CPC 100 - Universal testing device for electrical diagnostics

CPC 100 - Universal testing device for electrical diagnostics on transformers, current transformers, voltage transformers, grounding systems, lines and cables, and circuit breakers (photo by www.omicron.at)

Polarity Detection

This is needed for identifying the primary and secondary phasor polarities. It is a must for poly phase connections. Both a.c. and d.c methods can be used for detecting the polarities of the induced emfs.

The dot method is used to indicate the polarities.

The transformer is connected to a low voltage a.c. source with the connections made as shown in the Figure 1 (a). A supply voltage Vs is applied to the primary and the readings of the voltmeters V1, V2 and V3 are noted. V1 : V2 gives the turns ratio.

If V3 reads V1−V2 then assumed dot locations are correct (for the connection shown).

Transformer polarity test scheme

Figure 1 - Transformer polarity test scheme


The beginning and end of the primary and secondary may then be marked by A1 − A2 and a1 − a2 respectively. If the voltage rises from A1 to A2 in the primary, at any instant it does so from a1 to a2 in the secondary.

If more secondary terminals are present due to taps taken from the windings they can be labeled as a3, a4, a5, a6. It is the voltage rising from smaller number towards larger ones in each winding. The same thing holds good if more secondaries are present.

Figure 1 (b) shows the d.c. method of testing the polarity. When the switch S is closed if the secondary voltage shows a positive reading, with a moving coil meter, the assumed polarity is correct. If the meter kicks back the assumed polarity is wrong.

Reference: Electrical Machines I – Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao


About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on



8 Comments


  1. Kieran Gosal
    Jun 08, 2014

    Hanza Yunus,

    In the DC circuit when the switch closes to make the circuit the DC voltage across the winding changes from 0 to full battery voltage. This step change that occurs for only a few milliseconds is in fact an ac voltage (small change in voltage over small time period). Therefore we see this induced voltage lasting on the secondary (induced ac voltage lasting a few msec has a DC average that is picked up with DC meter). It is called a flicker test because you will only get DC voltage flick from 0V to the induced voltage very quickly before returning back to 0. If this induced voltage is positive then the polarity is shown as per the diagram and if its negative then the polarity is opposite (reverse polarity)


  2. Barbe
    Apr 21, 2014

    Its work to me using battery even on big transfo….to find the polarity of the transfo…


  3. James
    Dec 18, 2013

    Umm.. if you apply a DC voltage to the primary it will put a charge on the secondary side.. this is basic electronics people…. just because it is constant 25kv doesnt mean it wont induce a voltage on the opposite side…. the difference here is that DC will use the Peak Voltage and A/C will use an RMS Value because with the cycles you loose that peak….

    Do you not remember putting 12vDC on a coil then letting your buddy hold the ends of the coil to get that inrush shock when you remove the current?!? So fun in school….


  4. Hamza Yunus
    Oct 28, 2013

    to author,
    i m a bit confused here, if one energizes the primary of a T/F with DC voltage, due to constant flux, emf should not b induced in the secondary. how come can we get deflection in mechanical voltmeter ?
    would you mind elaborating a bit, huh ?


  5. satyanarayana adabala
    Sep 02, 2013

    A Dc supply applying to primary of transformer the winding is damaged,how can apply the dc supply for polarity test


    • mhkawi
      Sep 03, 2013

      i think it is a pulse not continous dc supply


  6. mhkawi
    Aug 29, 2013

    i think it is better for knowing the polarity to use DC method but use ammeter not voltmeter
    , please i want more article to show how to design polarities of the CT


    • javeed
      Jul 18, 2014

      haa. if you apply a DC voltage to the primary it will put a charge on the secondary side.. this is basic electronics people…. just because it is constant 25kv doesnt mean it wont induce a voltage on the opposite side…. the difference here is that DC will use the Peak Voltage and A/C will use an RMS Value because with the cycles you loose that peak….

      Do you not remember putting 12vDC on a coil then letting your buddy hold the ends of the coil to get that inrush shock when you remove the current?!? So fun in school…

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