FAQS

Frequently Asked Questions

Test Voltage Chart

Test Voltage Chart

The table below indicates the test voltages required to test the rotating machines.
Operating Voltage of Rotating Machines AC/DC H.V Test Voltage Surge Test Voltage
120 V 1,240 V 1,800 V
240 V 1,480 V 2,200 V
440 V 1,880 V 3,000 V
660 V 2,320 V 3,500 V
2.3 KV 5,600 V 7,000 V
3.3 KV 7,600 V 10,000 V
6.6 KV 14,200 V 20,000 V
11 KV 23,000 V 30,000 V
23 KV 47,000 V 49,000 V
 

NOTE:

Many other formulas are also used to calculate the test voltages for AC form wound coils. These are generally based on experience and theoretical arguments about the distribution of voltage in a coil, and the entire winding. The formulas are difficult to apply because of great diversity of coil specification and characteristics.

The other popular formula below also states the Min and Max test voltages:

MINIMUM TEST VOLTAGE = Number of Turns X 500
MAXIMUM TEST VOLTAGE = Operating Voltage X 1.5

AT WHAT VOLTAGE TO TEST WINDING OR A MOTOR?

The voltage test chart on the right gives a basic guiding formula for High Voltage insulation breakdown test and Surge Test as specified by the International standard organisations.

The formula used to calculate the Test voltages is as follows:
AC/DC H.V Test Voltage = (2×E + 1000)
Surge Test Voltage = √2 × (2×E + 1000)
where “E” is the operating voltage of the rotating machine

Principle of  Testing

The Principle of Surge Comparison Tester

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
    Balanced Winding
  • Turn short Winding

The Principle Of Bar To Bar Test

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 Principle Of DC Hi-Pot Test

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.

  • Test Voltage
  • System Capacitance and
  • 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.

Principle Of Rotor Tester

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.

Analyze Rotor waveform

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.

Fig. shows following defects of the rotor:-

  • defect due to Broken Bar
  • defect due to Bar to Bar shorting
  • defect due to Blow Hole or High Bar Resistance
  • Perfect rotor portion
  • defect due to Skew Angle

Testing Methods

Single Phase Windings

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
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

Three Phase Windings

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 comparedM
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.

Armature windings

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.

Transformers

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

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

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.

Assembled Motors

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

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.

Controls & Setup

Surge Tester – User Controls & Setup

CRT FRONT PANEL CONTROLS DESCRIPTION
INTENSITY Adjusts the brightness of the CRT display.
FOCUS & ASTIGMATISM Adjusts the clarity of the CRT from wide or thick trace to narrow or fine trace/wave pattern.
VERTICAL SHIFT Adjusts the position of trace/wave pattern in up and down movement.
HORIZONTAL SHIFT Adjusts the position of trace/wave pattern in left-to-right movement.
SWEEP Adjusts the Time base of the wave pattern. The waveform pattern can be contracted and expanded along the horizontal or time axis.
TRACE ROTATION Adjusts the Trace line in parallel with the horizontal line of the Graticule.
CRT Volts/Divison Determines the scale factor for reading the test voltage on the CRT display.
Surge Test Voltage applied = No. of Vertical divisons (X) VOLTS/DIV selected.
For eg:-
No. of divisons travesed by Surge test wave = 4
VOLTS/DIV Selected = 500V/DIV
SURGE TEST VOLTAGE APPLIED = 4 X 500 = 1000 Volts
SET TEST VOLTAGE This control when rotated clockwise will adjust the output test voltage from 0 to 100% of the equipment’s maximum rated output Test voltage capacity.
TEST SELECT SWITCH This Test Select switch determines which of the test leads are HOT or ENERGIZED and which are at Ground potential.
NOTE: The Windings connected to the corresponding Test leads will be compared with each other and resultant wavepattern will be displayed on the CRT.

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
HOT GND HOT GND
SENSITIVITY KNOB This control is only available with Automatic Surge Tester with Go/No-Go test.
This control helps in setting up the % variation difference wrt to Master winding.
NOTE: The sensitivity should be kept at 0-50 for Fan stators and around 0-70 for fridge stators.

Waveform Analysis

Waveform Analysis

The Fig. A below shows the Surge Test waveform of a coil of 13.3KV HT Motor and it has been highlighted with numbers to explain the important stages & charachteristics of the waveform.The Fig. A below shows the Surge Test waveform of a coil of 13.3KV HT Motor and it has been highlighted with numbers to explain the important stages & charachteristics of the waveform.

 

HIGHLIGHTED NO. DESCRIPTION OF WAVEFORM
1. The charging of the energy storage capacitor, is charge to the peak (Zero to Peak ) value of + ve half cycle of 50 Hz Mains frequency as shown in figure above for 10 milli sec.
2. Injection of Surge (impulse ) energy into the coils (impedance) under test.
3. The Impulse (surge) injection circuit is disconnected. i.e. the storage capacitor is disconnected from the coil (impedance) under test.
4. The energy which is injected into the coil under test, induced the feed back energy or we can say the voltage is generated in the coils. As you can see in the fig, the amplitude of the voltage peak is higher than the surge voltage injected in the coils.
The other way , we can say the loss is negligible or Q of the coil is good. So the IInd peak is Q-factor of the coil.
5. This voltage so induced charges the winding capacitance i.e. the Stray capacitance between the winding & core, as shown in fig. above.
6. The coil has three parameters such as:

  • Inductance L of the coil
  • Stray winding capacitance
  • The resistance of the coil

The coil will begin to oscillate with its natural frequency i.e. f = 1/2π √LC Hz.
These oscillations are the deciding factor in determining the quality of coil.

