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What does the dot mean on a single phase transformer wiring schematic?

Typically, a single phase transformer wiring schematic has a dot on both the primary and secondary windings.

The placement of these dots next to the ends of the primary and secondary windings informs us that the instantaneous voltage polarity seen across the primary winding will be the same across the secondary winding. In other words, the phase shift from primary to secondary will be zero degrees, which is important for some types of circuits. If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase.

  • What is Subtractive Polarity

      The relative polarities of voltages on single phase transformers is important when using the two units in parallel or connecting two or three units to create a three phase bank. Most single phase transformers are wired in Additive Polarity. Visualize two primary terminals H1 and H2 on top of a square representing a single phase transformer. Now visualize two secondary terminals, X1 and X2. If X2 is on the left size and X1 is on the right side, (reading X2 – X1 left to right) then this would be additive polarity. If the terminal X1 is on the left side and X2 is on the right side (reading X1-X2 left to right) then this would be subtractive polarity.

      Most single phase wiring diagrams have a dot on both the primary and secondary sides, typically on the H1 and X1 terminals. This dot represents the matching polarity on the primary and secondary sides of the single phase transformer. When wiring single phase units in parallel or banking, the polarities of the transformers used must be kept consistent.

  • What is Additive Polarity

      The relative polarities of voltages on single phase transformers is important when using the two units in parallel or connecting two or three units to create a three phase bank. Most single phase transformers are wired in Additive Polarity. Visualize two primary terminals H1 and H2 on top of a square representing a single phase transformer. Now visualize two secondary terminals, X1 and X2. If X2 is on the left size and X1 is on the right side, (reading X2 – X1 left to right) then this would be additive polarity. If the terminal X1 is on the left side and X2 is on the right side (reading X1-X2 left to right) then this would be subtractive polarity.

      Most single phase wiring diagrams have a dot on both the primary and secondary sides, typically on the H1 and X1 terminals. This dot represents the matching polarity on the primary and secondary sides of the single phase transformer. When wiring single phase units in parallel or banking, the polarities of the transformers used must be kept consistent.

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  • What are the advantages and disadvantages of an open delta transformer configuration ?

      Open-delta configurations are typically deployed by utilities, typically to loads with little single-phase requirements.

      Advantages:

      • Open-deltas only require the utility to install two transformers.
      • Future Capacity can be increased by simply installing a third similar sized transformer verses installing 2-3 larger transformers.

      Disadvantages:

      • While the line to line voltages will be equal, the line to neutral voltages will have two phases being equal and one phase being 1.732 times larger.
      • Unbalanced single phase loads can cause voltage fluctuations and additional, uneven transformer heating.
      • An open delta connection only has 58% of the capacity of a full set of three transformers, that is a 42% decrease in actual capacity event though the installed capacity only drops by 33%.

       

  • What is Parallel Operation?

      Single and three phase transformers may be operated in parallel by connecting similarly marked terminals, provided their ratios, voltages, resistances, reactance and ground connections are designed to permit parallel operation. Current and voltage angular displacements are also required to be the same in the case of three phase transformers.

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  • Can a transformer convert single-phase power to three-phase power?

      Single-phase power can be derived from a three-phase source. Transformers cannot convert a single-phase source to a three-phase source. The typical method to convert single-phase power to three-phase power is to utilize devices generally termed as rotary or static phase converters.

  • What does the dot mean on a single phase transformer wiring schematic?

      Typically, a single phase transformer wiring schematic has a dot on both the primary and secondary windings.

      The placement of these dots next to the ends of the primary and secondary windings informs us that the instantaneous voltage polarity seen across the primary winding will be the same across the secondary winding. In other words, the phase shift from primary to secondary will be zero degrees, which is important for some types of circuits. If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase.

  • Explain Balance Loading on Single and Three Phase Transformers?

      A single-phase transformer with a series/parallel 120/240V secondary winding has two separate 120V secondary windings and is usually connected into a 3-wire system. When the winders are wired in series for 240 VAC, 120 VAC can be obtained at either between the neutral and centerpoint or between the centerpoint and 240VAC. If both 240 VAC and 120 VAC are going to be used, care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total 120VAC load if the 120 VAC load exceeds 5% of the total tranformer rating.

      Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded with single phase loads.

      Failure to balance loads can cause secondary voltage imbalances, additional transformer losses and high neutral currents. Significantly unbalanced loads can reduce the life of a transformer.

  • What is the duty cycle of a transformer?

      Duty cycle is the amount of load over set periods of time. Transformers are designed to run continuously at full load without exceeding the insulation temperature limits provided that parameters such as ambient temperature, harmonic distortion, power factor, etc., are met. Transformers can also be designed to run for short term duty cycles which may result in a smaller unit. Short term duty cycles will need lower or no-load periods to aid in cooling. High load duty cycles can affect parameters such as impedance and voltage regulation.

  • Explain the term Phase?
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  • What is IR drop

      IR drops relates to Ohm’s Law: Voltage = Current x Resistance. Transformers utilize conductors which have resistance and can cause a slight voltage drop. Typically manufacturers compensate the winding ratio to mitigate any effects of IR voltage drop in the secondary voltage.

  • What is an “Electrostatic Shield”?

      Electrostatically shielded (Faraday Shield) transformers provides a copper electrostatic shield between the primary and secondary windings. The shield is grounded and thus shunts some noise and transients to the ground path rather than passing them through to the secondary. Transformers having a K-Rating are required to have an electrostatic shield.

      Electrostatically shielded transformers often preferred for electrical installations where electronic circuitry operating at low voltage DC is present and is very sensitive to ‘noise’. Recent testing of electrostatically shielded transformers has questioned their perceived effectiveness where the transformer’s secondary is grounded which would cover most applications.

  • What is a triplex transformer?

      A triplex transformer is composed of three separate single phase transformers which are banked and connected directly together to form a single three phase unit in a common enclosure. While slightly larger than a dedicated three phase unit. a triplex design can be broken down into three significantly smaller and lighter components when there are size and/or weight restrictions for transporting the transformer. These are often used in mining or high rise building installations where the unit must be transported in an elevator.

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  • What is a dual output Harmonic Mitigating Transformer

      Like a standard HMT, a dual put HMT minimizes the voltage distortion rise due to the transformer itself. Harmonic Mitigating Transformers (HMTs) are designed to reduce the impedance seen by the harmonic currents. This is accomplished through zero sequence flux cancellation and through phase shifting. The secondary winding configuration of the HMT cancels the zero sequence fluxes; those produced by the 3rd, 9th, 15th (triplen) current harmonics, without coupling them to the primary windings.

      This prevents the triplen current harmonics from circulating in the primary windings as they do in a delta-wye transformer. The flux cancellation also results in much lower impedance to the zero sequence currents and hence lower voltage distortion at these harmonics. In addition, the reduced primary winding circulating current will lower losses and allow the transformer to run cooler.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Define Series/Multiple?

      A winding consisting of two or more sections which can be connected for series operation or multiple (parallel) operation. Also referred to as dual voltage or series-parallel.

  • How to transformer taps adjust voltage?

      The ratio between the number of windings in the primary and secondary coil’s of a transformer determines the voltage ratio. If a transformer has 100 primary windings and 25 secondary windings, the ratio is 100:25 = 4:1. If the primary is fed with 480VAC, the ratio of primary to secondary voltage is 4:1 or 480:120. Taps are used to adjust for voltage differences at the transformer’s primary winding.A 5% adds or subtracts 5% of the windings.

  • Why do Buck-Boost transformers have 4 windings?

      A four winding buck-boost transformer with 2 primary and 2 secondary windings can be connected eight different ways to provide a multitude of voltages and kVA’s. This provides the flexibility necessary for the broad variety of applications. A two-winding transformer can only be connected in two different ways.

  • What are Electrostically Shielded Transformers and where are they used?

      Electrostatically shielded (Faraday Shield) transformers provides a copper electrostatic shield between the primary and secondary windings. The shield is grounded and thus shunts some noise and transients to the ground path rather than passing them through to the secondary. Transformers having a K-Rating are required to have an electrostatic shield.

      Electrostatically shielded transformers often preferred for electrical installations where electronic circuitry operating at low voltage DC is present and is very sensitive to ‘noise’. Recent testing of electrostatically shielded transformers has questioned their perceived effectiveness where the transformer’s secondary is grounded which would cover most applications.

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