Tan Delta Test on 6KV HT Motor

http://en.wikipedia.org/wiki/Tan_delta_test

If you don't have a spare what choice do you have? You run it and get a spare sorted quickly. If you can leave it shut down then you've all the time in the world, until you need it in a hurry!

It's your call, it's risk assessment!

You do not provide sufficient information for anyone to provide you with advice.

Yes, the motor is breaking out seriously into corona just below the operating voltage.

How soon will the motor break down - will depend on the insulation used in the motor and on the number, amplitude and frequency of over-voltages and surges appearing at the terminals of the motor.

The scarcity of the explanations allow two possibility. One is Olehwi's: the motor run long enough highly stressed, and low level of corona discharge is eating the insulation at the end of its life. The other is, that the motor was in storage long enough to absorb moisture in the insulation. That can be cooked out by running the motor without load for a couple of days, giving a chance the moisture to bake out.

BUT, I have a problem with the basic question itself. Any motor is a very lossy capacitor from the silicon iron lamination alone. What is the loss factor in a newly assembled motor? Without knowing that, deterioration - if any - can only be guesstimated.

One more note. Corona and other insulation failure modes are voltage threshold dependent. You will see them at higher than normal voltages. Attempting to measure them at low voltages, as reported, is meaningless.

Cannot give a specific answer. However , some thoughts below-

1) Assuming that the voltage was between line and earth, you have already reached near phase voltage 3468V which is likely to happen with earthed neutral system. So less problem in analysis and also with the motor if it is the case.

2) Take heart! Does not appear too bad as various factors are involved in diagnosis. One of them is the insulation system used. Older systems have higher tan delta on which IEC allowed approx.4.5 % at rated voltage. Olehvi & levles are right and you should have report of condition on new motor for better understanding.Otherwise, check the system used and results of same make from same manufacturer.

3) The tip up of current is not bad.They must have stopped further testing due to limitation on bridge to have more than 150/200 mA current. If the readings at rated voltage do not shoot up, there is less probability of catastropic failure.

4)Also tip up of tan delta between 20 % & approx 60 % rated voltage is not too bad considering that you cannot test a complete motor with guard rings at the ends of each coil. Most probably, the manufacturer must have used a stress grading tape at the end of the coils which increase tip up.

5) Cleaner and dry 0verhang can improve tan delta if the windings are found contaminated with conducting dust.

pl keep posted if further investigations and actions taken

regards

kindly give details how do u analysis the tan delta results.

AS YOU KNOW THE TAN DELTA TEST ALSO CALLED LOSS ANGLE OR DISSIPATION FACTOR TESTING , IS A DIAGNOSTIC METHOD OF TESTING MOTOR WINDINGS INSULATION QUALITY. IF THE INSULATION IS GOOD, FREE FROM DEFECTS LIKE WATER TREE, ELECTRICAL TREE, MOISTURE AND AIR POCKETS ETC THE WINDING APPROACHES THE PROPERTY OF A PERFECT CAPACITOR. IF THERE ARE IMPURITIES IN THE INSULATION, THE RESISTANCE OF INSULATION DECREASES, RESULTING THE INCREASE IN RESISTIVE CURRENT THROUGH THE INSULATION. THE CURRENT AND VOLTAGE WILL NO LONGER BE 90 DEGREE OUT OF PHASE AND THE DIFFERENCE IN OUT OF PHASE (FROM PEFECT POSITION OF 90 DEGREE) IS THE INDICATIVE LEVEL OF INSULATION CONTAMINATION/DETEORIATION. THIS IS CALLED TAN DELTA. ANALYSIS OF READINGS THE READINGS SHALL BE ANALYSED IN REFERENCE TO THE MANUFACTURER RESULTS/DESIGN RESULTS. THE INCREASING TREND W.R.T INCREASING TEST VOLTAGE AND BY AGAING GIVES THE INDICATION OF FUTURE FAILURE. NORMALY THE VALUE SHOULD BE BELOW .03 RADIAN (3%) HOWEVER THE INTERPRETATION SHALL BE CONCLUDED ON CHECKING PD,PI, DELTA C , DC LEAKAGE CURRENT & SURGE TEST ALSO. REGARDS AKD

IN CONTINUTIONTO ERLIER POSTING YOUR MOTORS SEEMS TO BE HEALTHY AND SEEMS TO GIVE NO PROBLEM FOR NEXT 3-4 YEARS.

YOU MUST MONITOR ITS PARAMETERS EVERY YEAR.