Transformer FAQ

    Transformer FAQ

    A transformer is an electrical static equipment 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 conditions, a long and trouble-free life. It consists, in it’s 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 level (or potential difference ratio) between the primary and secondary depends on the turns ratio of the two coils.

    Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating current & voltage so that it becomes extended and contracted twice during a full cycle of magnetization.

    The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect. The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbor, so you can see what a moving, writhing construction it is when excited.

    These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level.

    Reducing flux level helps to reduce noise? Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness.

    Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction equivalently. In technical terms the relationship is not linear.

    Taps are provided on some transformers on the high voltage winding to correct for low or high voltage conditions, and still deliver full rated voltages at the secondary terminals. Taps are generally set 1.25%,2.5% above and below the rated primary voltage.

    What is the difference between “Insulating”, “Isolating”, and “Shielded Winding” transformers? Insulating and isolating transformers are identical. These terms are used to describe the separation of the primary and secondary windings. A shielded transformer includes a metallic shield between the primary and secondary windings to attenuate (lessen) transient noise.

    In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated in excess of its nameplate rating unless tap changer provided. When operating below the rated voltage the KVA capacity is reduced correspondingly.

    Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to higher losses and resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz service.

    Single phase transformers can be used in parallel only in case of their 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.

    Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results.

    In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online.

    Taps are normally in the primary winding to adjust for varying incoming voltage. If the transformer is reverse fed, the taps are on the output side and can be used to adjust the output voltage.

    Transformer terminals are marked according to high and low voltage connections. An H terminal signifies a high voltage connection while an X terminal signifies a lower voltage connection. A common misconception is that H terminals are primary and X terminals secondary.

    This is true for step down transformers, but in a step up transformer the connections should be reversed. Low voltage primary would connect to X terminals while high voltage secondary would connect on the H terminals.

    No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phases.

    Voltage regulation in transformers is the difference between the full load voltage and the no load voltage. This is usually expressed in terms of percentage.

    Temperature rise in a transformer is the average temperature of the windings and oil & insulation above the existing ambient temperature.

    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.

    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.

    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.

    Impedance is the current limiting characteristic of a transformer and is expressed in percentage.

    The Efficiency of the transformer is defined as the ratio of useful power output to the input power, the two being measured in the same unit. Its unit is either in Watts (W) or KW. It is denoted by Ƞ.

    It is used for determining the interrupting capacity of switchgear employed to protect the primary of a transformer.

    On the basis of their use

    • 1. Power transformer: Used in transmission network, high rating
    • 2. Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.

    In any electrical machine, 'loss' can be defined as the difference between input power and output power. An electrical transformer is an static device, hence mechanical losses (like windage or friction losses) are absent in it. A transformer only consists of electrical losses (iron losses and copper losses). Transformer losses are similar to losses in a DC machine, except that transformers do not have mechanical losses.

    Losses in transformer are explained below :

    (I) Core Losses Or Iron Losses

    Eddy current loss and hysteresis loss depend upon the magnetic properties of the material used for the construction of core. Hence these losses are also known as core losses or iron losses.

    • Hysteresis loss in transformer:

      Hysteresis loss is due to reversal of magnetization in the transformer core. This loss depends upon the volume and grade of the iron, frequency of magnetic reversals and value of flux density. It can be given by, Steinmetz formula:

      Wh= ηBmax1.6fV (watts)
      where, η = Steinmetz hysteresis constant
      V = volume of the core in m3

    • Eddy current loss in transformer:

      In transformer, AC current is supplied to the primary winding which sets up alternating magnetizing flux. When this flux links with secondary winding, it produces induced emf in it. But some part of this flux also gets linked with other conducting parts like steel core or iron body or the transformer, which will result in induced emf in those parts, causing small circulating current in them. This current is called as eddy current. Due to these eddy currents, some energy will be dissipated in the form of heat.

    (Ii) Copper Loss In Transformer

    Copper loss is due to ohmic resistance of the transformer windings.  Copper loss for the primary winding is I12R1 and for secondary winding is I22R2. Where, I1 and I2 are current in primary and secondary winding respectively, R1 and R2 are the resistances of primary and secondary winding respectively. It is clear that Cu loss is proportional to square of the current, and current depends on the load. Hence copper loss in transformer varies with the load.

    Does you have “Zig-Zag” grounding transformers?

    Yes. This system can be used for either grounding or developing a fourth wire from a three phase 3 wire. (neutral)

    What is BIL and how does it apply to transformers?

    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 BIL of a transformer is a method of expressing the voltage surge that a transformer will tolerate without breakdown.

    What is exciting current?

    Exciting current 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 less to approximately 2% on larger sizes of 750 KVA.


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