Ventilated Dry Type, Encapsulated, Enclosed Dry Type, Encapsulated Core & Coil Bushing Potential Device Distribution Power Secondary Substation |
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What is a Transformer? What is a transformer and how does it work? A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle. A transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils. A transformer is a static electrical device that transfers energy by inductive coupling between its winding circuits. Transformers range in size from thumbnail-sized used in microphones to units weighing hundreds of tons interconnecting the power grid. A wide range of transformer designs are used in electronic and electric power applications. Transformers are essential for the transmission, distribution, and utilization of electrical energy. Types of Transformers Classification of transformers How are transformers classified? Transformers are frequently classified according to their uses; the details of construction depend on the intended application. 1.According to method of cooling a.Self-air–cooled (dry type) b.Air-blast–cooled (dry type) c.Liquid-immersed, self-cooled d.Oil-immersed, combination self-cooled and air-blast e.Oil-immersed, water-cooled f.Oil-immersed, forced-oil–cooled g.Oil-immersed, combination self-cooled and water-cooled 2.According to insulation between windings a.Windings insulated from each other b.Autotransformers 3.According to number of phases a.Single-phase b.Polyphase 4.According to method of mounting a.Pole and platform b.Subway c.Vault d.Special 5.According to purpose a.Constant-voltage b.Variable-voltage c.Current d.Constant-current 6.According to service a.Large power b.Distribution c.Small power d.Sign lighting e.Control and signaling f.Gaseous-discharge lamp transformers g.Bell ringing h.Instrument i.Constant-current j.Series transformers for street lighting |
| Classification parameters |
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Transformers
There are many different types of voltage transformers. A high voltage transformer operates with high voltages. Typically, these voltage transformers are used in power transmission applications, where voltages are high enough to present a safety hazard. High voltage transformers are built to handle elevated amounts of electrical energy in the range of 600 to 5,000 volts, although custom voltage transformers are available as well. A type of instrument transformer, high voltage transformers are often used for metering and protection in high-voltage circuits and in electrostatic industrial and scientific applications. Because they have the ability to step up primary voltage to a very high voltage, they are often also referred to as a power transformer. Due to the high voltage and frequency that it must handle, a high voltage transformer has a significantly different core geometry, winding techniques, and insulation methods that ordinary transformers. For example, factors such as the volts/turn ratings of the secondary wire, insulating material dissipation, and corona level must be carefully considered. What is the difference between a K-rated and a de-rated transformer? What's the difference between a distribution and a power transformer? What actions can manufacturers take to make transformers more efficient? What's the difference between a distribution and an industrial transformer? What will a manufacturer do if I do not specify loss evaluation factors? Do I need to specify a harmonic spectrum when buying a transformer? I know there will be harmonics, but don't know the spectrum Why should the temperature in the transformer not become too high? Are high efficiency transformers also more reliable? Is it necessary to replace all transformers by high efficiency ones? Why is the rating of transformers given in kVA and not in kW? What is the difference between a K-rated and a de-rated transformer? What's the difference between a distribution and a power transformer? Both power and distribution transformers are used for T&D applications (transmission & distribution). The difference between power and distribution transformers refers to size & input voltage. Distribution transformers vary between 25 kVA and 10 MVA, with input voltage between 1 and 36 kV. Power transformers are typically units from 5 to 500 MVA, with input voltage above 36 kV. A third segment in the transformer market are industrial transformers, used by industry to purchase electricity at high voltage and step it down for use on site. The size of industrial transformers varies, but typical units are 1.6, 2.5 and 5 MVA. What actions can manufacturers take to make transformers more efficient? A manufacturer has various options to lower losses. To lower no-load loss, the manufacturer can change the core material. Another option is to use thinner laminates, which reduces eddy currents in the core. Or the manufacturer can reduce the induction in the core. To reduce load losses, the manufacturer can choose for a conductor material with higher conductivity (copper instead of Aluminium). Or the resistance of the winding can be reduced by choosing a larger conduction cross section. What's the difference between a distribution and an industrial transformer? In principle, both types of transformers are the same. But