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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 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.

  • What is a Scott T Connection?
  • 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?
  • What is a Buck Boost transformer?
  • 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.

  • Where is a Scott-T Transformer used

      Typical applications for a Scott-T transformer include:

      • Used in an electric furnace installation where it is needed to operate two single-phase feeds and draw a balanced load from the three-phase supply.
      • Used to supply the single phase loads such as traction power. This helps to keep the load on the three-phase system as nearly balanced as possible.
      • Used to link a 3-phase system with a two–phase system with the flow of power in either direction.

      The Scott-T connection permits conversions of a 3-phase system to a two-phase system or vice versa. Since 2-phase generators are not available, the conversion from two phases to three phases is not a practical application.

  • Do different types of non-linear loads generate different harmonics?

      By far the majority of today’s non-linear loads are rectifiers with DC smoothing capacitors. These rectifiers typically come in 3 types:
      (i) single phase, line-to-neutral
      (ii) single phase, phase-to-phase
      (iii) three-phase

      Single-phase line-to-neutral rectifier loads, such as switch-mode power supplies in computer equipment, generate current harmonics 3rd, 5th, 7th, 9th and higher. The 3rd will be the most predominant and typically the most troublesome. 3rd, 9th and other odd multiples of the 3rd harmonic are often referred to as triplen harmonics and because they add arithmetically in the neutral are also considered zero sequence currents. Line-to-neutral non-linear loads can be found in computer data centers, telecom rooms, broadcasting studios, schools, financial institutions, etc.

      208V single-phase rectifier loads can also produce 3rd, 5th, 7th, 9th and higher harmonic currents but if they are reasonably balanced across the 3 phases, the amplitude of 3rd and 9th will be small. Because they are connected line-line, these loads cannot contribute to the neutral current. The largest current and voltage harmonics will generally be the 5th followed by the 7th. Typical single phase, 208V rectifier loads include the switch-mode power supplies in computer equipment and peripherals.

      Three-phase rectifier loads are inherently balanced and therefore generally produce very little 3rd and 9th harmonic currents unless their voltage supply is unbalanced. Their principle harmonics are the 5th and 7th with 11th and 13th also present. They cannot produce neutral current because they are not connected to the neutral conductor. The rectifiers of variable speed drives and Uninterruptible Power Supplies (UPS) are typical examples of three-phase rectifier loads.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Can a Scott-T connected transformer be fed to go from two phases to three phases?

      In theory, you can feed a Scott-T connected transformer with two phase power and derive three phases. However, the two phase feed would need to have the two phases offset by 90 degrees. In practicality, there are no two phase generators on the market that can provide this feed.

      In some cases, this is discussed with an open delta system. Since an open delta system does not have the two phases offset by 90 degrees, a Scott T connected transformers can not be used in this application.

  • Can a Scott-T connected transformer be fed from two phases of an open delta system to create a three phase system?

      No. The two phase input of a Scott-T connected transformer requires the two phases to be offset by 90 degrees. Since an open delta system does not have the two phases offset by 90 degrees, a Scott-T connected transformer can not be used in this application. There are currently no practical magnetics solutions to this application.

  • Can Buck-Boost transformers be used on 3 phase systems?
  • Should Buck-Boost transformers be used to develop 3-phase 4 wire Wye circuits from 3-phase 3 wire Delta circuits?

      No – a three-phase “Wye” buck-boost transformer connection should be used only on a 4-wire source of supply. A delta to Wye connection does not provide adequate current capacity to accommodate unbalanced currents fl owing in the neutral wire of the 4-wire circuit.

  • What is an Open Delta transformer?

      An open delta transformer is a three phase transformer that only has two primary and secondary windings, with one side of the delta phase diagram “open”. Open delta transformers are rare and are typically only used for small loads where cost is important. More common is critical loads being wired with three single phase transformers in a banked configuration. Should one of these transformers fail, the three phase circuit can remain active although the two remaining transformers are limited to about 57% of the total load. This allows a circuit to remain powered during a failure of a transformer, albeit at a lower overall load factor.

      Open Delta