The primary guideline was that any leads taken out of the casing (high voltage or low voltage) should be numbered 1, 2, 3, 4, and so on. The lowest and highest values reflect the entire winding, while the intermediate numbers represent winding fractions or taps. For example, if the secondary is also divided into four parts, they are labeled S1, S2, S3, and S4.
A transformer's markings can be very helpful in identifying what type of transformer you have, but they can also be misleading. A three-phase transformer with six legs may look like it would supply power to six lamps, but that isn't possible with standard wiring. The number of lamps that can be powered by this transformer depends on how it is wired. If all the legs are connected in parallel, then only one lamp can be plugged in. If half the legs are connected in parallel and the other half in series, then three lamps can be plugged in.
A transformer's secondary rating is found by multiplying the number of primaries by the number of secondaries. For example, if there are four secondaries and each secondary has enough capacity for two lamps, then the total load capacity is eight lamps. Transformer makers often use smaller symbols for the secondaries to help people identify the size of the transformer.
The voltage across any winding will be the same, regardless of which way the current flows.
Designations for wind connections
The parameters are derived from the secondary side's short-circuit test reading. The input voltage is increased until the current in the short-circuited winding equals the rated value. The voltage, current, and power inputs are all measured. The magnetic field strength can then be calculated using Faraday's law: B = μ0I where B is in teslas (T), I is in amps (A), and μ0 is 0.0035 H/mm.
If you know the primary voltage and the primary current, then you can calculate the secondary voltage and current. The secondary voltage is just equal to the primary voltage times the turns ratio of the transformer. The secondary current is just equal to the primary current times the turns ratio of the transformer.
The power delivered to the secondary is simply the product of the secondary voltage and current. That is, P_sec = V_sec × I_sec.
For example, if you have a transformer with a primary voltage of 120 volts and a secondary voltage of 48 volts when there is no load on the secondary side, then the turns ratio is about 5 : 1.
Transformer 1 has a connected primary with a line-to-line rating of 69,000 V and a winding rating of the same. With a line-to-line rating of 12,470 V and a winding rating of 7,200 V, the secondary is grounded-Y connected. This transformer's TTR is equal to the ratio of the winding ratings: 69,000/7200 = 9.583. The TFR is then 3.417.
The second transformer, which is also ground-Y connected on its secondary, has a different line-to-line voltage ratio of 48,000/12,470 = 3.9. Its TFR is then 10.067, and its TFR is then 10.067.
The third transformer has a line-to-line voltage ratio of 36,000/5,400 = 6.667. Its TFR is then 5.333.
The fourth transformer has a line-to-line voltage ratio of 27,000/4,050 = 6.6. Its TFR is then 5.8.
The fifth transformer has a line-to-line voltage ratio of 18,000/2,700 = 6.0. Its TFR is then 5.6.
The sixth transformer has a line-to-line voltage ratio of 12,470/1,800 = 6.7. Its TFR is then 6.3.
The main, secondary, and tertiary voltages, as well as their power ratings at those voltages, are listed in the center right area. MVA is the unit of measurement for power ratings (Mega Volt-Amperes). When there is a tap changer, the voltage of each tap is displayed on a table or tables. The main voltage is found by selecting the lowest tap without shutting off the generator.
When there is no tap changer, the secondary voltage is always higher than the tertiary voltage. The ratio between them is called the transformation factor. This factor can be calculated with this formula: main voltage / (secondary voltage + tertiary voltage).
In practice, however, the factor will not be exactly equal to this number; instead it will usually be within 10% of this value. Therefore, to find the actual voltage of the transformer when no tap changer is used, simply divide the rating of the transformer by the transformation factor. In this case, the power rating of the transformer must be greater than or equal to the desired output voltage multiplied by the transformation factor. For example, if the main voltage is 120 volts and the transformation factor is 0.7, then the power rating of the transformer must be at least 147 MVAr or greater.
As an example, say that the transformer has a power rating of 1000 VA and that you want to know its voltage when no tap changer is used. First, find the transformation factor by dividing 1000 by 2.
The nameplate of a transformer contains information such as voltage rating (V), kilovoltampere rating (KVA), frequency (f), number of phases, temperature increase, cooling class, percent impedance, manufacturer's name, and so on. They also provide Basic Impulse Level, Phasor Diagrams for 3-phase operation, and tap changing information on the huge power transformer nameplates.
Nameplates were first used by American engineers in the early 20th century. Before that time, there was no way to identify the quality or type of transformer other than by its physical size. Size differentiation was achieved by dividing large transformers into multiple sections, each representing a different grade of insulation resistance. The number of these sections indicated the height of the barrier between secondary and primary windings; the higher the number, the better the insulation. A transformer's ability to carry current without damage was not considered important before the development of power systems; therefore, it did not matter how many milliohm of resistance it had in its insulation system.
In modern power systems, however, high voltages require high-quality, thick insulation layers between the primary and secondary sides of the transformer. This prevents any electrical connection except that which passes energy from one side of the transformer to the other. Transformers are thus divided into two categories based on the resistance of their insulation: low-voltage transformers have insulation values greater than 1 megavoltmeter (MVM) while high-voltage transformers have insulation values of 5 MVM or more.
Some coil symbols are labeled with letters and/or numbers to represent electrical connections, while others are labeled with dots to signify polarity. Dot convention marks make use of dots on the transformer schematic symbol to identify the winding direction between input and output, and hence the polarity of windings. For example, if a coil has two dots next to each other, it means that the current in those coils flows in opposite directions.
Dot conventions were first adopted by the electronics industry in the 1950s. Before then, all electrical connections to coils were always indicated by either marking tape or wire. Coils could not be identified by direction alone, so if you wanted to know which way a coil was wired, you had to look at its physical arrangement on the transformer core or case. Dots added to the schematic symbol made this information available electronically, so it could be referenced by computer programs. They also helped to avoid mistakes when wiring multiple transformers in parallel.
There are several different dot conventions used in circuit diagrams. These include plain dots, hyphens, circles, triangles, and crosses. Some manufacturers mark their components with more than one method, while others do not use any special markings at all. It's up to the designer which method they prefer and what symbols they choose to use for their components.