transformers used in industry are operated differently than distribution transformers. In industry, transformers are often more heavily loaded (during short time intervals even above 100%). In addition, due to non-linear loads in industry, the harmonic component in the load current will be higher. What will a manufacturer do if I do not specify loss evaluation factors? If you do not specify values for the A & B factor (see minute lecture TCO of transformers), the manufacturer has no means to assess whether you prefer low losses during the lifetime of the transformer, or a low purchase price. Most likely, the manufacturer will assume A = B = 0 euro/W. As a result, the capitalisation formula will include only the purchase price. The result of the purchasing process is likely to be a lowest-first cost transformer, but with high losses, and probably higher cost of ownership. Do I need to specify a harmonic spectrum when buying a transformer? If you know that the transformer will be loaded during its lifetime with harmonic currents, it is important to provide the harmonic spectrum to the manufacturer. The manufacturer needs to verify in the design stage, for the harmonic spectrum provided, whether the temperature in the transformer will not become too high due to the extra losses. Why should the temperature in the transformer not become too high? Besides destruction, the lifetime of a transformer is determined by its highest temperature (so-called hot-spot temperature). For an oil-cooled transformer according to IEC 60354, lifetime is reduced by a factor 2 for every 6 degrees increase in hot-spot temperature. For dry transformers, according to IEC 60905, the increase is 10 degrees. In particular for transformers serving harmonic loads, the hot-spot location is difficult to determine. Are high efficiency transformers also more reliable? There is no relation between transformer reliability, and losses. But one needs to take into account to provide sufficient cooling for losses, including harmonic losses, to avoid loss of lifetime. Is it necessary to replace all transformers by high efficiency ones? Over the past decades, losses in transformers have continued to decrease. Old transformers therefore have relatively high losses. In some cases, it may be worthwhile to consider to replace these older transformers. The cost for purchasing these new transformers needs to be recovered through electricity savings. When replacing existing transformers by high efficiency models, you need to verify whether the additional volume or weight of transformers will not cause problems. Why is the rating of transformers given in kVA and not in kW? kVA is the unit for apparent power. Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again. It occurs when on account of some »inertia« in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only, regardless of whether it aligns with the voltage or not. Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated (and selected) by apparent power. Can I use my several appliances at once with voltage transformer? Yes, As long as total wattage of all your appliances do not exceed than the wattage capacity of a voltage transformer. Also consider only 80% wattage capacity of voltage converter than wattage rating of your appliance(s) to avoid damage caused by power fluctuation. Yes, since there is no worldwide standard for the plug configuration, plug shapes and plug holes (electrical outlets) differ from _____ to _____. What is voltage regulation in a transformer? The voltage difference between loaded and unloaded output. To provide the proper secondary load voltage, extra primary windings cause the no-load secondary voltage to be 3-5% higher than the load voltage. Also known as "compensated windings." How do I figure how much chinking or caulking I need? Usage depends on the width of the joint and the thickness of application. The following chart is based on ÂĽ inch thickness of chinking and a no backer-rod standard "V" groove for caulking: What is the effect of under, or over voltage on the ratings of a transformer? If a transformer has lower than rated voltage applied, the KVA is reduced by the same percentage as the voltage is reduced. 1 or 2 percent overage will generally be tolerated by most transformers without reduction in KVA. Voltage in excess of 2% should not be applied unless overvoltage taps are provided. What is an electronic low voltage transformer? Conventional low voltage transformers, also called magnetic core & coil low voltage transformers can be extremely large and heavy, consist of an iron core and two sets of wires as described in the previous paragraph. An electronic low voltage transformer, on the other hand, also contains an electronic device, called an inverter, which allows the size of the low voltage transformer to be substantially smaller. Can I use several appliences at once on the same voltage converter/transformer? Yes, as long as total wattage of all your appliences does not exceed the wattage capacity of your transformer/converter. Try to set up your transformer to run at 80% of its wattage to avoid demaging your appliences by power flunctuations. What does voltage converter or transformer mean? In speaking general Voltage Converters and Voltage Transformers are used to convert electricity. North