How To Interpret TAN DELTA Test Result In A Rotating Machine

Dear sir,

Greetings.

We are having an induction motor with following specifications. This motor is failed and the test details are given for your information.

INDUCTION MOTOR

Make: Crompton Greaves

KW: 2000

Amps: 223

M/C No: 2081741

Stater Volts: 6600

Rotor Volts: 1825

RPM: 993

Efficiency: 95.8

Year of mfd: 06/2009

Bearings

DE: NU234C3

NDE: 232C3 + 6232C3

Weight: 11000kg

Frame: 13TPR630G

The motor is failed and the primary observation report of the failed motor is as given below.

PRIMARY OBSERVATION

· Stator slot wedges are found opened at some slots.

· Rotor NDE side binding was found carbonized due to flash over occurred at overhang portion.

· Rotor main leads were fount burnt.

· Both DE & NDE side bearing housings were found loose to bearings.

· Rotor found slightly rubbed to stator core.

· DE side bearing inner race found damaged.

WINDING CONDITION ANALYSIS

· Rotor binding on both sides were removed to see for any physical damage to windings.

· Found one particular rotor top coil was burnt and melted due to heavy flash over creating a cavity in rotor core as well.

· Further, removed the connecting lugs to windings and tested each coil by isolating and found the bottom coil of the flashed area as well as the surrounding coil are also burnt,

· Confirmed the Rotor winding is burnt.

· Tested stator with SURGE COMPRASION TESTER and found the resultant waveforms were satisfactory, again tested the POLARIZATION INDEX(PI) value of the stator winding and found OK, resulting that the stator winding was found GOOD.

· But, found the stator insulation was found brittle at the portion(18-22 places Approx) where wedges were opened.

ROTOR WINDING FAILURE CAUSE

· Primarily, the bearings housings were worn out, causing to reduce the AIR GAP for stator and rotor.

· Which thereby rubbed stator core, erupting the wedges.

· While running at full RPM, the erupted wedge punctured the rotor insulation, causing the winding to fail at an early stage.

RECTIFICATION AND REMEDIAL WORKS TO BE CARRIED OUT

· Rotor is highly recommended to be completely re-winded as the flash has affected the whole area.

· Stator needs to be de wedged completely and using a standard procedure of re-insulating by providing H class insulation paper, liquid insulating and providing with new EPOXY GLASS wedges etc.

· Reconditioning of both DE & NDE bearing housings.

· Replacement of both side bearings with new onces.

After the primary observation, we have gone for the rewinding of rotor and repairing of stator and the scope of work by the motor rewinding party is as given below.

SCOPE OF WORK FOR ROTOR REWINDING

· De-brazing the end series jumper connections.

· Removing the wedges and stripping out the old burnt copper strip from the rotor core.

· Cleaning of slots with appropriate cleaning solvents.

· Fabricating of new rotor coils with 99.9% ELECTROLYTIC & DOUBLE FIBER GLASS COVERED COPPER CONDUCTOR.

· Providing of `H’ Class inter turn insulation to each strip with NOMEX and again a layer of POLY GLASS MICA INSULATION tape on the strip, Then providing slot insulation with `H’ CLASS HYPOTHERM(10 mill).

· Shaping the rotor strips as per the pitch and inserting into the rotor slots.

· Providing overhang insulation with VARNISHED GLASS TAPE.

· Providing of NEW CONNECTION LEADS made of tin coated copper conductor, insulated with DOUBLE FIBER GLASS & SILICON RUBBER and a layer POLY GLASS MICA AND TERLIN tape insulation along with copper lugs.

· Providing copper clips for series connections on both sides.

· Brazing of end series connections and jumper connections with leads on both sides with L&T MAKE 1805 GRADED SILVER BRAZING RODS.

· Insulating he brazed connections with a layer of KAPTON and two layers of PGMP & TERLIN insulations.

· Binding of overhang portions with unvarnished Resi glass yarn by applying Dr.Beck make HARDNER EH 408 and DEBKOT 505C resin bond insulation material for locking.

· Put into hot air circulated heat chamber for curing of Resi glass and getting proper IR values.

· Baking with Dr.Beck make ISONAL-51 J Varnish, again put into heat chamber for drying the varnish, applying Dr.Beck make Becktol red and gel coat to avoid the entry of moisture.

· DYNAMIC BALNCING OF ROTOR on computerized dynamic balancing machine along with cooling fans, slipring and accessories.

SCOPE OF WORK FOR STATOR REPAIR & OVERHAULING

· Motor complete dismantling including removing of air cooler, cooling Fans, slipring unit and slipring cover, both end Covers, bearings with housings on both sides.

· Threading out the rotor from the stator, checking the bearing seating dimensions on both sides on shaft with technical parameters.

· Carefully DE WEDGING of all slots by special tools, without damaging the windings.

· Cleaning of Burnt paper insulation and carbon deposits with CCL4.

· Keeping of stator in Hot air circulating heat chamber for 48Hrs to remove all moisture and then pouring DR.BECK make ISONEL71J liquid insulating varnish into each slot and again baking in oven upto drying @ 120 Deg.

· Providing PURE NOMEX insulation paper along with EPOXY slot liners into windings and then inserting new wedges made of TAPERED & EPOXY GLASS material.

· Again putting in to hot air circulated heat chamber for getting proper IR Values and firming up of windings

· Applying Dr.Beck make EPOXY GEL COAT red to avoid entry of moisture.

· After completion of work inserting the rotor into the stator, fixing of bearings on both sides, fixing of housings, end covers, Fans and air cooler, complete replacement of gaskets and fastners, assembling the motor in complete shape.

· Conducting all routine tests i.e. Insulations resistance, winding resistance, inductance, surge comparison and tan delta tests etc.

After completion of rewinding of rotor and repairing of stator,we did the tests before assembling and test results are given below.

ROTOR

1. INSULATION RESISTANCE(IR)

2. HIGH VOLTAGE TEST

3. RESISITANCE & INDUCTION VALUES TEST

4. POLARISATION INDEX(PI)

5. SURGE COMPARISON TEST( Testes with 15KV DC HIPOT Surge Test Kit)

· Applied Voltage 500V in between R,Y,B phases to Earth and the wave forms displayed in the Oscilloscope are found equal, Hence the results are satisfactory.

STATOR

1. INSULATION RESISTANCE(IR)

2. RESISITANCE & INDUCTION VALUES TEST

3. POLARISATION INDEX(PI)

4. SURGE COMPARISON TEST (Testes with 15KV DC HIPOT Surge Test Kit)

· Applied Voltage 500V in between R,Y,B phases to Earth and the wave forms displayed in the Oscilloscope are found equal, Hence the results are satisfactory.

5. TAN DELTA

I request members to kindly guide us about these test results and their opinions. It is observed that dissipation factor in TAN DELTA test is 0.09994, 0.0937 and 0.0982.Is there any guide line about this factor? Whether Tan Delta test is applicable for rotating machines? How much should be dissipation factor? Since, stator, all other tests , viz; PI test, Surge comparator test ,IR test ,HV test, Resistance and inductor values are OK, is there any need for TAN Delta test? Exactly what is the purpose and how the results can be inferred towards healthiness of motor, Kindly give your guidance. Give your comments about the other test results also. After rewinding, how much life I can expect for this motor?

I was told by the repairer, the insulation cost for the rotor is about 75% of the cost of the copper used for rotor. Can any body react on this statement? What is the advantage of going for silver brazing compared to copper brazing?How much cost difference will come for both brazings. Please help us.

Thanks and Regards,

NVRSrinivas