7. Voltage Induced decay oscillations of the coil.

 

  • Fig1. Turn short in the coil
  • Fig2. Complete Ground
  • Fig3. Open Coil Connection
  • Fig4. Phase-to-Phase Short

Calibration Procedure

Calibration Procedures

The circuit diagram shown below gives an overview of a basic Surge Tester and the significance of each component in the circuit. The user should have this basic knowledge before performing the calibration.

 

Circuit Components Description
Vc Supply Voltage Source 230V/50Hz
Ds Half Wave Rectifier
Rs Charging Current Limiting Resistance
Ts Discharge Switch
Rc Surge Discharging Current Limiting Resistance
C Charging Capacitor
Lc Coil under Test
Cp High Voltage Pick-up Capacitor
Ca & Rx Attenuation Capacitor & Resistance

OPTION I (X1000 times Probe):

  • Connect the Surge Test leads to the Stator or coils.
  • Set the Test Select switch to Position A for 3φ winding or Position 1φ for 1-phase coils.
  • Make sure the Surge tester is connected to a constant voltage source with 230V/50Hz.
  • Set the Test voltage control to zero.
  • Set the VOLTS/DIV to 500 position on the Surge Tester.
  • Set the VOLTS/DIV to 0.5 V on the Oscilloscope. As we are using X1000 times probe it will be equivalent to 500 volts.
  • Connect the X1000 Oscilloscope probe to the RED test lead and Ground clip of the probe to the Ground Test Lead.
  • Now slowly increase the Surge test voltage so that the charging peak of waveform equals (exact) one vertical divison on the oscilloscope. The waveform on the Oscilloscope will be Inverted wrt to display on Surge tester.
  • Verify that the waveform also shows one vertical divison on the CRT display of Surge tester.
  • If there is a discrepancy between the peak amplitude of two waveforms then surge test waveform can be calibrated by adjusting the value of Attenuation Capacitance connected to the 500V/DIV switch internally.
  • There are Individual Ca (Attenuation Capacitor) for each 500, 1000, 2000 & 4000 VOLTS/DIV therefore each range has to be calibrated wrt oscilloscope. The procedure remains the same as mentioned above.

OPTION II (X10 times Probe):

  • Connect the Surge Test leads to the Stator or coils.
  • Set the Test Select switch to Position A for 3φ winding or Position 1φ for 1-phase coils.
  • Make sure the Surge tester is connected to a constant voltage source with 230V/50Hz.
  • Set the Test voltage control to zero.
  • Set the VOLTS/DIV to 500 position on the Surge Tester.
  • Connect the X1000 Oscilloscope probe to the RED test lead and Ground clip of the probe to the Ground Test Lead.
  • Now slowly increase the Surge test voltage so that the charging peak of waveform equals (exact) to 500 volts on the oscilloscope. The waveform on the Oscilloscope will be Inverted wrt to display on Surge tester.
  • Make sure you disconnect the probe of the oscilloscope from the Surge test leads. This is X10 probe, the high voltage may damage the probe and the scope.
  • Verify that the waveform also shows one vertical divison on the CRT display of Surge tester.
  • If there is a discrepancy between the peak amplitude of two waveforms then surge test waveform can be calibrated by adjusting the value of Attenuation Capacitance connected to the 500V/DIV switch internally.
  • Once the 500V/DIV is calibrated, slowly increase the Surge test voltage so that it shows Two vertical divisons on the display. Now press the 1000V/DIV switch, the amplitude should drop to exact one division.
  • If there is any discrepancy adjust the Attenuation Capacitor for 1000 position internally.
  • Once the 1000V/DIV is calibrated, slowly increase the Surge test voltage so that it shows Two vertical divisons on the display. Now press the 2000V/DIV switch, the amplitude should drop to exact one division.
  • If there is any discrepancy adjust the Attenuation Capacitor for 2000 position internally.
  • Once the 2000V/DIV is calibrated, slowly increase the Surge test voltage so that it shows Two vertical divisons on the display. Now press the 4000V/DIV switch, the amplitude should drop to exact one division.
  • If there is any discrepancy adjust the Attenuation Capacitor for 4000 position internally.

SURGE TESTER – CALIBRATION

There are two procedures for doing the calibration of Surge Tester:

  • Any Standard 50MHz or above Oscilloscope and X1000 times, 10KV AC Probe.
  • Any Standard 50MHz or above Oscilloscope and X10 times AC Probe.

Pre-requisites for Calibration:

    Calibrated Oscilloscope (Min 50MHz)

  • A balanced 3φ stator or two 1φ balanced coils

NOTE: The Oscilloscope can also be calibrated using a square wave generator having a rise time of 0.1 micro sec, min. output voltage of 100 volts (peak-to-peak) and frequency of oscillation of 1KHz.

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