America and some other __________ operate on 110/120 Volt AC 60 HZ. While most of the world operates on 220/240 volts AC 50 HZ. Voltage Converters change the electricity to make work electrical appliances in different __________. Voltage converters/transformers are used to convert electricity. North America and some other countries operate on 110V/120V AC 60Hz, while most of other countries operate on 220/240 volts AC 50Hz. Voltage transformers convert the power into different voltage, so your equipement can be powered at different countries without being demaged. What is the Voltage and do I need an adaptor? Voltage is 220 volts, but you will need an adaptor for Thai plug which can be found in most convenience stores. What Effect Will 50 Cycle Voltage Have On A Transformer Rated For 60 Cycles Of Like Voltage? The effect will be higher no-load losses or iron losses, which in turn will tend to raise the temperature of the transformer. Many small types of transformers are rated for 50/60 cycles. This can be done quite successfully, as the amount of heat can be dissipated from small transformers without too much difficulty. How do I figure how much ridge vent I need to use? Take the square footage of house under roof X .48 divide this number by 20 (for V-600 products), by 17 (for V-400E or X-5) or by 13.5 (for V-300 or Fold-A-Vent products) this will give the lineal feet of product needed to meet a 1/150 vent ratio. If you are only going for a 1/300 (code minimum) vent ratio divide the above result by two. REMEMBER there MUST be an equal amount of soffit vent (in NFVA) to balance the ridge vent. How do I figure out how much bandwidth I need? You can never have enough! The same advice people give for PC processor speed can be given for bandwidth: buy as much as you can reasonably afford, since nobody ever has "too much" bandwidth. One of the main factors though, is price. What is a magnetic core & coil low voltage transformer? A low voltage transformer is an electrical device that reduces 120 volts (line voltage) into 12 volts or 24 volts (low voltage). It is sometimes made by winding two wires around an iron core with one wire connected to the primary side (line voltage side) and the second wire connected to the secondary side (low voltage side). How does an electronic low voltage transformer work? The inverter conditions the voltage to change direction at a frequency of about 20,000 times per second (called Hertz or Hz) as opposed to the "normal" power from your wall outlet, which changes direction at a frequency of 50Hz or 60Hz. The higher the frequency, the smaller the low voltage transformer can be. Most electronic low voltage transformers provide high frequency AC output. What is the advantage of an electronic low voltage transformer? Electronic low voltage transformers are very small and light compared to magnetic low voltage transformers, in most cases small enough that fixture manufacturers can often incorporate them within their lighting fixture rather than leaving the customer to find a hiding place. Even when not incorporated within the lighting fixture an electronic low voltage transformer is very easy to install in a small hidden location. If the drapery motors are low voltage controlled, do I have to plug in a transformer? No. The term “low voltage” refers only to the control aspects of the motor, which are handled by the built in electronics. Typically no, but you need to get clarification from the contractor or control system subcontractor. Usually, in these situations, the contractor is mistaken (out of lack of knowledge) in requesting that you provide remote controlled motors. How far can I run a low voltage cable from a transformer to a garden lighting product? Cable loses voltage according to its length and the number & power of lights you connect to it, so keep low voltage cable runs as short as possible to optimise the performance of your garden lighting products. To minimise cable voltage drop to the luminaires, identify groups of lights which can be supplied from one central transformer. Why do you do switching on the low voltage side of the transformer instead of the 240V mains side? Switching on the low voltage side is much safer and seems to work just as well, if not better. Also I suspect that the actual reaction time for the lights is quicker, since the transformer is already "charged". During testing, when I manually was switching the transformer on/off at the powerpoint, there was a noticeable (although slight) lag from when I hit the switch and the lights coming on completely, since the transformer had to get power through it first. Can Electro Flow increase the voltage in the secondary side of the transformer? A : Yes. The ElectroFlow's microprocessor is able to sense and regulate the voltage level desired by you. This is to say that if the transformer is tapped at 495V but you are receiving 480V, the standard ElectroFlow will be able to increase the voltage near to 495V at the secondary side or load center. This can provide an opportunity to reduce the voltage on the transformer tap setting to pick up additional savings. How many watts does the Generator need to be? How do I figure out how many Watts voltage transformer do i need? How do I know which transformer I need for a low voltage pump? How many watts do you think that you need? What are taps and when are they used? Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Standard tap arrangements are at two -and - one -half and five percent of the rated primary voltage for both high and low voltage conditions. For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating. The standard ASA and NEMA designation for taps are “ANFC” (above normal full capacity) and “BNFC” (below normal full capacity). What is the difference between “Insulating,” “Isolating,” and “Shielded Winding” transformers? Insulating and isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two. A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise. This is especially important in critical applications such as computers, process controllers and many other microprocessor controlled devices. All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety. Can transformers be operated at voltages other than nameplate voltages? In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated at a voltage in excess of its nameplate rating, unless taps are provided for this purpose. When operating below the rated voltage, the KVA capacity is reduced correspondingly. For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts. If the transformer was originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage. Can 60 Hz transformers be operated at 50 Hz? ACME transformers rated below 1 KVA can be used on 50 Hz service. Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service, due to the higher losses and resultant heat rise. Special designs are required for this service. However, any 50 Hz transformer will operate on a 60 Hz service. Transformer Questions & Answers Can transformers be used in parallel? Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5 % of each other. For example: Transformer A has an impedance of 4%, trans- former B which is to be parallel to A must have an impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers,the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical. Can Acme Transformers be reverse connected? ACME dry-type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phase can be reverse connected without any adverse effects or loss in KVA capacity. The reason for this limitation in KVA size is, the turns ratio is the same as the voltage ratio. Example: A transformer with a 480 volt input, 240 volt output— can have the output connected to a 240 volt source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will become the output or 480 volt secondary. On transformers rated below 1 KVA single phase, there is a turns ratio compensation on the low voltage winding. This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load. For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts secondary, would actually have a no load voltage of approximately 250 volts, and a full load voltage of 240 volts. If the 240 volt winding were connected to a 240 volt source, then the output voltage would consequently be approximately 460 volts at no load and approximately 442 volts at full load. As the KVA becomes smaller, the compensation is greater— resulting in lower output voltages. When one attempts to use these transformers in reverse, the transformer will not be harmed; however, the output voltage will be lower than is indicated by the nameplate. Can a Single Phase Transformer be used on a Three Phase source? Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4 -wire. The transformer output will be single phase. Can Transformers develop Three Phase power from a Single Phase source? No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phase. How do you select transformers? (1) Determine primary voltage and frequency. (2) Determine secondary voltage required. (3) Determine the capacity required in volt-amperes. This is done by multiplying the load current (amperes) by the load voltage (volts) for single phase. For example: if the load is 40 amperes, such as a motor, and the secondary voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10 KVA (10,000 All of these insulation systems will normally have the same number of years operating life. A well designed transformer, observing these temperature limits, will have a life expectancy of 20-25 years. Why should Dry-Type Transformers never be over-loaded? Overloading of a transformer results in excessive temperature. This excessive temperature causes overheating which will result in rapid deterioration of the insulation and cause complete failure of the transformer coils. Are temperature rise and actual surface temperature related? No. This can be compared with an ordinary light bulb. The filament temperature of a light bulb can exceed 2000 degrees, yet the surface temperature of the bulb is low enough to permit touching with bare hands. What is meant by “impedance” in transformers? Impedance is the current limiting charac- teristic of a transformer and is expressed in percentage. Why is impedance important? It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer. Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source. Calculate as follows: Normal Full Load Current = Nameplate Volt Amps = 10,000 VA = Line Volts 480 V 20.8 Amperes Maximum Short Circuit Amps = Full Load Amps 20.8 Amps 4% = 4% = 520 Amps The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts. 7 volt-amperes) transformer is required. ALWAYS SELECT THE TRANSFORMER LARGER THAN THE ACTUAL LOAD. This is done for safety purposes and allows for expansion, in case more load is added at a later date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of 3) then divide by 1000. ( 4) Determine whether taps are required. Taps are usually specified on larger transformers. (5) Use the selection charts in Section I. What terminations are provided? Primary and Secondary Terminations are provided on ACME Dry-Type Transformers as follows: No lugs — lead type connection on 0-25 KVA single phase 0-15 KVA three phase Bus-bar terminations (drilled to NEMA standards) 37.5 -250 KVA single phase 25 - 500 KVA three phase Can 60 Hz transformers be used at higher frequencies? ACME transformers can be used at frequencies above 60 Hz up through 400 Hz with no limitations provided nameplate voltages are not exceeded. However, 60 Hz trans- formers will have less voltage regulation at 400 Hz than 60 Hz. What is meant by regulation in a transformer? Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usually expressed in terms of percentage. For example: A transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the regulation would be 5%. ACME dry-type distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for which they are used. What is temperature rise in a transformer? Temperature rise in a transformer is the temperature of the windings and insulation above the existing ambient or surrounding temperature. What is “Class” in insulation? Insulation class was the original method used to distinguish insulating materials operating at different temperature levels. Letters were used for different designations. Letter classifications have been replaced by insulation system temperatures in degrees Celsius. The system temperature is the maximum temperature at the hottest spot in the winding (coil). Graphical representations of six insulation systems recognized by Underwriters’ Laboratories, Inc. are shown in Figure A. These systems are used by Acme for a large part of the product line. Is one insulation system better than another? Not necessarily. It depends on the application and the cost benefit to be realized. Higher temperature class insulation systems cost more and larger transformers are more expensive to build. Therefore, the more expensive insulation systems are more likely to be found in the larger KVA units. Referring to Figure A, small fractional KVA transformers use insulation class 130°C. Compound filled transformers use insulation class 180°C. Larger ventilated transformers are designed to use 220°C insulation. 10 Total Winding Temperature °C COIL HOT SPOT DIFFERENTIAL AV. WINDING RISE AMBIENT 105 130 180 220 30 25 10 55 80 115 150 AGENCY: UL/ANSI 1561 MARCH 1987 40 40 40 40 Figure A 25. Can tr ansformers listed in this catalog be reconnected as autotransformers to increase their KVA rating? Several standard single phase transform- ers listed in this catalog can be connected as autotransformers. The KVA capacity will be greatly increased when used as an autotransformer, in comparison to the nameplate KVA as an insulating transformer. Examples of autotransformer applications are changing 600 volts to 480 volts in either single phase or three phase; changing 480 volts to 240 volts single or three phase or vice versa; or the developing of a fourth wire (neutral) from a 480 volt three phase three wire system for obtaining 277 volts single phase. This voltage is normally used for operating fluorescent lamps or similar devices requiring 277 volts. For further details showing KVA and voltage combinations for various autotrans- former connections refer to Page 30 and 31 in this catalog. Are ACME Transformers shown in this catalog U.L. Listed? All of the transformers, with few exceptions, are listed by Underwriters’ Laboratories and have met their rigorous requirements. We are also prepared to have transformers, which are not presently listed, submitted for listing to Underwriters’ upon the customer’s request. Please contact the factory for details. Is CSA certification available for transformers shown in this catalog? They have been designed and tested in accordance with the latest specifications. Please contact the factory if further details are required. What is BIL and how does it apply to transformers listed in this catalog? BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The Basic Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that a transformer will tolerate without breakdown. All transformers manufactured in this catalog, 600 volts and below, will withstand the NEMA standard BIL rating, which is 10 KV. This assures the user that he will not experience break- downs when his system is properly protected with lightning arrestors or similar surge protection devices. What is polarity, when associated with a transformer? Polarity is the instantaneous voltage obtained from the primary winding in rel ation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage windings on the left hand side are connected together, refer to diagram below. This leaves one Example: Determine the interrupting capacity, in amperes, of a circuit breaker or fuse required for a 75 KVA, three phase transformer, with a primary of 480 volts delta and secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If the secondary is short circuited (faulted), the following capacities are required: Normal Full Load Current = Volt Amps 75,000 VA v 3 x Line Volts v 3 x 480 V 90 Amps Maximum Short Circuit Line Current = Full Load Amps = 90 Amps 5% 5% 1,800 Amps The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480 volts. NOTE: The secondary voltage is not used in the calculation. The reason is the primary circuit of the transformer is the only winding being interrupted. Can Single Phase Transformers be used for Three Phase applications? Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer. For example: Three 10 KVA single phase transformers will accommodate a 30 KVA three phase load. Does ACME provide “Zig - Zag” Grounding Transformers? Yes. Please refer to Page 31 for a special diagram which can be used to connect standard single phase off - the-shelf transformers in a three phase zig-zag manner. This system can be used for either grounding or developing a fourth wire from a three phase neutral. An example would be to change a 480 V — three phase — three wire system to a 480Y/277 V — three phase — four wire system. What color are ACME Dry-Type Transformers? ASA 61 (NEMA) light gray is used on all enclosed transformers from .050 to 500 KVA. How do you select a transformer to operate in an ambient higher than 40° centigrade? When the ambient exceeds 40°C use the following chart for de-rating standard transformers. Instead of ordering custom built transformers to operate in ambients higher than 40°C, it is more economical to use a standard transformer of a larger KVA rating. 8 Maximum Ambient Maximum Percentage Temperature of Loading 40°C (104°F) 100% 50°C (122°F) 92% 60°C (140°F) 84% H1 H2 x2 x1 120 VOLT OUTPUT 240 VOLT INPUT VOLT METER 360 VOLT READING ADDITIVE POLARITY high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase trans- formers in parallel for three phase operations. Polarity is a term used only with single phase transformers. What is exciting current? Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA . The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and is referred to as KVAR. Will a transformer change Three Phase to Single Phase? A transformer will not act as a phase changing device when attempting to change three phase to single phase. There is no way that a transformer will take three phase in and deliver single phase out while at the same time presenting a balanced load to the three phase supply system. There are, however, circuits available to change three phase to two phase or vice versa using standard dual wound transformers. Please contact the factory for two phase applications. Can air cooled transformers be applied to motor loads? This is an excellent application for air cooled transformers. Even though the inrush or starting current is five to seven times normal running current, the resultant lower voltage caused by this momentary overloading is actually beneficial in that a cushioning effect on motor starting is the result. The tables on Pages 11 and 12 illustrate some typical transformer requirements for use with motor applications. How is an Acme Drive Isolation Transformer (DIT) different than a General Purpose Transformer? DITs, as the name implies, are designed to be used with motor drives (AC and DC) and to provide isolation from the service line. They are specifically designed to withstand the“short circuit like”duty imposed by the firing of the thyristors. Harmonics generated by drives create added loads on the transformer. Therefore, it is important that a transformer of equal or greater KVA to that recommended by the drive manufacturer be installed for a particular motor application. 34. How are transformers sized to operate Three Phase induction type squirrel cage motors? The minimum transformer KVA rating required to operate a motor is calculated as follows: Minimum Transformer KVA = Running Load Amperes x 1.73 x Motor Operating Voltage 1000 NOTE: If motor is to be started more than once per hour add 20% additional KVA. Care should be exercised in sizing a transformer for an induction type squirrel cage motor as when it is started, the lock rotor amperage is approximately 5 to 7 times the running load amperage. This severe starting overload will result in a drop of the transformer output voltage. When the voltage is low the SECTION DRY-TYPE DISTRIBUTION TRA NSFORMERS 1 9 torque and the horsepower of the motor will drop proportion- ately to the square of the voltage. For example: If the voltage were to drop to 70 % of nominal, then motor horsepower and torque would drop to 70 % squared or 49 % of the motor nameplate rating. If the motor is used for starting a high torque load, the motor may stay at approximately 50% of normal running speed as illustrated by the graph below: The underlying problem is low voltage at the motor terminals. If the ampere rating of the motor and transformer overcurrent device falls within the motor’s 50% RPM draw requirements, a problem is likely to develop. The overcurrent device may not open under intermediate motor ampere loading conditions. Overheating of the motor and/or transformer would occur, possibly causing failure of either component. This condition is more pronounced when