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PRINCIPLE OF TESTING
  • Surge Comparison Test
  • Bar-To-Bar Test
  • DC Hi-Pot Test
  • Rotor Test
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    The Surge Comparison tester checks the strength of ground insulation that consists of enameled insulation. It detects the insulation failures such as turn-to-turn shorts, layer-to-layer shorts, coil-to-coil shorts, winding-to-winding shorts, and phase-to-phase shorts. Open circuit and ground detection are other benefits of surge testing.
    The Surge tester uses the principle of impedance balance to test the quality of electrical windings.The Surge tester works as a capacitive-discharge system. A capacitor is charged with high voltage and then discharged into the winding, through a solid state assembly. This sequence is repeated thus stressing the insulation of the winding with high voltage pulses.
    The resulting voltage decay pattern of two winding is then displayed on the CRT. The waveform pattern will be perfectly superimposed for good windings but in case of a defective winding a double wave pattern will appear on the screen as one wave pattern from the good winding plus the erratic pattern from the faulty winding.
    The Surge Tester stresses the whole winding system as the current is applied as series of pulses. The voltage of these pulses rises in microseconds and produces a voltage distribution across the coil. For instance, when the pulse has penetrated to turn number 10, it may be at 2000volts while other turns (20,30 etc) have not been pulsed. It is at a lower voltage and if this difference is greater than Dielectric strength of the turn insulation, one or more turns may be shorted out of the circuit. If this shorted circuit is compared to the master winding, the two patterns will not match.


    Balanced Winding

     
    Turn short Winding

     
     
    The bar-to-bar test is used to test the armatures of Large DC motors without over-stressing the group insulation. Historically, span test method was used to test the DC Armatures by applying a very high test voltage across the coils to generate required inter-bar voltage. High voltages could result in break down of ground insulation of the first coil in the series.
    The Bar-to-Bar test method eliminates the limitation of span test method by applying High Surge current at significantly low voltage to produce the necessary inter-bar voltage across the adjacent commutator bars. Therefore the danger of ground insulation break-down is significantly reduced as the Bar-to-Bar test voltage never exceeds 1000 volts.
    A special low impedance cable and test head assembly is required to perform the Bar-to-Bar test in order to apply the Inter-bar voltages across different types of armatures.

  • 100-300 volts for small armatures
  • 200-500 volts for medium armatures
  • 500-800 volts for large traction armatures


  •  
    The DC Hi-Pot test is extremely important to identify the deterioration in the insulation as early as possible to take corrective measures. The high potential is applied between isolated parts of a circuit or a product, the behaviour of electrical parameters such as Leakage current helps identify broken or poor insulation, stray wire strands or braided shielding, spacing problem between terminals, tolerance errors, etc. During the Hi-Pot test, a high voltage is applied to the device under test (DUT) that causes a small leakage current (microamperes) to flow from the conductor and insulation. This small leakage current depends on three main factors i.e.
    (i) test voltage
    (ii) system capacitance and
    (iii) temperature of the material
    The leakage current is also a combination of three sub-currents :
    I) Conductive leakage current. Conductive current is a small current that normally flows through insulation, between conductors or from a conductor to ground. This current increases as insulation deteriorates and becomes predominant after the absorption current vanishes.
    II) Capacitive charging leakage current. When two or more conductors are run together in a raceway, these act as a capacitor. Due to this capacitive effect, a leakage current flows through conductor insulation.
    III) Polarisation absorption leakage current. Absorption current is caused by polarisation of molecules within the dielectric material.
    The Hi-pot test therefore helps in making informed decision on the safety and quality of electrical circuits and eliminates the possibility of having a life-threatening short-circuit or short-to-ground faults.

     
    The Rotor tester uses the principle of electrical and magnetic effects caused by the circulation of induced currents into the bars of a rotor, which is made to turn at a constant speed, with in a magnetic field produced by a permanent magnet. The induced magnetic field produced by permanent magnet, acts on one slot at a time. The direction of magnetic field is such that the turns of a moving rotor cut the lines of force normally.
    The induced currents, which are proportional to the field intensity (distance of magnet from the rotor, on the speed of rotation constant) & on the rotor characteristics, produce magnetic fields, the symmetry of which enable a clear observation of the turns in the short-circuited condition. A fixed probe subjected to the magnetic fields produced by the rotor under test, provides the signals, which after being amplified, are applied to the monitor.
    The synchronized sinusoidal wave pattern from the rotor bars is displayed on the monitor screen. The waveform identifies typically occurring faults in the rotor bars & there is a distant correlation between the result of the inductive test of the rotor bars & the starting torque of a motor.



    Analyze Rotor waveform
      Fig. shows following defects of the rotor:-
  • A - defect due to Broken Bar
  • B - defect due to Bar to Bar shorting
  • C - defect due to Blow Hole or High Bar Resistance
  • D - Perfect rotor portion
  • E - defect due to Skew Angle

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