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TESTING METHODS
Jabbals Surge Comparison Tester is a universal type of device capable of testing single and three phase windings.
The Surge testing methods for various types of windings and motors is as follows:
  • Single Phase Windings
  • Three Phase Windings
  • Armatures
  • Transformers
  • Field Coils
  • Form Wound Coils
  • Assembled Motors
  • Large AC Stators
    		•
    Surge testing is generally a comparison test therefore two windings of the same nature are required to perform the surge test. The Test Select Switch is set to 1φ and the Surge Test Voltage is slowly increased.
    If the waveform on the CRT display remains stable upto the desired voltage rating of the winding then the insulation of the windings or coils is adequate but if the pattern changes and becomes erratic, then intermediate shorting or arcing occurs resulting due to poor insulation.
    When testing 1φ windings the magnetic material (iron, ferrite) close to the coils under test must be same for both coils.

    TEST LEADS
    TEST POSITION RED YELLOW BLUE BLACK
    1φ HOT GND HOT GND

    As shown in the table above the RED and BLUE Surge Test Leads are HOT when the Test Select switch is set to Position of 1φ therefore connect test leads as follows:
    COIL (I): RED AND YELLOW
    COIL (II): BLUE AND BLACK

    Surge testing for 3φ windings does not need a reference as the windings are automatically tested in pairs.
    Perform the test, connect the RED test lead to U-Phase, the YELLOW test lead to V-Phase and Blue test lead to W-Phase either in the Star or Delta formation.
    Set the Test Select switch to Position A and slowly apply the Surge Test Voltage upto the desired rating and follow the same for Position B and Position C.
    As the table below suggests the Test leads are switched HOT and GND automatically by Test Select switch so that all the phases are compared without removing the test leads.

    TEST LEADS
    TEST POSITION RED YELLOW BLUE BLACK
    A (3φ) HOT HOT GND GND
    B (3φ) HOT GND HOT GND
    C (3φ) GND HOT HOT GND

    AT POSITION A, U & V Phases are compared
    AT POSITION B, U & W Phases are compared
    AT POSITION C, V & W Phases are compared
    NOTE: If Surge Test waveform is erratic for Position A and B but is perfect for Position C, this signifies that V and W phase are perfectly balanced but the U-phase is unbalanced with both V and W phase.

    For testing Armature windings a fixture with three bush pick-up on an adjustable Yoke is used.
    Set the Test Select Switch to Position 1φ.
    Connect the Red Test lead to (1), Yellow to the centre (2) & Blue to (3) as shown in the fig. on the right.
    Two segments with an equal number of bars are then compared. If the two segments are balanced and contain no faults, a single pattern will appear on the CRT display. Faults are located by noting the change in the pattern as the armature is rotated bar-to-bar.
    VOLTAGE STRESS ON ARMATURES
    Voltage stressed on the Arismature is measured by differential drop between each bar. For eg. a 10 bar span center with 1,000 volts applied will give a 100 volt stress. If the span is lowered to 5 bars from center connection then the bar voltage will double to 200 volts. It is useful to keep the span as low as possible and still get a good ringing waveform. A 10 bar span at 1000-1500 Surge test volt age is optimum.

    The 3φ Transformers can be tested both during the Pre-varnish and Post-varnish stages.
    During the Pre-Varnish stage (without core), the transformer coils can be tested as a 1φ windings with one coil as a master reference coil and the other coils compared against it.
    The Post-varnish stage is when the Transformer coils have core inside it with Primary and Secondary windings then the transformer will be tested as 3φ windings as mentioned above.
    Imp. Testing Guidelines:
  • When the three phases of the Primary winding of the transformer has to be tested then the three phases of the Secondary windings has to be all shorted together to eliminate the inductive coupling effect of the secondary on the primary.
  • Similarly, when the three phases of the Secondary winding of the transformer has to be tested then the three phases of the Primary windings has to be all shorted together to eliminate the inductive coupling effect of the primary on the secondary.


    • Field coils have multiple poles so the comparison is done as 1φ windings.
    To perform the test do the connection of Surge Test leads as follows:
  • Connect RED and BLUE test leads to the field leads
  • Connect YELLOW test lead to the Midpoint of the field coil. With a sharp awl, simply pierce the insulation between the fields being tested and connect the test lead to the shaft of the awl. This will become the common path of Surge.
  • Connect BLACK lead has to be connected to the frame of field coil
  • The Test Select Switch should be set Position of 1φ.
    		•
    If the Fault is detected, it can be isolated by comparing individual coils. If this test is done, be sure all the remaining coils are either completely disconnected or all the coils are in series.
    The Shunt coils usually have a small error in turns count. Some mismatch or separation of the wave patterns should be accepted.

    Form wound coils are also tested by following the same procedure of 1φ windings.
    Set the Test Select Switch to Position 1φ and connect the test leads as follows:
    COIL (I): RED AND YELLOW
    COIL (II): BLUE AND BLACK
    NOTE: Some mismatch of waveforms may result due to Inductive coupling of the two coils. This is not an indication of faulty insulation. When the insulation failure occurs, a great deal os separation will be noted.

    When testing the assembled motors, the rotor can influence the shape of surge waveform as it causes the rapid damping of the wave pattern on the screen. The natural unbalance between the rotor and stator windings can also cause the two good phases to be mis-aligned. By turning the rotor slowly by hand this unbalance can be eliminated.
    Affect of Rotor:
  • The rotor affects the magnetic field and depending on it's position relative to each phase will determine the unbalance in the traces.
  • The shape of the trace on CRT display will be determined by the design of the motor and the number of poles.

    • Testing Guidelines:
  • If the Ground fault exists, one or both the waveforms will collapse.
  • As the test voltage is slowly increased observe the waveform for any instability. If the waveform begins to shift and change shape, then momentary shorting is starting at that voltage. Many incipient turn-to-turn short will not be visible at low voltage, but will become apparent at higher voltages.
  • The shape of the trace on CRT display will be determined by the design of the motor and the number of poles.
  • Record the voltage at which pattern becomes unstable. This voltage is the Di-electric limit of turn or phase Insulation.
  • In case of one winding being Open there will be either no pattern or a charachteristic squared off pattern in the beginning of the trace.


    • TESTING ASSEMBLED MOTORS FROM SWITCHGEAR
    Testing Assembled motors from the switchgear has few limitations so proper guidelines have to followed that are as follows:
  • First step is to de-energize the Motor.
  • Any Power factor capacitors in the circuit should be disconnected.
  • The Surge test circuit gets loaded by the cable capacitance as well as the motor therefore higher output test voltage will be required.

    • After this the same test procedure is then followed as mentioned above.

    The Large AC stators have high Capacitance and Inductance therefore extra care has to be observed while doing the test.
    The 3φ Stator can be tested as 3φ windings section above.
    The Sweep rate of the CRT display has to be adjusted carefully.

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