one transformer is used to power one motor and the running amperes of the motor is in the vicinity of the full load ampere rating of the transformer. The following precautions should be followed: (1) When one transformer is used to operate one motor, the running amperes of the motor should not exceed 65% of the transformer’s full load ampere rating. (2) If several motors are being operated from one transformer, avoid having all motors start at the same time. If this is impractical, then size the transformer so that the total run- ning current does not exceed 65% of the transformer’s full load ampere rating. Why are Small Distribution Transformers not used for ________ Control Applications? _______ control equipment demands a momentary overload capacity of three to eight times normal capacity. This is most prevalent in solenoid or magnetic contactor applications where inrush currents can be three to eight times as high as normal sealed or holding currents but still maintain normal voltage at this momentary overloaded condition. Distribution transformers are designed for good regulation up to 100 percent loading, but their output voltage will drop rapidly on momentary overloads of this type making them unsuitable for high inrush applications. _______ control transformers are designed especially for maintaining a high degree of regulation even at eight times normal load. This results in a larger and generally more expensive transformer. For a complete listing of ACME industrial control transformers, refer to Section V. 50 100 150 200 250 20 40 60 80 100 SPEED (PERCENT OF SYNCHROUS SPEED) TORQUE (PERCENT OF FULL LOAD TORQUE) SPEED vs TORQUE FOR A TYPICAL THREE PHASE INDUCTION TYPE SQUIRREL CAGE MOTOR STALL ZONE Can 4-Winding Single Phase Transformer be auto-connected? Yes. There are occasions where 480 volts single phase can be stepped down to 240 volts single phase by autoconnecting a standard 4 - winding isolating transformer as shown in Figure 1. If connected in this manner, the nameplate KVA is doubled. For example: A 10 KVA load can be applied to a 5 KVA 4 - winding transformer if connected per Figure 1. What about balanced loading on Three Phases? Each phase of a three phase transformer must be considered as a single phase transformer when determining loading. For example: A 45 KVA three phase transformer with a 208Y/120 volt secondary is to service 4 loads at 120 volts single phase each. These loads are 10 KVA, 5 KVA, 8 KVA,and 4 KVA. NOTE: that maximum loading on any phase does not exceed 10 KVA. Each phase has a 15 KVA capacity. 45 KVA 3 phase = 15 KVA per phase If incorrect method is used, phase B will have an 18 KVA load which is 3 KVA above its normal capacity of 15 KVA and failure will result even though we only have a total load of 27 KVA on a 45 KVA transformer. SECTION DRY-TYPE DISTRIBUTION TRA NSFORMERS 1 What is meant by “Balanced Loading” on Single Phase Transformer applications? Since most single phase transformers have a secondary voltage of 120/240, they will be operated as a three wire system. Care must be taken in properly distributing the load as the transformer secondary consists of 2 separate 120 volt windings. Each 120 volt winding is rated at one-half the nameplate KVA rating. For example: A 10 KVA transformer, 120/240 volt secondary is to service an 8 KVA load at 240 volts and two 1 KVA loads at 120 volts each. If the incorrect method is used, winding A will be loaded at 6 KVA, and winding B will be loaded at 4 KVA. These do total 10 KVA but, since each winding is only rated at 5 KVA (1/2 of nameplate rating), we have an overloaded transformer and a certain failure. What are typical applications for transformers? ACME transformers should be specified to: (1) Distribute power at high voltage. (2) Eliminate double wiring. (3) Operate 120 volt equipment from power circuits. (4) Insulate circuits/establish separately derived circuits. (5) Provide 3-wire secondary circuits. (6) Buck and Boost (See Section VII). (7) Provide electrostatic shielding for transient noise protection. Figure 1 H1 480V H3 H2 H4 X1 X3 X2 X4 240V Enclosure Definitions Type 1 Enclosures — are intended for indoor use, primarily to provide a degree of protection against contact with the enclosed equipment. Type 2 Enclosures How are power transformers used? Power transformers are generally used to transmit power at a constant frequency. What is the purpose of an audio transformer? Audio transformers are designed to operate over a wide range of frequencies with a nearly flat response i.e. a nearly constant ratio of input to output voltage. What is the simplest type of transformer? The simplest type of transformer consists of two coils of wire electrically insulated from one another and arranged so that a change in the current in one coil (the primary) will produce a change in voltage in the other (the secondary). What is the purpose of Radio frequency transformers? Radio frequency (RF) transformers are designed to operate efficiently within a narrow range of high frequencies. In transformers what is the core of the coil made of? In many transformers the coils are wound on a core made of a material with high magnetic permeability; this intensifies the magnetic field induced by the current in the primary increasing the transformer's efficiency. |